leaf_core/

doc.rs

//! The document model: a `twig::Editor` plus a byte-offset caret and selection.
//!
//! Where bough moves a selection through the *tree*, leaf moves a *caret*
//! through the *characters* — a normal text editor's model — and expresses
//! every mutation as one of twig's offset-addressed ops:
//!
//!   - typing / delete  → `edit_range(start, end, text)`   (P0)
//!   - re-anchoring      → the returned `Change`            (P1)
//!   - cursor context    → `node_at` / `ancestors_at`       (P3)
//!   - the toolbar       → `wrap_range`/`toggle_inline`/`set_block`,
//!                         `toggle_block_container`/`insert_link`   (P5)
//!
//! twig reparses after every edit and leaves everything outside the splice
//! byte-for-byte untouched, so the document stays a live, navigable AST while
//! you type into it.

// `PathBuf` names the `path` field and the untitled marker on every build;
// `Path` is only touched by the filesystem I/O gated behind the `fs` feature.
// The docs in this file lay their `- key → meaning` lists out in aligned
// columns, which puts a continuation line further right than clippy's
// list-indent rule likes. A lazy continuation renders as the same paragraph
// either way, and the alignment is what makes those tables readable, so the
// layout wins over the lint.
#![allow(clippy::doc_overindented_list_items)]

use std::collections::HashMap;
use std::ops::Range;
#[cfg(feature = "fs")]
use std::path::Path;
use std::path::PathBuf;

#[cfg(feature = "fs")]
use anyhow::Context;
use anyhow::{Result, anyhow};
use twig::{
    Alignment, BlockContainerKind, BlockKind, Change, Editor, FlatNode, Format, Gesture,
    InlineKind, Kind, MarkdownExtensions, NodeId, QueryMatch,
};
use unicode_segmentation::GraphemeCursor;

use crate::counts::{self, TextCounts};
use crate::html;
use crate::source::{self, SourceMap};
use crate::style::{Align, FontFace, FontSize, LineHeight, MarkColor, TextColor};
use crate::wysiwyg::{self, MediaKind, MediaStop, Reveal, VisualMap};

/// Which view the body shows.
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub enum View {
    /// The raw document with a caret in source bytes.
    Source,
    /// Markup resolved to real styles, caret riding the rendered glyphs.
    Wysiwyg,
}

/// How much of the source markup the WYSIWYG view exposes — a per-editor
/// preference, orthogonal to [`View`]. Named for markup rather than for Markdown
/// because leaf is grammar-agnostic: twig hands it Djot, HTML and XML on the same
/// terms, and every rung below is about *delimiters*, whatever grammar spells
/// them. The examples are Markdown only because that is what most documents are.
///
/// A single ladder over two underlying axes, because only three of their four
/// combinations are coherent:
///
/// | | authoring off | authoring on |
/// |---|---|---|
/// | delimiters hidden | [`None`](Self::None) | [`Shortcuts`](Self::Shortcuts) |
/// | caret line revealed | *incoherent* | [`Full`](Self::Full) |
///
/// The empty quadrant would show delimiters on the caret's line and then escape
/// the ones you type — a surface that displays a syntax it refuses to accept.
/// Someone who wants to read raw markup without authoring it has
/// [`View::Source`], which is the better tool for it.
///
/// The two axes are read separately by the code that cares — see
/// [`reveals_caret_line`](Self::reveals_caret_line) and
/// [`authors`](Self::authors) — so neither behaviour has to know it's spelled
/// as a ladder.
#[derive(Clone, Copy, PartialEq, Eq, Debug, Default)]
pub enum MarkupMode {
    /// Delimiters stay hidden even on the caret's line, and typed syntax stays
    /// literal — twig escapes anything that would open markup, so formatting
    /// comes from commands (⌘b, the toolbar) instead of from spelling. The clean
    /// reading surface for people who don't write markup by hand; the default,
    /// and what Diaryx ships.
    #[default]
    None,
    /// Delimiters stay hidden, but typing them authors real markup: `*x*`
    /// becomes italic and the asterisks disappear into the styling
    /// (Typora/Bear-shaped). For someone who knows the syntax but wants the
    /// clean surface back once it has been applied.
    Shortcuts,
    /// The caret's line shows its raw markup while every other line renders
    /// resolved (Obsidian live-preview-shaped), and typed syntax authors markup
    /// — for people fluent in the document's grammar who want to see and edit
    /// the delimiters they type.
    Full,
}

impl MarkupMode {
    /// Whether the rich view shows raw delimiters on the line holding the caret.
    /// The rendering axis — read by [`Doc::reveal_line`] and threaded into the
    /// WYSIWYG builder.
    pub fn reveals_caret_line(self) -> bool {
        matches!(self, MarkupMode::Full)
    }

    /// Whether typed markup characters author real formatting. The editing axis
    /// — read by [`Doc::insert`], which escapes typed syntax when this is false.
    pub fn authors(self) -> bool {
        !matches!(self, MarkupMode::None)
    }
}

/// How the WYSIWYG view treats a *soft break* — a bare newline inside a
/// paragraph. An axis of its own, orthogonal to [`MarkupMode`] (which governs
/// inline-markup delimiters) and to [`View`]: any reveal preference pairs with
/// either flow. The renderer consults it when it lays a block's inline content
/// into visual rows.
#[derive(Clone, Copy, PartialEq, Eq, Debug, Default)]
pub enum LineFlow {
    /// A soft break folds into a space and the paragraph reflows to the
    /// viewport width — flowing prose, where the source's line wrapping is
    /// insignificant. The default, and what Diaryx ships.
    #[default]
    Fold,
    /// A soft break renders as a line break exactly where it was written, so
    /// the author's source line structure shows on screen unchanged — the mode
    /// for people who lay out their prose deliberately (one sentence or clause
    /// per line, semantic line breaks). The break is still a soft break in the
    /// source; only its rendering changes.
    Preserve,
}

/// What the file behind a document looks like right now, against the bytes leaf
/// last read from it or wrote to it — the question a frontend asks before it
/// saves (a `Changed` file plus a `dirty` document is an overwrite about to
/// happen) or when its window regains focus. See [`Doc::disk_state`].
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum DiskState {
    /// The file holds exactly the bytes leaf last read or wrote.
    Unchanged,
    /// Someone else wrote the file since. Saving overwrites their work; see
    /// [`Doc::reload`] for the other direction.
    Changed,
    /// The file is gone — deleted or renamed away. A save recreates it.
    Missing,
    /// There is a path, but the file couldn't be read (permissions, a directory
    /// in the way): leaf can't tell, and won't guess.
    Unreadable,
    /// No file behind this document yet — see [`Doc::blank`]. Nothing can have
    /// changed under a document that was never on disk.
    Untitled,
}

/// The inline marks in force at a point in the document — what a toolbar
/// lights up. A `Copy` bitset rather than a `HashSet`, because
/// [`Doc::active_inline_marks`] is called on every frame that draws a toolbar
/// and a set that allocates to answer "is Bold on?" is a set that shouldn't.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub struct InlineMarks(u8);

impl InlineMarks {
    /// Every kind, in the order [`InlineMarks::iter`] yields them.
    const ALL: [InlineKind; 8] = [
        InlineKind::Strong,
        InlineKind::Emph,
        InlineKind::Verbatim,
        InlineKind::Mark,
        InlineKind::Superscript,
        InlineKind::Subscript,
        InlineKind::Insert,
        InlineKind::Delete,
    ];

    pub const fn empty() -> Self {
        InlineMarks(0)
    }

    /// Private: the set is an *answer*, and adding a mark to it doesn't mark
    /// anything ([`Doc::toggle`] does that). `FromIterator` is the way in.
    fn insert(&mut self, kind: InlineKind) {
        self.0 |= Self::bit(kind);
    }

    /// Flip `kind` in the set — the sticky-marks toggle at a collapsed caret.
    fn flip(&mut self, kind: InlineKind) {
        self.0 ^= Self::bit(kind);
    }

    /// The symmetric difference: which marks differ between the two sets. Used
    /// to resolve the marks already in force at the caret against the pending
    /// delta — a bit set in the delta flips the base mark for the next keystroke.
    fn xor(self, other: InlineMarks) -> InlineMarks {
        InlineMarks(self.0 ^ other.0)
    }

    /// Whether `kind` is in force — the toolbar's "is Bold active?".
    pub fn contains(self, kind: InlineKind) -> bool {
        self.0 & Self::bit(kind) != 0
    }

    pub fn is_empty(self) -> bool {
        self.0 == 0
    }

    /// The marks in force, for a frontend that renders whatever is on rather
    /// than asking after a fixed list.
    pub fn iter(self) -> impl Iterator<Item = InlineKind> {
        Self::ALL.into_iter().filter(move |&k| self.contains(k))
    }

    fn bit(kind: InlineKind) -> u8 {
        1 << match kind {
            InlineKind::Strong => 0,
            InlineKind::Emph => 1,
            InlineKind::Verbatim => 2,
            InlineKind::Mark => 3,
            InlineKind::Superscript => 4,
            InlineKind::Subscript => 5,
            InlineKind::Insert => 6,
            InlineKind::Delete => 7,
        }
    }
}

impl FromIterator<InlineKind> for InlineMarks {
    fn from_iter<I: IntoIterator<Item = InlineKind>>(iter: I) -> Self {
        let mut m = InlineMarks::empty();
        for k in iter {
            m.insert(k);
        }
        m
    }
}

/// What kind of edit produced an undo group. Same-kind edits in a row coalesce
/// into one undo step (a run of typed characters undoes together); `Other` never
/// coalesces, so a paste, format toggle, or block change is always its own step.
#[derive(Clone, Copy, PartialEq, Eq)]
enum EditKind {
    Insert,
    Delete,
    /// One step of an IME composition — see [`Doc::edit_composing`]. Its own kind
    /// rather than `Insert`'s because a composition is not typing: each step
    /// *replaces* the last (`か` → `かん` → `感`), so the run has to coalesce even
    /// though no two steps insert the same bytes, and it must not fold into the
    /// typed characters on either side of it.
    Compose,
    Other,
}

/// Which side of the caret a delete looks for an in-cell `<br>` break to swallow
/// whole — see [`Doc::cell_break_at`]. `Backward` is Backspace (a break ending at
/// the caret), `Forward` is Delete (one starting at it).
#[derive(Clone, Copy)]
enum BreakEdge {
    Backward,
    Forward,
}

/// A re-spelling of one inline mark run, held ready in case the edit about to
/// happen breaks it — see [`Doc::mark_edge_fix`] and [`Doc::repair_mark_edges`].
/// Every offset in it is in the coordinates the document will have *after* the
/// plain edit, since that is when it may be applied.
struct MarkEdgeFix {
    /// The run's kind, and an offset inside what was its content: together they
    /// answer "did the plain edit actually break this mark?" — the question that
    /// decides whether any of this is applied at all.
    kind: InlineKind,
    probe: usize,
    /// The byte range to re-spell (the run's delimiters included) and its new
    /// spelling, with the edge whitespace moved outside the delimiters.
    start: usize,
    end: usize,
    text: String,
    /// Where the caret belongs afterwards — the same place on screen it would
    /// have had, which is now on the other side of a delimiter.
    caret: usize,
    /// The marks in force for text typed at that caret. The caret can land
    /// outside a run it was inside, and the marks have to survive the move or
    /// the toolbar goes dark mid-word.
    want: InlineMarks,
}

/// The caret and selection at one moment — the part of a history step twig's
/// `Change` cannot carry, because the caret is leaf's state and twig only knows
/// about bytes. leaf serializes it into the opaque per-state blob twig now
/// stores in its own undo history (see `record_caret`), so undo and redo hand
/// back the caret that matches the source they restore.
#[derive(Clone, Copy)]
struct CaretState {
    caret: usize,
    anchor: Option<usize>,
}

impl CaretState {
    /// Pack into the fixed 17-byte blob leaf hands twig: the caret as a u64,
    /// then an anchor-present flag and the anchor. twig copies these bytes and
    /// never reads them.
    fn to_blob(self) -> [u8; 17] {
        let mut b = [0u8; 17];
        b[..8].copy_from_slice(&(self.caret as u64).to_le_bytes());
        if let Some(a) = self.anchor {
            b[8] = 1;
            b[9..].copy_from_slice(&(a as u64).to_le_bytes());
        }
        b
    }

    /// Recover a state from twig's blob, or `None` when it is empty or the wrong
    /// length — a state twig restored that never had a caret set on it, which
    /// leaves the caller to fall back to the edit site.
    fn from_blob(b: &[u8]) -> Option<Self> {
        let b: &[u8; 17] = b.try_into().ok()?;
        let caret = u64::from_le_bytes(b[..8].try_into().unwrap()) as usize;
        let anchor = (b[8] != 0).then(|| u64::from_le_bytes(b[9..].try_into().unwrap()) as usize);
        Some(CaretState { caret, anchor })
    }
}

/// A footnote reference and the note it names — the answer to
/// [`Doc::footnote_at`].
///
/// The two `Option`s move together: a reference whose definition is missing has
/// neither a body to show nor a place to jump to, and one that resolved has
/// both.
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct FootnoteRef {
    /// The reference's label — the `1` of `[^1]`, with neither the `^` that
    /// spells it a footnote nor the brackets around it.
    pub label: String,
    /// The note's body as source bytes (see
    /// [`wysiwyg::footnote_body_span`](crate::wysiwyg)), or `None` when the
    /// document defines no `[^label]:` to read one from.
    pub text: Option<String>,
    /// Where the note's *body* starts, for a "go to note" that moves the caret
    /// there. `None` alongside a `None` `text`.
    ///
    /// The body rather than the definition, because this is an offset to put a
    /// caret on and the `[^1]:` marker is decoration the caret can't occupy —
    /// aiming at the definition's first byte snaps to the nearest real stop,
    /// which is up in the paragraph above the note. It is also simply where a
    /// reader following a reference wants to land: at the note's first word,
    /// ready to read or amend it.
    pub offset: Option<usize>,
    /// Where the note's body ends, exclusive — so a frontend can ask which
    /// *rendered rows* the note occupies and draw those instead of [`text`](Self::text).
    ///
    /// The rows are the note with its markup resolved: `see *later*` reaches a
    /// frontend as an italic run, not as asterisks. `text` is the source bytes
    /// and stays the honest answer for anything that wants the note as written
    /// (a search index, a copy); this pair of offsets is for anything that wants
    /// it as *read*. `None` alongside a `None` `offset`.
    pub end: Option<usize>,
}

/// A footnote definition and the reference that sends a reader to it — the
/// answer to [`Doc::footnote_definition_at`], and the other half of the round
/// trip [`FootnoteRef`] starts.
///
/// A note is a place a reader *arrives*, so the useful thing to know while
/// standing in one is the way back. Without this the jump to a note is a
/// one-way door: the definitions sit at the foot of the document, so returning
/// by hand means scrolling back up and finding the sentence again.
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct FootnoteDef {
    /// The definition's label — the `1` of `[^1]: …`, marker and colon stripped,
    /// spelled exactly as [`FootnoteRef::label`] spells the same footnote's.
    pub label: String,
    /// Where the reference's *label* is, for a "back to reference" that moves
    /// the caret there. `None` for a note nothing refers to — an orphan, which
    /// is worth being able to say rather than silently doing nothing.
    ///
    /// The label rather than the reference's first byte, for
    /// [`FootnoteRef::offset`]'s reason: a reference's brackets are decoration
    /// and its label is the only part of it the caret can rest on.
    ///
    /// The *first* reference, when a label is cited more than once: a repeated
    /// citation has no one true home, and the first is both the one a reader
    /// most likely came from and the only choice that doesn't depend on how
    /// they got here.
    pub offset: Option<usize>,
}

/// twig's cap on retained undo steps (`MAX_UNDO` in its splicer), which
/// [`Doc::can_undo`]'s count follows: past it twig drops the oldest step.
const UNDO_CAP: usize = 200;

/// An open [`Doc::begin_undo_group`]: how deeply it is nested, and whether an
/// edit made inside it has put its one step on the history yet.
#[derive(Clone, Copy, Debug)]
struct UndoGroup {
    depth: usize,
    has_step: bool,
}

/// What [`Doc::set_unrevealed`] sets aside: the screen's map, what it was
/// built from, and the caret state a paper build's clamp must not move.
struct ScreenMap {
    vmap: VisualMap,
    key: Option<(u64, Option<usize>, Option<Reveal>)>,
    caret: usize,
    anchor: Option<usize>,
    goal_col: Option<usize>,
}

/// Where a locator lands — the answer to [`Doc::locate`].
///
/// A locator (the `v2` of a `chapter.dj#v2`) names a *place* rather than a
/// document, and a place is a span rather than a point: a reader following one
/// wants the caret at its first byte, and a reader merely *peeking* at one wants
/// the block it covers drawn. Both are served by carrying the whole span, and
/// only one of the two can be recovered from an offset alone.
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct Landing {
    /// The first byte of the block the locator names — where a caret goes.
    pub start: usize,
    /// One past its last byte, so a frontend can map the pair through
    /// [`VisualMap::row_range_for`](crate::wysiwyg::VisualMap::row_range_for) to the rendered rows the block occupies and draw
    /// those, the way a footnote peek draws a note ([`FootnoteRef::end`]).
    pub end: usize,
}

/// The heading a place in the document sits under — the answer to
/// [`Doc::heading_at`].
///
/// What a host writing a link *to* a position needs: the `#its-slug` half of a
/// reference is made from `text`, and a peek at the target draws `span`.
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct Heading {
    /// The heading's inline content with its markup stripped — `A *big* day`
    /// is `A big day` — which is what a slug is made from.
    pub text: String,
    /// 1 for `#`, 2 for `##`, and so on.
    pub level: u32,
    /// The heading block's own source span, marker and all — the heading, not
    /// the section it opens.
    pub span: Range<usize>,
}

/// Where a dragged block would land — the answer to [`Doc::drop_target_at`].
///
/// A drop is aimed at a *row* (the one under the pointer) and lands at a
/// *boundary* (between two blocks), and a frontend needs both halves: the
/// offset to hand [`Doc::move_block`], and the row to draw the indicator
/// above — which is not the row aimed at, since a drop on the lower half of a
/// block lands below it.
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub struct DropTarget {
    /// The boundary offset — `move_block`'s `to`.
    pub offset: usize,
    /// The rendered row the indicator is drawn above; `rows.len()` for a drop
    /// below everything.
    pub row: usize,
}

/// A selection cited out of the source: the text itself, up to a requested
/// number of characters either side, and the byte range it came from. See
/// [`Doc::selection_quote`].
///
/// The prefix and suffix are what make the quote *re-findable*: the same text
/// can occur twice, and a little of what surrounded it is how a later reader —
/// or the same document after an edit — tells the occurrences apart. The Web
/// Annotation model calls this a `TextQuoteSelector`; the shape is older than
/// the name.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Quote {
    /// The selected source, verbatim.
    pub exact: String,
    /// What immediately preceded it — possibly empty, at the document's start.
    pub prefix: String,
    /// What immediately followed it — possibly empty, at the document's end.
    pub suffix: String,
    /// Byte offset in the source where the selection begins.
    pub start: usize,
    /// Byte offset where it ends (exclusive).
    pub end: usize,
}

/// A host-painted range of the source — an annotation's footprint, a search
/// hit, a reviewer's mark. Leaf renders it (a background wash behind the
/// glyphs whose source falls inside it) and hands back the `id` when the
/// reader activates it; what the range *means* is entirely the host's.
///
/// Ranges are source bytes, like the caret and the selection, so a host that
/// anchors quotes against the source ([`Doc::selection_quote`] is the other
/// half of that loop) can paint what it found without any coordinate
/// conversion. A range that drifts off the text it meant is the host's to
/// re-anchor; leaf draws what it is told.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Highlight {
    /// Byte offset in the source where the wash begins.
    pub start: usize,
    /// Byte offset where it ends (exclusive).
    pub end: usize,
    /// The host's name for it, handed back on activation. Opaque to leaf.
    pub id: String,
    /// A rendering hint the frontend maps — a `#RRGGBB` hex string, or
    /// nothing for the theme's default wash.
    pub color: Option<String>,
    /// A margin glyph's name, or nothing for wash-only ink. A highlight with
    /// a marker gets a small glyph in the margin beside its first line, and
    /// the glyph — not the wash — is what activates it: the wash is ink, the
    /// marker is the control, which is what lets a reader put a caret in (or
    /// copy from) annotated text without a card leaping at them. The name is
    /// opaque to leaf; an Apple frontend reads it as an SF Symbol, a web one
    /// as a class.
    pub marker: Option<String>,
}

impl Highlight {
    /// The range covering source `offset` in a list [`Doc::set_highlights`]
    /// sorted, first by start where several overlap — the one place that
    /// question is answered, for the frontends that paint by asking it as well
    /// as for [`Doc::highlight_at`].
    ///
    /// The list is sorted by `(start, end)`, so the scan can stop at the first
    /// range starting past `offset` rather than running to the end. A painter
    /// asking once per glyph wants [`HighlightCursor`] instead; this is the
    /// one-shot form, for the host asking what the reader just activated.
    pub fn covering(highlights: &[Highlight], offset: usize) -> Option<&Highlight> {
        highlights
            .iter()
            .take_while(|h| h.start <= offset)
            .find(|h| offset < h.end)
    }
}

/// [`Highlight::covering`] for a caller walking the document in order — which
/// is every painter, since a frontend draws rows top to bottom and glyphs left
/// to right.
///
/// The one-shot form is a scan from the front of the list per glyph, and a
/// document with two hundred search hits pays that two hundred times a row. A
/// range that ends at or before an offset can never cover that offset *or any
/// later one*, so the cursor retires those permanently and each glyph costs the
/// ranges that actually reach it. The answer is identical to
/// [`Highlight::covering`]'s, offset for offset — this is the same scan with
/// the part that was being redone dropped, not a cheaper approximation.
///
/// Offsets are expected to arrive non-decreasing. One that goes backwards is
/// still answered correctly: the cursor re-seats to the front, since a painter
/// that revisits a row is asking a question the retired ranges may own again.
pub struct HighlightCursor<'a> {
    highlights: &'a [Highlight],
    /// The first range not yet retired.
    at: usize,
    /// The last offset asked about, to notice a caller going backwards.
    last: usize,
}

impl<'a> HighlightCursor<'a> {
    pub fn new(highlights: &'a [Highlight]) -> Self {
        HighlightCursor {
            highlights,
            at: 0,
            last: 0,
        }
    }

    /// The range covering `offset`, advancing the cursor past every range that
    /// can no longer cover anything.
    pub fn at(&mut self, offset: usize) -> Option<&'a Highlight> {
        if offset < self.last {
            self.at = 0;
        }
        self.last = offset;
        while self
            .highlights
            .get(self.at)
            .is_some_and(|h| h.end <= offset)
        {
            self.at += 1;
        }
        Highlight::covering(&self.highlights[self.at..], offset)
    }
}

/// The identity of a built [`VisualMap`] — see [`Doc::visual_key`]. Opaque on
/// purpose: the only useful question is whether two of them are the same map,
/// and what is behind it — which `Doc` built it, and the (revision, wrap,
/// reveal line) it was built from — is core's business.
///
/// The document is part of it because the rest is not unique to one: two
/// documents opened at the same width are both at revision zero with no reveal
/// line, and a frontend holding one copy of a map across the two would take
/// the second's key for the first's and paint the wrong document.
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct VisualKey(u64, Option<(u64, Option<usize>, Option<Reveal>)>);

pub struct Doc {
    editor: Editor,
    pub format: Format,
    pub path: PathBuf,
    /// Current source, refreshed from the editor after every successful edit.
    pub source: String,
    /// The caret, as a byte offset into `source` (always on a char boundary).
    pub caret: usize,
    /// The selection's fixed end, if a selection is active; the moving end is
    /// the caret. `None` means no selection.
    pub anchor: Option<usize>,
    pub dirty: bool,
    pub status: Option<String>,
    pub view: View,
    /// Whether the document refuses to change — a *reading* surface over the
    /// same rendering, selection, and navigation the editor has.
    ///
    /// Enforced here rather than by each frontend hiding its input paths,
    /// because every mutation funnels through a few doors —
    /// [`splice_exact`](Self::splice_exact), [`undo`](Self::undo),
    /// [`redo`](Self::redo), and the handful of inserts that go to twig's own
    /// verbs directly rather than through the splice (a typed literal, a link,
    /// an image, a rule, a footnote, a cell's line break) — and guarded doors
    /// are a guarantee where a frontend's suppressed keyboard is a hope. A
    /// gated door reports exactly like a rolled-back splice, a path every
    /// caller already handles. `a_read_only_document_refuses_every_door` is
    /// the list; a new `self.editor.insert_*` call belongs on it.
    read_only: bool,
    /// The host-painted ranges, kept sorted by start — see [`Highlight`].
    /// State like the selection rather than like the text: no edit history,
    /// no dirty bit, redrawn from whatever the host last set.
    highlights: Vec<Highlight>,
    /// How much of the source markup the rich view exposes — a frontend preference (see
    /// [`MarkupMode`]). Its two axes are read apart: the rendering one by
    /// [`reveal_line`](Self::reveal_line), the editing one by
    /// [`insert`](Self::insert).
    markup_mode: MarkupMode,
    /// Whether soft breaks fold into the reflowed paragraph or render where
    /// they were written (see [`LineFlow`]) — an independent frontend
    /// preference the WYSIWYG builder consults when it lays out a block.
    line_flow: LineFlow,
    /// The kind of the last edit, for coalescing: twig owns the undo *history*
    /// (see `undo`/`redo`), but "what counts as one undo step" is a frontend-UX
    /// call, so leaf decides when a run continues and tells twig to coalesce.
    last_edit_kind: Option<EditKind>,
    /// The inline marks the user has toggled *at a collapsed caret* with no
    /// selection — "start typing bold here". Held as the XOR delta from the marks
    /// already in force at [`pending_at`](Self::pending_at): a set bit means
    /// "flip this kind for the next typed text", so it both turns a mark on where
    /// none is (type into bold) and off where one already covers the caret (type
    /// past the bold you're standing in). [`Doc::insert`] realises it onto the
    /// freshly typed text and then clears it — a mark once realised is carried by
    /// the caret sitting inside the run, not by this delta.
    pending_marks: InlineMarks,
    /// The caret offset [`pending_marks`](Self::pending_marks) applies to. The
    /// delta is live only while the caret still stands here with no selection;
    /// any motion or edit ([`move_to`](Self::move_to), a splice, a click) drops
    /// it, so a toggled-but-never-typed format doesn't leak onto text elsewhere.
    pending_at: Option<usize>,
    /// The source as of the last open/save — `dirty` is `source != clean_source`,
    /// so undoing back to the saved state correctly clears the modified flag.
    clean_source: String,
    /// A hash of the bytes leaf last read from `path` or wrote to it; `None`
    /// while the document has no file behind it. [`Doc::disk_state`] compares
    /// the file against this to catch an edit made *outside* leaf before a save
    /// silently overwrites it — `clean_source` only knows what leaf itself did.
    ///
    /// A hash, not an mtime: mtime is the cheap answer and the wrong one — two
    /// writes inside one filesystem timestamp tick are indistinguishable, a
    /// clock that steps backwards (or a writer that restores an mtime) hides a
    /// real change, and a `touch` invents one. The whole point of the watermark
    /// is to not clobber someone's work, so it reads the bytes and compares what
    /// is actually there. That costs a file read per question, which is why the
    /// question is asked on a user event (focus, save) and not every frame.
    disk_hash: Option<u64>,
    /// The "sticky" display column vertical motion aims for, in the active
    /// view's grid. Set on the first `move_up`/`move_down` of a run and
    /// reused by every subsequent one in that run, so passing through a
    /// shorter line doesn't permanently forget the original column. Any
    /// horizontal motion or edit clears it.
    ///
    /// A column, not a character index: dropping down a line of `你好` onto one
    /// of ASCII has to land under the glyph the caret was drawn beneath, which
    /// is the only thing the user can see to aim by. Where the goal falls inside
    /// a wide character on the target line, the mapping resolves it to that
    /// character — the caret lands on it rather than between its cells.
    goal_col: Option<usize>,
    /// The rendered map for the WYSIWYG view; empty in the source view. Movement
    /// and clicks read it to stay in visible space.
    pub vmap: VisualMap,
    /// The syntax map for the source view; empty in the WYSIWYG view, which
    /// styles resolved glyphs instead. Built by [`Doc::build_source`] — a
    /// frontend that never calls it paints raw source unstyled, which is what
    /// every frontend did before this map existed.
    pub smap: SourceMap,
    /// The revision `smap` was built from, or `None` before the first build.
    /// The map is a pure function of the text alone — no width, no caret, no
    /// reveal line — so unlike [`vmap_key`](Self::vmap_key) the revision is the
    /// whole key.
    smap_key: Option<u64>,
    /// Everything the map is built from, as one number: bumped whenever the
    /// document's text changes, and never by a motion, a selection, or a save.
    /// A frontend can hold work against it — see [`Doc::revision`].
    revision: u64,
    /// How many history steps stand behind the caret, and how many ahead of
    /// it — the answer to a native Edit menu's "may Undo be enabled?", which
    /// twig's history does not answer itself. A mirror of twig's two stacks,
    /// kept exact by following every move twig makes to them: a step onto the
    /// undo stack at [`refresh`](Self::refresh), the funnel every edit comes
    /// through, capped where twig caps it ([`UNDO_CAP`]); one fewer at each
    /// [`coalesce_last_undo`](Self::coalesce_last_undo), under twig's own
    /// two-step rule; and moved back and forth by [`undo`](Self::undo) and
    /// [`redo`](Self::redo). A counter that only ever went up — which this was
    /// — answered `true` after a coalesced run had been undone whole. Should
    /// it ever drift anyway, the first undo twig refuses sets it to zero.
    undo_steps: usize,
    redo_steps: usize,
    /// The undo group a host has open, if any — see
    /// [`begin_undo_group`](Self::begin_undo_group). `None` outside one.
    undo_group: Option<UndoGroup>,
    /// Make the next literal insert fail as twig would refuse one — a test's
    /// way to reach the rollback no document provokes.
    #[cfg(test)]
    refuse_next_literal: bool,
    /// What `vmap` was built from, or `None` before the first build. The map is
    /// a pure function of `(revision, wrap, reveal line)`, so when those haven't
    /// moved, rebuilding it produces the identical map — see
    /// [`Doc::build_visual`].
    ///
    /// The reveal line ([`Doc::reveal_line`]) is the caret's, and is `None` in
    /// every mode but [`MarkupMode::Full`] — so outside that mode the key is
    /// text and width alone, and a caret motion still rebuilds nothing.
    vmap_key: Option<(u64, Option<usize>, Option<Reveal>)>,
    /// The screen's map, set aside while [`Doc::set_unrevealed`] lays the
    /// document out as paper, and put back when it stops — `Some` exactly
    /// while the map in `vmap` is the page's.
    screen: Option<Box<ScreenMap>>,
    /// Which `Doc` this is, distinct from every other one built in this
    /// process. Folded into [`VisualKey`] so that a map stashed by a frontend
    /// can never be mistaken for another document's — see
    /// [`Doc::visual_key`]. Nothing else reads it.
    identity: u64,
    /// Per-block row cache backing the incremental rebuild: when the text
    /// changes, only the top-level blocks whose bytes moved are re-rendered and
    /// the rest are reused shifted (see [`wysiwyg::BlockCache`]). Persists across
    /// builds; a pure accelerator, so it's never read for correctness.
    block_cache: wysiwyg::BlockCache,
    /// What the frontend has said about itself — how tall its pictures came
    /// out, keyed by destination or by TeX, and whether it paints a picture in
    /// a line — set through [`Doc::set_media_rows`], [`Doc::set_math_rows`] and
    /// [`Doc::set_inline_pictures`]. Core does no I/O and lays out in glyphs,
    /// so this is the only way it learns a height or a capability. Threaded
    /// into every build; a change drops both caches, since none of it is in a
    /// block's bytes.
    surface: wysiwyg::Surface,

    // View geometry the renderer stamps each frame, so mouse events can map a
    // screen cell back to a byte offset.
    pub scroll: usize,
    pub body_origin: (u16, u16),
    /// Width of the body rectangle last painted by the frontend. Zero means
    /// unknown (used by tests or a frontend that has not drawn yet).
    pub body_width: u16,
    pub body_height: u16,
    /// The caret as of the last frame drawn, or `None` before the first.
    ///
    /// Scrolling is the viewport's business, not the caret's: the view follows
    /// the caret when the caret *moves*, but a wheel that doesn't touch the
    /// caret has to be free to scroll away from it — otherwise the view is
    /// pinned to the caret and stops dead at the edge of the document you can
    /// see. Comparing against this is what tells the two apart, and it catches a
    /// caret set by any route, including a frontend assigning the field itself.
    pub drawn_caret: Option<usize>,
}

/// The Markdown extensions every leaf document is parsed with — five of them,
/// each departing from twig's defaults for a reason leaf can state.
///
/// `html_elements` promotes embedded raw HTML (`<img>`, `<picture>`,
/// `<source>`, …) into semantic AST nodes, so a picture becomes a real `image`
/// node the frontends can frame and rasterize instead of opaque `raw_block`
/// text. `directives` turns on generic `:::name{.class}` fenced-div containers
/// (`directive` nodes), which a host app uses for its own semantics (diaryx's
/// `:::vis{.audience}` visibility blocks) — core renders any directive as a
/// plain tinted container, agnostic of `name`.
///
/// `highlight` and `highlight_colors` are the pair that makes Markdown read
/// `==text==` as a `mark` node, and `==🔴 text==` as one carrying a
/// `data-color`. leaf already had somewhere to put both: the
/// [`Mark`](crate::Role::Mark) role and the ⌘⇧M highlight button predate them,
/// and until twig 3.3 a Markdown document could only ever *receive* a highlight
/// from a Djot one it was converted from — the button wrote `==…==` and the
/// reparse read it straight back as text.
/// They are on together because a colour is inert without the highlight itself,
/// and a document that writes `==🔴 x==` means the colour by it.
///
/// `math` makes Markdown read `$…$` as an `inline_math` node and `$$…$$` as
/// a `display_math` one — what djot reads natively and what leaf has
/// somewhere to put: a formula typesets to a picture, or reveals to its TeX
/// on the caret's line. Without it a `$$` block is a paragraph whose `\,`
/// twig has already read as an escaped comma, and an author who types a
/// backslash in it is authoring Markdown, not TeX. The flag is bounded by
/// twig's own rule that a dollar followed by whitespace never opens math, so
/// `$5 and $6` stays prose.
///
/// Every flag is inert for non-Markdown formats, so it's safe to pass them
/// unconditionally. Threading this through every constructor (not just `open`)
/// keeps `from_source`, `blank`, and `reload` parsing the same document the same
/// way — twig reparses with these same flags after each edit.
pub(crate) fn parse_extensions() -> MarkdownExtensions {
    MarkdownExtensions {
        html_elements: true,
        directives: true,
        highlight: true,
        highlight_colors: true,
        math: true,
    }
}

/// Build an editor over `bytes` in `format` with leaf's [`parse_extensions`],
/// mapping twig's error into the `anyhow` context every constructor shares.
fn new_editor(bytes: &[u8], format: Format) -> Result<Editor> {
    Editor::new_ext(bytes, format, parse_extensions()).map_err(|e| anyhow!("twig parse: {e}"))
}

/// Does `format` spell a table as a **pipe table** — the one grid twig's table
/// editor knows how to emit?
///
/// This is the single capability leaf still has to answer for itself, and the
/// only hand-maintained format list left in this file. Every other gesture is
/// [`Format::supports`], which is twig's own answer read across the C ABI — but
/// twig deliberately leaves the table ops out of that query, because they read
/// no `Syntax` table at all. They rewrite a grid that is already in the source
/// and refuse on *position*, never on format. Handed a caret inside an HTML
/// `<table>`, `table_insert_row` therefore re-emits the whole element as
/// `| a | b |` and reports success — a real splice, a clean reparse, an honest
/// `dirty` flag, and nothing downstream able to tell it from a good edit.
///
/// So the list is narrow on purpose. `Format` is `#[non_exhaustive]`, and the
/// wildcard answers "no" for a format leaf has never heard of: a new twig
/// language that *does* spell pipe tables loses its grid controls until this
/// line is updated, which shows up as a missing button. The other default hands
/// it to [`Doc::table_op`], which rewrites documents it cannot spell.
fn spells_pipe_tables(format: Format) -> bool {
    matches!(format, Format::Markdown | Format::Djot)
}

/// Which of leaf's authoring controls this document's format can actually
/// spell — one flag per toolbar button, resolved once so a frontend can build
/// its chrome instead of discovering each refusal on a click.
///
/// Every field but [`table`](Self::table) is `Format::supports_with` on the
/// gesture the matching [`Doc`] method calls, so this record cannot drift from
/// what the ops do; `table` is [`spells_pipe_tables`], the one answer twig
/// doesn't export.
///
/// `supports_with` rather than `supports` because two of these are facts about
/// the *parse options* as much as about the format. `Format::supports` answers
/// for twig's defaults, and leaf never parses with those — it parses with
/// [`parse_extensions`], and a document's toolbar has to describe the document
/// it is over. A Markdown editor holding `highlight` authors `==text==`; one
/// without it would mint bytes its own reparse hands back as plain text, which
/// is why twig asks before it writes.
///
/// **The formats are ragged, and that is the point.** A single per-document
/// boolean was enough while the two authorable formats were Markdown and djot
/// and everything else spelled nothing. HTML is neither: it writes seven of the
/// eight inline marks as a tag pair, plus `<code>`, `<hr>`, an in-cell
/// `<br>`, and — since twig 3.4 — a heading or paragraph rebuilt as its tag
/// pair, and since 3.5 a quote, a list, a code block, a link and an image
/// printed as fresh nodes; it spells no task box (a form control there) and
/// no footnote, and its `<table>` is one twig reads but will not write. So
/// ⌘B, ⌘1 and the quote button work in an HTML document and the task and
/// table buttons do not, and no one flag can say that. Markdown and djot
/// differ from each other too:
/// `^superscript^` is djot-only, and an in-cell `<br>` is Markdown-only.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct Capabilities {
    /// ⌘B — `InlineKind::Strong`.
    pub bold: bool,
    /// ⌘I — `InlineKind::Emph`.
    pub italic: bool,
    /// Inline code — `InlineKind::Verbatim`.
    pub code: bool,
    /// Highlight — `InlineKind::Mark`. Djot spells it, and so does Markdown
    /// under the `highlight` extension [`parse_extensions`] turns on: the
    /// button writes `==text==`, which is what the reparse reads back.
    pub mark: bool,
    /// ⌘U — `InlineKind::Insert`, which every format that marks at all spells.
    pub underline: bool,
    /// Strikethrough — `InlineKind::Delete`. Markdown spells GFM's `~~text~~`
    /// out of the box, since twig parses it out of the box.
    pub strike: bool,
    /// The highlight *palette* — [`Doc::set_mark_color`]. Narrower than
    /// [`mark`](Self::mark) and deliberately its own flag: Markdown spells a
    /// colour on a highlight (`==🔴 text==`) and djot spells only the highlight,
    /// so a toolbar offering the swatches wherever the button lights would offer
    /// them in a document that cannot write one. Pair with
    /// [`Doc::caret_in_mark`], which asks the other question — the palette needs
    /// a highlight to colour as much as a format that spells one.
    pub mark_color: bool,
    pub superscript: bool,
    pub subscript: bool,
    /// Heading levels and "make this a paragraph" — [`Doc::set_block`].
    pub heading: bool,
    pub blockquote: bool,
    pub bullet_list: bool,
    pub ordered_list: bool,
    /// The checkbox controls: giving an item a box, and ticking one.
    pub task: bool,
    pub link: bool,
    /// Covers [`Doc::insert_media`] too — see the note there on why the three
    /// media kinds stand or fall together.
    pub image: bool,
    /// The horizontal-rule button. HTML spells this one (`<hr>`).
    pub thematic_break: bool,
    /// The footnote button — [`Doc::insert_footnote`]. Markdown and djot spell
    /// the pair; HTML has no footnote of its own, so the button goes away rather
    /// than writing brackets that would render as brackets.
    pub footnote: bool,
    /// The code-block button — [`Doc::toggle_code_block`], twig's
    /// `Gesture::ToggleCodeBlock`. Markdown and djot spell the fence; HTML
    /// rebuilds the block as `<pre><code>`.
    pub code_block: bool,
    /// Setting a fenced block's language — a control only ever offered with the
    /// caret already in a fence.
    pub code_language: bool,
    /// The grid controls: insert/delete/move a row or column, set a column's
    /// alignment. Pair with [`Doc::caret_in_table`], which asks the other
    /// question — an HTML `<table>` holds the caret and still can't be edited.
    pub table: bool,
    /// Shift+Return inside a cell. Markdown and HTML spell it; djot has no
    /// idiomatic in-cell break.
    pub cell_line_break: bool,
    /// The alignment control — [`Doc::set_alignment`], twig's
    /// `Gesture::SetBlockAttrs`. Every format leaf opens but XML spells a
    /// block's attributes, Markdown under the `html_elements`
    /// [`parse_extensions`] turns on (a `<div>` around the block) and AsciiDoc
    /// through its `[…]` line.
    pub alignment: bool,
    /// The line-spacing menu — [`Doc::set_line_spacing`]. The same gesture as
    /// [`alignment`](Self::alignment) and so the same answer, and its own flag
    /// because a toolbar dims controls one at a time and the pair may yet
    /// diverge.
    pub line_spacing: bool,
    /// The size menu — [`Doc::set_font_size`], twig's `Gesture::WrapRangeAttrs`
    /// over a selection. **Narrower than the block pair**: AsciiDoc's
    /// `[#id.role]#text#` keeps an id and a role and has no slot for a
    /// `data-` key, so twig refuses the span there and this is `false` while
    /// [`alignment`](Self::alignment) is `true`. The block-level form of the
    /// same property — the caret in a paragraph, no selection — goes through
    /// `SetBlockAttrs` and still works, which is why the flag describes the
    /// control rather than the caret.
    pub font_size: bool,
    /// The face menu — [`Doc::set_font_family`]. `WrapRangeAttrs`, as
    /// [`font_size`](Self::font_size) is.
    pub font_family: bool,
    /// The text-colour swatches — [`Doc::set_text_color`]. `WrapRangeAttrs`,
    /// and not to be confused with [`mark_color`](Self::mark_color): that is a
    /// highlight's background and rides the `mark` node twig already owns,
    /// this is a run's foreground and rides an attributed span.
    pub text_color: bool,
    /// The page-break button — [`Doc::insert_page_break`], twig's
    /// `Gesture::InsertDirective`. Markdown under the `directives` extension
    /// [`parse_extensions`] turns on (`::page-break`), djot, which spells it
    /// as an empty `::: page-break` fence, HTML (`<page-break></page-break>`)
    /// and AsciiDoc (`<<<`) — each drawn as the same placeholder row.
    pub page_break: bool,
    /// Moving a block — [`Doc::move_block`] and the Alt+↑/↓ pair, twig's
    /// `Gesture::MoveBlock`. Every format with blocks a caret can name; XML
    /// has none.
    pub move_block: bool,
}

impl Capabilities {
    /// Resolve every flag for `format`, as leaf parses it. Pure and cheap —
    /// twig computes each from a static table — but a frontend that wants to
    /// hold them can.
    ///
    /// The extensions are not a parameter because they are not a choice a
    /// caller makes: every leaf document is parsed with [`parse_extensions`],
    /// so the format is the whole of what varies.
    pub fn of(format: Format) -> Self {
        let exts = parse_extensions();
        let supports = |g| format.supports_with(exts, g);
        let inline = |k| supports(Gesture::ToggleInline(k));
        let container = |k| supports(Gesture::ToggleBlockContainer(k));
        Self {
            bold: inline(InlineKind::Strong),
            italic: inline(InlineKind::Emph),
            code: inline(InlineKind::Verbatim),
            mark: inline(InlineKind::Mark),
            underline: inline(InlineKind::Insert),
            strike: inline(InlineKind::Delete),
            mark_color: supports(Gesture::SetMarkColor),
            superscript: inline(InlineKind::Superscript),
            subscript: inline(InlineKind::Subscript),
            heading: supports(Gesture::SetBlock),
            blockquote: container(BlockContainerKind::BlockQuote),
            bullet_list: container(BlockContainerKind::BulletList),
            ordered_list: container(BlockContainerKind::OrderedList),
            // Both halves of the checkbox story, and leaf offers no control that
            // needs only one: the item gesture mints the box, the checked one
            // ticks it, and a format spelling a `task_marker` spells both.
            task: supports(Gesture::ToggleTaskItem) && supports(Gesture::ToggleTaskChecked),
            link: supports(Gesture::InsertLink),
            image: supports(Gesture::InsertImage),
            thematic_break: supports(Gesture::InsertThematicBreak),
            footnote: supports(Gesture::InsertFootnote),
            code_block: supports(Gesture::ToggleCodeBlock),
            code_language: supports(Gesture::SetCodeLanguage),
            table: spells_pipe_tables(format),
            cell_line_break: supports(Gesture::InsertLineBreak),
            // The presentation vocabulary, one gesture per level: the two
            // line-level properties are a block's attributes and the three
            // run-level ones a span's. They are asked separately because the
            // formats answer differently — AsciiDoc spells the block and not
            // the span — and a toolbar that dimmed all five together would dim
            // three controls that work.
            alignment: supports(Gesture::SetBlockAttrs),
            line_spacing: supports(Gesture::SetBlockAttrs),
            font_size: supports(Gesture::WrapRangeAttrs),
            font_family: supports(Gesture::WrapRangeAttrs),
            text_color: supports(Gesture::WrapRangeAttrs),
            // Every format twig spells the directive in: the walker draws
            // HTML's `<page-break>` and AsciiDoc's `<<<` as the same
            // placeholder Markdown's `::page-break` and djot's fence get.
            page_break: supports(Gesture::InsertDirective),
            move_block: supports(Gesture::MoveBlock),
        }
    }
}

/// The source of [`Doc::identity`], one per document ever built.
static NEXT_IDENTITY: std::sync::atomic::AtomicU64 = std::sync::atomic::AtomicU64::new(0);

impl Doc {
    #[cfg(feature = "fs")]
    pub fn open(path: PathBuf) -> Result<Self> {
        let bytes = std::fs::read(&path).with_context(|| format!("reading {}", path.display()))?;
        Self::from_disk_bytes(path, bytes)
    }

    /// An empty document *named* `path`, for a file that isn't there yet — what
    /// every other terminal editor gives you when you name a file that doesn't
    /// exist. It is a real named document, not a [`Doc::blank`]: `is_untitled`
    /// is false, so ⌘S writes straight to `path` with no Save As detour, and
    /// the header shows the name the user asked for.
    ///
    /// The format comes from the extension, exactly as [`Doc::open`] reads it —
    /// so `leaf notes.dj` starts a djot buffer rather than the Markdown
    /// [`Doc::blank`] has to assume for want of a name. An extension leaf can't
    /// parse is still an error: a mistyped flag or a stray argument should say
    /// so, not open a buffer promising to save somewhere.
    ///
    /// The watermark is the hash of *no bytes*, not `None`, and that is the
    /// whole trick: `None` means untitled, and would leave [`Doc::disk_state`]
    /// answering [`DiskState::Untitled`] for a document that has a path and
    /// intends to write to it. Hashing `""` instead makes the answers the true
    /// ones — [`DiskState::Missing`] while the file still isn't there (a save
    /// recreates it, which is exactly what this is for), and
    /// [`DiskState::Changed`] if somebody creates it underneath us between
    /// launch and save, so the frontend's overwrite prompt guards a new file as
    /// it guards an opened one.
    ///
    /// Nothing is written here. A buffer that is never typed into never touches
    /// the filesystem, and a `path` whose directory doesn't exist is allowed to
    /// open — the write is where that fails, and it says so then.
    #[cfg(feature = "fs")]
    pub fn create(path: PathBuf) -> Result<Self> {
        Self::from_disk_bytes(path, Vec::new())
    }

    /// [`Doc::open`] when the file is there, [`Doc::create`] when it isn't —
    /// the call a CLI frontend wants for its path argument.
    ///
    /// The decision is made from the failed read itself rather than a `exists()`
    /// check first, so there is no window between the two for the file to appear
    /// or vanish in. Only `NotFound` opens a new buffer: a permissions error or
    /// a directory in the way is still an error, because pretending those are
    /// "no file yet" would offer to save over something leaf couldn't read.
    #[cfg(feature = "fs")]
    pub fn open_or_create(path: PathBuf) -> Result<Self> {
        match std::fs::read(&path) {
            Ok(bytes) => Self::from_disk_bytes(path, bytes),
            Err(e) if e.kind() == std::io::ErrorKind::NotFound => Self::create(path),
            Err(e) => Err(e).with_context(|| format!("reading {}", path.display())),
        }
    }

    /// The shared body of [`Doc::open`] and [`Doc::create`]: bytes that are (or
    /// stand in for) the file at `path`, parsed as the format its extension
    /// names. Keeping the two on one path is what makes a new file's document
    /// identical in every respect to an opened one but its contents.
    #[cfg(feature = "fs")]
    fn from_disk_bytes(path: PathBuf, bytes: Vec<u8>) -> Result<Self> {
        let format = detect_format(&path)?;
        let editor = new_editor(&bytes, format)?;
        let source = String::from_utf8(bytes).map_err(|_| anyhow!("document is not UTF-8"))?;
        let disk_hash = Some(hash_bytes(source.as_bytes()));
        // Store the document's *absolute* path. A relative one (`leaf README.md`)
        // has an empty parent, so a frontend can't resolve a relative image
        // destination (`![](pic.png)`) against the document's directory and the
        // picture silently falls back to its text placeholder. `absolute` is
        // purely lexical — it prefixes the current directory and normalizes, but
        // reads nothing and resolves no symlinks — so `file_name` and save are
        // unchanged; it only gives `path.parent()` something to join against.
        let path = std::path::absolute(&path).unwrap_or(path);
        Ok(Doc::from_parts(editor, format, path, source, disk_hash))
    }

    /// Build a document from an in-memory string, the format named explicitly —
    /// the portable, filesystem-free counterpart to [`Doc::open`] (which reads a
    /// path and sniffs the format from its extension). A wasm or FFI host, which
    /// has no path to read, uses this: it hands over bytes it fetched however it
    /// could, and later persists [`Doc::source`] however it can (a browser
    /// download, `localStorage`, a backend `PUT`) and calls [`Doc::mark_saved`].
    ///
    /// No file backs the result, so it starts untitled ([`Doc::is_untitled`] is
    /// true) exactly like a [`Doc::blank`] that has been given content.
    pub fn from_source(source: String, format: Format) -> Result<Self> {
        let editor = new_editor(source.as_bytes(), format)?;
        Ok(Doc::from_parts(
            editor,
            format,
            PathBuf::new(),
            source,
            None,
        ))
    }

    /// An untitled, empty document — the `+` button and a `leaf` launched with
    /// no file argument. Nothing on disk backs it until a [`Doc::save_as`].
    ///
    /// It is Markdown, because a format has to be chosen before a name exists to
    /// read one from: `detect_format` reads the extension and an untitled
    /// document has neither. Markdown is what leaf's own files are, what its
    /// block markers are already written for (`insert_block_prefix`), and the
    /// extension a Save As will overwhelmingly pick — a wrong guess here would
    /// mean typing djot into a buffer parsing it as Markdown. Note that Save As
    /// *doesn't* revisit this: see [`Doc::save_as`].
    pub fn blank() -> Result<Self> {
        let format = Format::Markdown;
        let editor = new_editor(b"", format)?;
        // An empty `path` is the untitled marker (`path` is a public `PathBuf`
        // field two frontends already read; making it an `Option` to say this
        // would break both). `is_untitled` is the question to ask, not the
        // representation to copy.
        Ok(Doc::from_parts(
            editor,
            format,
            PathBuf::new(),
            String::new(),
            None,
        ))
    }

    /// The fields every constructor agrees on, so `open` and `blank` can't drift
    /// apart in the ones neither of them has an opinion about.
    // `identity` is taken from a counter rather than from the `Doc`'s address,
    // which moves — a session that holds one is moved into and out of
    // containers freely, and an identity that changed with it would defeat the
    // one comparison it exists for.
    fn from_parts(
        editor: Editor,
        format: Format,
        path: PathBuf,
        source: String,
        disk_hash: Option<u64>,
    ) -> Self {
        Doc {
            editor,
            format,
            path,
            disk_hash,
            clean_source: source.clone(),
            source,
            caret: 0,
            anchor: None,
            dirty: false,
            status: None,
            read_only: false,
            highlights: Vec::new(),
            // leaf opens in the rich-text (WYSIWYG) view by default — the
            // markup-resolved surface is leaf's differentiator. Frontends can
            // still start in source view explicitly (e.g. a CLI flag), and ⌘e/⌥w
            // toggles at runtime.
            view: View::Wysiwyg,
            // `None` by default — the clean surface Diaryx ships, with typed
            // syntax kept literal; a markup-fluent frontend can climb the
            // ladder to `Shortcuts` or `Full`.
            markup_mode: MarkupMode::default(),
            // Fold by default — flowing prose that reflows to the viewport, the
            // behaviour every frontend had before this preference existed.
            line_flow: LineFlow::default(),
            last_edit_kind: None,
            pending_marks: InlineMarks::empty(),
            pending_at: None,
            goal_col: None,
            vmap: VisualMap::default(),
            smap: SourceMap::default(),
            // No map yet — the first `build_source` always builds.
            smap_key: None,
            revision: 0,
            undo_steps: 0,
            redo_steps: 0,
            undo_group: None,
            #[cfg(test)]
            refuse_next_literal: false,
            // No map yet — the first `build_visual` always builds.
            vmap_key: None,
            screen: None,
            identity: NEXT_IDENTITY.fetch_add(1, std::sync::atomic::Ordering::Relaxed),
            block_cache: wysiwyg::BlockCache::default(),
            surface: wysiwyg::Surface::default(),
            scroll: 0,
            body_origin: (0, 0),
            body_width: 0,
            body_height: 0,
            drawn_caret: None,
        }
    }

    /// Whether this document has no file behind it yet — a [`Doc::blank`] that
    /// has never been saved. The question a ⌘S handler asks to know it should
    /// open a Save As picker instead ([`Doc::save`] won't guess a name), and the
    /// header asks to know the name it shows is a placeholder.
    pub fn is_untitled(&self) -> bool {
        self.path.as_os_str().is_empty()
    }

    pub fn toggle_view(&mut self) {
        self.view = match self.view {
            View::Source => View::Wysiwyg,
            View::Wysiwyg => View::Source,
        };
        self.scroll = 0;
        self.status = None;
        // Entering WYSIWYG, the caret may be sitting in now-hidden frontmatter;
        // lift it to the first rendered offset.
        self.clamp_caret();
    }

    /// The current markup-exposure preference (see [`MarkupMode`]).
    pub fn markup_mode(&self) -> MarkupMode {
        self.markup_mode
    }

    /// Set the markup-exposure preference. Both of its axes take effect at
    /// once: the editing one on the next [`insert`](Self::insert), and the
    /// rendering one on the next build — which is why this drops the cached
    /// visual map and the per-block render cache, exactly as
    /// [`set_line_flow`](Self::set_line_flow) does.
    pub fn set_markup_mode(&mut self, mode: MarkupMode) {
        if self.markup_mode == mode {
            return;
        }
        self.markup_mode = mode;
        // Neither cache is keyed on the mode, and moving between `Full` and the
        // hidden modes changes every row the caret's line renders to — so
        // invalidate both explicitly.
        self.vmap_key = None;
        self.block_cache = wysiwyg::BlockCache::default();
    }

    /// The line the caret sits on, when that line should render something
    /// raw — `None` when nothing on it would, which is what the builder reads
    /// as "reveal nothing" and what keeps caret motion from costing a build.
    ///
    /// Two things ask for it. Under [`MarkupMode::Full`] every delimiter on
    /// the caret's line shows ([`Reveal::full`]). In the two hidden modes a
    /// *formula* on it still shows its TeX ([`Reveal::math`]), because a
    /// formula's content is not its picture and hiding the `$` alone would
    /// leave nothing to edit; there the line is threaded through only when it
    /// meets a block that holds one, which the last build's layout knows
    /// ([`wysiwyg::BlockCache::math_meets`]) — so a document with no math
    /// keeps the `None` it always had, and one with math pays a rebuild only
    /// while the caret is in the formula's block.
    ///
    /// A *source* line (newline to newline), not a visual row: a wrapped
    /// paragraph and a `LineFlow::Preserve` soft break both split one source
    /// line across several rows, and revealing half a delimiter pair because the
    /// other half wrapped would be worse than revealing neither. The range
    /// excludes the terminating newline and is empty-but-present on a blank
    /// line, which reveals nothing but still keys the caches correctly.
    ///
    /// Only in [`View::Wysiwyg`]: source view already shows every byte, so
    /// there is nothing there to reveal.
    pub(crate) fn reveal_line(&self) -> Option<Reveal> {
        // A page has no caret, so no line of it is the caret's.
        if self.view != View::Wysiwyg || self.screen.is_some() {
            return None;
        }
        let line = source_line_range(&self.source, self.caret);
        if self.markup_mode.reveals_caret_line() {
            return Some(Reveal::full(line));
        }
        self.block_cache
            .math_meets(&line)
            .then_some(Reveal::math(line))
    }

    /// The current soft-break flow preference (see [`LineFlow`]).
    pub fn line_flow(&self) -> LineFlow {
        self.line_flow
    }

    /// Set the soft-break flow preference. The mode changes how every block lays
    /// out, so a change drops the cached visual map and the per-block render
    /// cache, forcing the next [`build_visual`] to rebuild under the new flow.
    ///
    /// [`build_visual`]: Self::build_visual
    pub fn set_line_flow(&mut self, mode: LineFlow) {
        if self.line_flow == mode {
            return;
        }
        self.line_flow = mode;
        // Both caches are keyed on `(revision, wrap)`, neither of which moved —
        // so invalidate them explicitly, or the next build would reuse rows laid
        // out under the old flow.
        self.vmap_key = None;
        self.block_cache = wysiwyg::BlockCache::default();
    }

    pub fn view_name(&self) -> &'static str {
        match self.view {
            View::Source => "source",
            View::Wysiwyg => "wysiwyg",
        }
    }

    /// Rebuild the WYSIWYG visual map for the current tree at `width` columns
    /// (called by the renderer each frame it's in the WYSIWYG view).
    /// Build the WYSIWYG map, wrapped at `width` display columns.
    ///
    /// Cheap to call every frame, which is what both frontends do: the map is a
    /// pure function of the document and the wrap width, so a call that would
    /// rebuild the same map returns the one already built. Only an edit (or a
    /// resize) pays.
    ///
    /// That isn't a micro-optimisation. A frontend repaints for reasons that have
    /// nothing to do with the text — a blinking caret, a scroll, a focus change —
    /// and rebuilding here is O(document): 23 ms on a 1 MB file, of which 5 ms is
    /// marshalling twig's AST across the C ABI. Paid twice a second by the GUI's
    /// blink timer, that was 14% of a core spent redrawing an unchanged document.
    /// (`cargo run --release -p leaf-core --example bench` for the numbers.)
    pub fn build_visual(&mut self, width: usize) {
        self.build_map(Some(width));
    }

    /// Build the WYSIWYG map with each block as a single unwrapped row — for a
    /// frontend (the GUI) that wraps at its own proportional pixel width rather
    /// than a fixed character column.
    pub fn build_visual_unwrapped(&mut self) {
        self.build_map(None);
    }

    /// Lay the document out as paper shows it, with no line revealed — or,
    /// with `false`, go back to the screen's map.
    ///
    /// The reveal is core's, folded into the rows before a frontend sees them:
    /// under [`MarkupMode::Full`] the caret's line shows its delimiters, and in
    /// every mode a formula on it is its TeX rather than its picture. A page
    /// has no caret, so a PDF or a printout laid out from the screen's map
    /// printed the one line the reader happened to be standing on as source.
    /// While this is on, [`build_visual`](Self::build_visual) and its twin
    /// build as though nothing were revealed.
    ///
    /// Kept apart from the screen's build rather than replacing it: turning
    /// this on sets the screen's map aside — and the caret, which a build
    /// clamps against the map it builds — and turning it off puts both back
    /// as they were, so the screen's next build costs nothing it would not
    /// have cost anyway. Meant to bracket one read of the map, on and then
    /// off; an edit in between is not the paper's to make.
    pub fn set_unrevealed(&mut self, on: bool) {
        match (on, self.screen.take()) {
            (true, None) => {
                self.screen = Some(Box::new(ScreenMap {
                    vmap: std::mem::take(&mut self.vmap),
                    key: self.vmap_key.take(),
                    caret: self.caret,
                    anchor: self.anchor,
                    goal_col: self.goal_col,
                }));
            }
            (false, Some(screen)) => {
                let ScreenMap {
                    vmap,
                    key,
                    caret,
                    anchor,
                    goal_col,
                } = *screen;
                self.vmap = vmap;
                self.vmap_key = key;
                self.caret = caret;
                self.anchor = anchor;
                self.goal_col = goal_col;
            }
            // Already in the state asked for.
            (_, screen) => self.screen = screen,
        }
    }

    /// Build the source view's syntax map ([`Doc::smap`]) — the styling for
    /// [`View::Source`], the way [`build_visual`](Self::build_visual) is the
    /// styling for [`View::Wysiwyg`].
    ///
    /// A frontend calls this before painting raw source. One that doesn't gets
    /// an empty map and paints unstyled text, so this is additive: nothing
    /// breaks by not calling it.
    ///
    /// Built at most once per revision, and the revision is the whole key — the
    /// map has no width and no caret in it, so it survives every resize, every
    /// motion, and every selection change.
    ///
    /// The builds it does do cost a whole-arena marshal, which is precisely what
    /// the WYSIWYG path works to avoid, so this has no incremental path where
    /// that one has two. From `cargo run --release -p leaf-core --example
    /// bench`, per keystroke, against the WYSIWYG build the source view is
    /// *not* doing:
    ///
    /// |  size |  nodes | marshal | `source::build` | (`wysiwyg::build`) |
    /// |------:|-------:|--------:|----------------:|-------------------:|
    /// |  10 KB|    613 |  0.16 ms|         0.07 ms |            0.28 ms |
    /// | 100 KB|  6 097 |  0.84 ms|         0.38 ms |            2.43 ms |
    /// |   1 MB| 60 601 |  5.67 ms|         3.12 ms |           23.39 ms |
    ///
    /// Linear, two thirds of it the marshal, and the build itself five to seven
    /// times cheaper than the one it stands in for at every size. Comfortable
    /// well past any document a person edits in a terminal — a megabyte is where
    /// it would want [`Editor::dirty_range`] and the same splice treatment
    /// `build_spliced` gives the other map. The door is open; nothing has needed
    /// it yet.
    pub fn build_source(&mut self) {
        if self.smap_key == Some(self.revision) {
            return;
        }
        let nodes = self.nodes();
        self.smap = source::build(&nodes, &self.source);
        self.smap_key = Some(self.revision);
    }

    /// Tell the model how many visual rows each block image should reserve, keyed
    /// by the image's destination. A terminal frontend calls this once it has
    /// decoded and measured its pictures — core does no image I/O, so this is the
    /// only way it learns a height — and the next [`Doc::build_visual`] lays each
    /// placeholder out that tall (the label row plus blank filler rows the
    /// frontend paints the raster over). A destination left out of the map falls
    /// back to the bare one-row placeholder, which is also what a frontend that
    /// can't draw pictures (or lays them out in its own units, like the GUI) gets
    /// by never calling this.
    ///
    /// Cheap to call every frame with the same map: only a *change* invalidates
    /// the built map (and the block-row cache, since a height isn't part of a
    /// block's bytes and so wouldn't otherwise re-render it). Steady state is a
    /// no-op, so a frontend can just hand over its current measurements each frame.
    pub fn set_media_rows(&mut self, rows: HashMap<String, usize>) {
        if self.surface.media_rows == rows {
            return;
        }
        self.surface.media_rows = rows;
        self.surface_changed();
    }

    /// Tell the model how many visual rows each display formula should
    /// reserve, keyed by the formula's TeX exactly as the map's
    /// [`MathInfo::tex`](wysiwyg::MathInfo::tex) handed it over. The peer of
    /// [`set_media_rows`](Self::set_media_rows) for the terminal, which
    /// typesets the picture, measures it in cells, and reports back; a
    /// frontend that lays formulas out in pixels never calls this and gets
    /// the one-row placeholder to paint over.
    pub fn set_math_rows(&mut self, rows: HashMap<String, usize>) {
        if self.surface.math_rows == rows {
            return;
        }
        self.surface.math_rows = rows;
        self.surface_changed();
    }

    /// Tell the model whether the frontend can paint a picture *inside* a line
    /// of text. When it can, an inline formula renders to one atom glyph the
    /// frontend draws its typeset picture over — see
    /// [`MathInfo`](wysiwyg::MathInfo) — and when it cannot (a terminal), to
    /// the code-styled TeX it always showed. Off until a frontend says
    /// otherwise, so a host that has not caught up sees what it saw.
    pub fn set_inline_pictures(&mut self, on: bool) {
        if self.surface.inline_pictures == on {
            return;
        }
        self.surface.inline_pictures = on;
        self.surface_changed();
    }

    /// A height or a capability lives outside a block's source bytes, so the
    /// content-keyed block cache would hand back the old rows on a hit. Drop
    /// it (and the splice layout it carries) so the next build re-renders
    /// every block against the new surface, and force that build by clearing
    /// the map key.
    fn surface_changed(&mut self) {
        self.block_cache = wysiwyg::BlockCache::default();
        self.vmap_key = None;
    }

    /// The revision the document's text is at — bumped by every edit, undo,
    /// redo, and reload, and by nothing else. A frontend caches against this to
    /// tell a repaint that needs new work from one that doesn't.
    ///
    /// It counts *edits*, not distinct texts: typing `x` and deleting it again
    /// lands on the same text two revisions later. Work is only ever rebuilt
    /// needlessly, never wrongly reused.
    pub fn revision(&self) -> u64 {
        self.revision
    }

    /// The identity of the map presently in [`vmap`](Self::vmap) — what the last
    /// [`build_visual`](Self::build_visual) built it from, or the identity of an
    /// unbuilt map before the first one.
    ///
    /// This is *not* [`revision`](Self::revision). The revision says where the
    /// text is; this says where the map is, and the two part company the moment
    /// an edit lands, until something rebuilds. A frontend that keeps its own
    /// copy of the map — leaf-ratatui stashes core's before splicing filler rows
    /// under an oversized heading — compares this against the value it held when
    /// it took the copy, and learns whether `vmap` is still the map it stashed
    /// or one somebody else has since rebuilt. Restoring a copy over a newer
    /// map would paint a stale document; restoring nothing hands core's
    /// incremental rebuild a map it never built.
    ///
    /// "Somebody else" includes another document. The key names the `Doc`
    /// as well as the build, so a frontend that draws two documents through
    /// one stash — a host with several buffers, or one that opens the next
    /// document where the last one stood — never has the copy it took of one
    /// accepted by the other, however alike their builds are.
    pub fn visual_key(&self) -> VisualKey {
        VisualKey(self.identity, self.vmap_key.clone())
    }

    /// The map, built at most once per `(revision, wrap)`. `clamp_caret` still
    /// runs on every call: the caret moves without the document changing, and
    /// keeping it on a legal stop is this function's job either way.
    fn build_map(&mut self, wrap: Option<usize>) {
        // Under `MarkupMode::Full` the map is a function of the caret's *line*
        // as well as the text, so the line joins the key: moving within a line
        // still reuses the map, and crossing into another one rebuilds it. In
        // every other mode `reveal_line` is `None` and the key is what it was,
        // so caret motion goes on costing nothing.
        let reveal = self.reveal_line();
        let key = (self.revision, wrap, reveal.clone());
        if self.vmap_key.as_ref() != Some(&key) {
            self.build_map_with(wrap, reveal);
            self.vmap_key = Some(key);
            // In a hidden mode the reveal line was decided from the *previous*
            // build's layout, whose spans are stale across an edit: the
            // keystroke that closes a new `$…$` on the caret's line asked "is
            // there math here?" of a layout that had none, and the formula
            // would snap to its picture under the caret until the next
            // motion. Ask again of the layout just built, and go once more if
            // the answer moved. Between edits the first answer is exact and
            // this is one comparison.
            let again = self.reveal_line();
            if again != self.vmap_key.as_ref().and_then(|k| k.2.clone()) {
                self.build_map_with(wrap, again.clone());
                self.vmap_key = Some((self.revision, wrap, again));
            }
        }
        self.clamp_caret();
    }

    /// One build of the map at `wrap` under `reveal`, incremental where it can
    /// be — the body of [`build_map`](Self::build_map), which decides whether
    /// to call it.
    fn build_map_with(&mut self, wrap: Option<usize>, reveal: Option<Reveal>) {
        {
            // Enumerate the top-level blocks cheaply — no whole-arena marshal.
            // A subtree is pulled only for the block(s) that actually changed, so
            // the FFI marshal shrinks from O(document) to O(edited block).
            let top = self.top_blocks();

            // Fast path: when twig reports a dirty byte range, try to patch the
            // previous map in place — a single-block edit moves the prefix,
            // shifts the suffix, and re-renders only one block. `build_spliced`
            // returns `None` (and we fall back to the always-correct full rebuild)
            // whenever the edit reshaped the block structure, hit a table, or
            // there's no previous map to patch.
            // Preserve soft breaks as written when the flow preference asks for
            // it — the builder renders each as its own visual row instead of
            // folding it into the reflowed paragraph.
            let preserve_soft = self.line_flow == LineFlow::Preserve;
            let spliced = match self.editor.dirty_range() {
                Some(dirty) => {
                    let prev = std::mem::take(&mut self.vmap);
                    let source = &self.source;
                    let cache = &mut self.block_cache;
                    let surface = &self.surface;
                    let editor = &mut self.editor;
                    wysiwyg::build_spliced(
                        prev,
                        source,
                        wrap,
                        preserve_soft,
                        &top,
                        dirty,
                        surface,
                        reveal.clone(),
                        cache,
                        |id| editor.subtree(NodeId(id)).unwrap_or_default(),
                    )
                }
                None => None,
            };
            self.vmap = spliced.unwrap_or_else(|| {
                let source = &self.source;
                let cache = &mut self.block_cache;
                let surface = &self.surface;
                let editor = &mut self.editor;
                wysiwyg::build_cached(
                    &top,
                    source,
                    wrap,
                    preserve_soft,
                    surface,
                    reveal,
                    cache,
                    |id| editor.subtree(NodeId(id)).unwrap_or_default(),
                )
            });
            // Acknowledge the dirty range so the next edit's range starts fresh.
            self.editor.clear_dirty();
        }
    }

    fn nodes(&mut self) -> Vec<FlatNode> {
        self.editor.nodes().unwrap_or_default()
    }

    /// The document's top-level blocks for the incremental render. See
    /// [`wysiwyg::top_blocks`] for why this isn't simply `child_spans(None)`.
    fn top_blocks(&mut self) -> Vec<QueryMatch> {
        wysiwyg::top_blocks(&mut self.editor)
    }

    pub fn format_name(&self) -> &'static str {
        // `Format` is `#[non_exhaustive]` as of twig 3.0, so the wildcard is
        // required. It also covers `Asciidoc`, which twig parses but cannot
        // serialize — leaf never opens a document in it (see `Doc::open`).
        match self.format {
            Format::Djot => "djot",
            Format::Markdown => "markdown",
            Format::Xml => "xml",
            Format::Html => "html",
            _ => "unknown",
        }
    }

    /// Whether this document's format offers *any* door in — `false` only for a
    /// wholly parse-only format (XML, AsciiDoc), where every gesture refuses and
    /// a frontend may as well open the file read-only.
    ///
    /// This is a much weaker claim than the name suggests, and driving per-button
    /// state from it is exactly the mistake to avoid: HTML answers `true` because
    /// it spells the inline marks with a tag pair (`<strong>`, `<em>`, `<code>`)
    /// while a heading, a quote, a list, a task box, a link and a code fence all
    /// remain unspellable there. Ask [`capabilities`](Self::capabilities) — or
    /// [`supports`](Self::supports) — per control.
    pub fn authorable(&self) -> bool {
        self.format.is_authorable()
    }

    /// Whether this document can spell `gesture`, which is twig's own answer
    /// rather than a copy of it: `Format::supports_with` reads the same
    /// `Syntax` table the `Editor` method consults before refusing, chosen by
    /// the very [`parse_extensions`] this document's editor reparses with — so
    /// what the toolbar offers and what the splice will accept are one table.
    ///
    /// It is a fact about the *document*, not about the caret. `true` does not
    /// promise the gesture succeeds where it is standing — a link over a table
    /// border still fails — only that it will not fail with
    /// `UnsupportedFormat`. Gray out on `false`; don't read `true` as "this
    /// will work here".
    pub fn supports(&self, gesture: Gesture) -> bool {
        self.format.supports_with(parse_extensions(), gesture)
    }

    /// Every control's enabled state in one read — what a toolbar builds itself
    /// from when a document opens or its format changes. See [`Capabilities`].
    pub fn capabilities(&self) -> Capabilities {
        Capabilities::of(self.format)
    }

    /// Refuse a gesture this document's format cannot spell, saying so in the
    /// status line. `true` means the caller must return without calling twig.
    ///
    /// Most of these refusals duplicate one twig would make anyway, and they are
    /// made here regardless because a message naming the *document's* format
    /// reads better than one naming twig's internals. Two of them are not
    /// duplicates and are the reason this is a guard rather than an error
    /// translation:
    ///
    /// - The table family (see [`table_op`](Self::table_op)) consults no
    ///   `Syntax` table, so twig does not refuse it at all.
    /// - [`toggle`](Self::toggle) at a collapsed caret never reaches twig — it
    ///   arms a sticky mark for text not yet typed, which is a promise `insert`
    ///   could not keep.
    fn refuse_unsupported(&mut self, what: &str, gesture: Gesture) -> bool {
        self.refuse_unless(what, self.supports(gesture))
    }

    /// [`refuse_unsupported`](Self::refuse_unsupported) against a capability leaf
    /// answers itself — today only [`spells_pipe_tables`].
    fn refuse_unless(&mut self, what: &str, supported: bool) -> bool {
        if supported {
            return false;
        }
        self.status = Some(format!("{what}: not supported in {}", self.format_name()));
        true
    }

    /// The name to show for this document. An untitled one has no file to name
    /// it, and both frontends put this straight on screen — an empty path
    /// renders as an empty header, so it says so instead.
    pub fn file_name(&self) -> String {
        if self.is_untitled() {
            return "untitled".into();
        }
        self.path
            .file_name()
            .map(|s| s.to_string_lossy().into_owned())
            .unwrap_or_else(|| self.path.display().to_string())
    }

    /// The selection as an ordered `[start, end)` byte range, or `None` when the
    /// caret and anchor coincide (an empty selection is no selection).
    pub fn selection(&self) -> Option<(usize, usize)> {
        self.anchor
            .map(|a| (a.min(self.caret), a.max(self.caret)))
            .filter(|(s, e)| s != e)
    }

    /// The selected text, or `None` when there's no selection — the source
    /// slice a copy/cut hands to the system clipboard.
    pub fn selected_text(&self) -> Option<&str> {
        self.selection().map(|(s, e)| &self.source[s..e])
    }

    /// The selection as a quote with a little of what surrounds it — the shape
    /// a host that cites, annotates, or searches for a passage wants, cut from
    /// the **source** rather than from anything rendered, so the quote is
    /// findable in the document again by plain string search.
    ///
    /// `context` is a count of characters (not bytes) on each side, clipped at
    /// the document's edges; the slices land on char boundaries by
    /// construction. `None` when nothing is selected.
    pub fn selection_quote(&self, context: usize) -> Option<Quote> {
        let (start, end) = self.selection()?;
        let mut before = start;
        for _ in 0..context {
            match self.source[..before].chars().next_back() {
                Some(c) => before -= c.len_utf8(),
                None => break,
            }
        }
        let mut after = end;
        for _ in 0..context {
            match self.source[after..].chars().next() {
                Some(c) => after += c.len_utf8(),
                None => break,
            }
        }
        Some(Quote {
            exact: self.source[start..end].to_string(),
            prefix: self.source[before..start].to_string(),
            suffix: self.source[end..after].to_string(),
            start,
            end,
        })
    }

    /// Words, characters, and paragraphs over the whole document — the numbers
    /// a status bar or an inspector puts next to a piece of writing.
    ///
    /// Counted over the text a **reader** sees, not the markup that spells it:
    /// `**bold**` is one word and four characters, a link is its label and not
    /// its destination, a block picture's `🖼 alt` placeholder is a picture and
    /// counts nothing, and leading frontmatter — which the WYSIWYG view does
    /// not render at all — is not writing. [`crate::counts`] states the rules
    /// in full; [`TextCounts`] states them per field.
    ///
    /// The same numbers in both views. They have to be: a word count that fell
    /// when you pressed ⌘E would be telling you the view had changed, which
    /// you knew already. So this reads neither [`Doc::view`] nor the map the
    /// frontend last built — it renders the source afresh, unwrapped, with
    /// soft breaks folded and no line revealed, and counts that. A narrower
    /// window, a different [`MarkupMode`], a different [`LineFlow`], and the
    /// source view all give the identical answer, because none of them is an
    /// input.
    ///
    /// That costs a reparse and an unwrapped layout — O(document), about 4 ms
    /// on a 45 KB file in release and 36 ms on half a megabyte. Fine on a
    /// settle and wrong in a paint loop, so a frontend should ask when the
    /// typing stops rather than once a keystroke. Caching it against
    /// [`revision`](Self::revision) is the obvious next move if that is ever
    /// not enough; nothing has needed it yet.
    pub fn counts(&self) -> TextCounts {
        self.count_over(None)
    }

    /// The same statistics over the selection alone — `None` when nothing is
    /// selected, since an empty selection is no selection.
    ///
    /// Same rules, over the same rendering, narrowed to the glyphs whose
    /// source byte falls inside [`selection`](Self::selection)'s range. A
    /// block the selection only clips still counts as one paragraph, and one
    /// it enters without catching a visible character counts as none — a
    /// selection that starts on a hidden `**` gains no paragraph from it.
    pub fn selection_counts(&self) -> Option<TextCounts> {
        let (start, end) = self.selection()?;
        Some(self.count_over(Some(start..end)))
    }

    /// The rendering both counters tally, and the tally itself.
    ///
    /// A fresh parse rather than `self.editor`, because these take `&self` and
    /// twig's arena is reached through `&mut`. A document that will not
    /// reparse is a "cannot happen" — the source came out of an editor that
    /// had already accepted it — and answers zero rather than panicking in
    /// what is very likely a paint path.
    fn count_over(&self, range: Option<Range<usize>>) -> TextCounts {
        let Ok(mut editor) = new_editor(self.source.as_bytes(), self.format) else {
            return TextCounts::default();
        };
        let Ok(nodes) = editor.nodes() else {
            return TextCounts::default();
        };
        // A surface that paints pictures in a line, so an inline formula is
        // an atom here and never its TeX: a formula is a picture to a reader
        // whichever way it is written, and the count says so consistently.
        let surface = wysiwyg::Surface {
            inline_pictures: true,
            ..Default::default()
        };
        let map = wysiwyg::build(&nodes, &self.source, None, false, &surface, None);
        counts::tally(&map, range)
    }

    /// Whether the document refuses to change — see the field.
    pub fn read_only(&self) -> bool {
        self.read_only
    }

    /// Turn the read-only gate on or off. A frontend preference like
    /// [`set_markup_mode`](Self::set_markup_mode): nothing about the document
    /// itself changes, only what may be done to it from here on.
    pub fn set_read_only(&mut self, on: bool) {
        self.read_only = on;
    }

    /// The host-painted ranges, sorted by start — see [`Highlight`].
    pub fn highlights(&self) -> &[Highlight] {
        &self.highlights
    }

    /// Replace the host-painted ranges wholesale. The whole set each time,
    /// rather than add/remove verbs: the host owns the list (it derives it
    /// from its own state — annotations, search hits), and a replace can
    /// never leave the two disagreeing about what should be on screen.
    pub fn set_highlights(&mut self, mut highlights: Vec<Highlight>) {
        highlights.retain(|h| h.start < h.end);
        highlights.sort_by_key(|h| (h.start, h.end));
        self.highlights = highlights;
    }

    /// The highlight covering source `offset`, if one does — first by start
    /// when several overlap, which makes overlapping washes resolvable rather
    /// than undefined. What a frontend asks when the reader activates a spot.
    ///
    /// [`Highlight::covering`] is the whole of it: the frontends paint by
    /// asking the same question per glyph, against a slice they were handed
    /// rather than against a `Doc`, and one answer for both is what keeps a
    /// wash and an activation agreeing about which range a spot is in.
    pub fn highlight_at(&self, offset: usize) -> Option<&Highlight> {
        Highlight::covering(&self.highlights, offset)
    }

    /// The AST breadcrumb at the caret (root → deepest), e.g.
    /// `doc › para › strong`. Read live from twig via `ancestors_at`.
    pub fn breadcrumb(&mut self) -> String {
        match self.editor.ancestors_at(self.caret) {
            Ok(chain) => chain
                .iter()
                .map(|m| m.kind.as_str())
                .collect::<Vec<_>>()
                .join(" › "),
            Err(_) => String::new(),
        }
    }

    // ── editing ──────────────────────────────────────────────────────────────

    /// Replace the byte range `[start, end)` with `text`, re-anchoring the caret
    /// after it. The public form of the internal splice — a pixel frontend that
    /// hit-tests to a byte offset (or an IME that hands back an explicit range)
    /// edits through this, the same twig `edit_range` the caret ops use.
    pub fn edit(&mut self, start: usize, end: usize, text: &str) {
        self.splice(start, end, text, EditKind::Other);
    }

    /// Replace `[start, end)` with `text` in place, behind the caret — an
    /// automatic substitution a host makes as the user types: a misspelling
    /// corrected, a text replacement expanded, a straight quote curled, a `--`
    /// made a dash. Whether it did: `false` for a read-only document, a range
    /// that is not one, or an edit twig refused.
    ///
    /// Unlike [`edit`](Self::edit), the caret and the selection stay where they
    /// were, moved along by the edit — a word corrected behind the caret does
    /// not pull the caret back to it — and so does a sticky mark armed at the
    /// caret. It is an undo step of its own, folded into neither the typing
    /// before it nor the typing after, so one undo puts back exactly what was
    /// typed, with the caret where it stood. The bytes are replaced exactly,
    /// snapping neither end, so a word at the edge of `**bold**` keeps its
    /// delimiters. `text` is written the way typing writes it: literally, with
    /// anything that would open markup escaped, where typing is literal
    /// ([`MarkupMode::None`] in the rendered view).
    pub fn substitute(&mut self, start: usize, end: usize, text: &str) -> bool {
        if self.read_only
            || start > end
            || end > self.source.len()
            || !self.source.is_char_boundary(start)
            || !self.source.is_char_boundary(end)
        {
            return false;
        }
        let before = self.source.len();
        let (caret, anchor) = (self.caret, self.anchor);
        let (pending_marks, pending_at) = (self.pending_marks, self.pending_at);
        let group_had_step = self.undo_group.map(|g| g.has_step);
        self.last_edit_kind = None;
        let literal = !self.markup_mode.authors()
            && self.view == View::Wysiwyg
            && !text.is_empty()
            && self.supports(Gesture::InsertLiteral);
        let landed = if literal {
            let deleted = self.source[start..end].to_owned();
            if start != end && !self.splice_exact(start, end, "", EditKind::Other) {
                false
            } else if self.insert_literal_at(start, text, EditKind::Other, start != end) {
                true
            } else {
                // The deletion landed and the text did not: take the deletion
                // back rather than leave half a substitution.
                if start != end {
                    self.restore_deleted(start, &deleted, group_had_step);
                }
                false
            }
        } else {
            self.splice_exact(start, end, text, EditKind::Other)
        };
        if !landed {
            self.caret = caret;
            self.anchor = anchor;
            self.record_caret();
            return false;
        }
        let delta = self.source.len() as isize - before as isize;
        let new_end = (end as isize + delta).max(start as isize) as usize;
        // A place after the range moves with it, one inside it goes to the end
        // of what replaced it, and one before it stays.
        fn shift(p: usize, start: usize, end: usize, new_end: usize, delta: isize) -> usize {
            if p >= end {
                (p as isize + delta).max(0) as usize
            } else if p > start {
                new_end
            } else {
                p
            }
        }
        self.caret = shift(caret, start, end, new_end, delta);
        self.anchor = anchor
            .map(|a| shift(a, start, end, new_end, delta))
            .filter(|&a| a != self.caret);
        if pending_at == Some(caret) {
            self.pending_marks = pending_marks;
            self.pending_at = Some(self.caret);
        }
        self.goal_col = None;
        self.last_edit_kind = None;
        self.clamp_caret();
        // The caret the step leaves, so a redo puts it back here too.
        self.record_caret();
        true
    }

    /// Put back `deleted`, the bytes a half-made [`substitute`](Self::substitute)
    /// took out at `at`, and leave nothing of the pair on the history.
    ///
    /// Not [`undo`](Self::undo): that closes an open group and takes the whole
    /// of it back — every substitution of the keystroke made before this one
    /// — and leaves a redo step that would delete the bytes again. Instead the
    /// bytes go back as an edit of their own, and the two edits, which change
    /// nothing together, are folded away: inside a group that already had a
    /// step they fold into it as they land; as a group's first step, or
    /// outside a group, into the step before. With no step before it there is
    /// nothing to fold into, and one step that changes nothing is left.
    /// `group_had_step` is what the group said before the deletion — `None`
    /// outside one.
    fn restore_deleted(&mut self, at: usize, deleted: &str, group_had_step: Option<bool>) {
        if !self.splice_exact(at, at, deleted, EditKind::Other) {
            // twig will not take its own bytes back: the history's way, which
            // at least leaves the document as it was.
            self.undo();
            return;
        }
        match group_had_step {
            Some(true) => {}
            Some(false) => {
                let steps = self.undo_steps;
                self.fold_last_undo();
                if self.undo_steps < steps
                    && let Some(g) = &mut self.undo_group
                {
                    g.has_step = false;
                }
            }
            None => {
                self.fold_last_undo();
                self.fold_last_undo();
            }
        }
        self.last_edit_kind = None;
    }

    /// Insert typed `text` at the caret, replacing the selection if there is one.
    /// A single typed character coalesces with the run of typing before it; a
    /// newline or a multi-character insert is its own undo step.
    ///
    /// Typed input only — clipboard text goes through [`paste`](Self::paste).
    pub fn insert(&mut self, text: &str) {
        // The read-only gate, up front: the paths below reach twig by several
        // verbs, not all of them through the splice — see the field.
        if self.read_only {
            return;
        }
        // Typing against a block picture would dissolve it, and typing past a
        // table would grow it a row — see `open_paragraph_at_block_edge`. Give
        // the text a paragraph first, so what the caret was standing beside
        // stays what it was.
        self.open_paragraph_at_block_edge(text);
        // Armed sticky marks (⌘b with no selection) turn the next typed text
        // bold/italic/… and then retire — see `insert_with_marks`. Whitespace is
        // the exception: it takes no mark of its own and keeps the delta armed
        // for the character behind it — see `insert_space_with_marks`.
        let pending = self.pending_here();
        if !pending.is_empty() && self.selection().is_none() && !text.is_empty() {
            if text.trim().is_empty() {
                self.insert_space_with_marks(self.caret, text, pending);
            } else {
                self.insert_with_marks(self.caret, text, pending);
            }
            return;
        }
        // `MarkupMode::None`: typed syntax stays literal — twig escapes
        // anything that would open markup, so a Diaryx user never mints
        // formatting by keyboard (it comes from commands instead). The other two
        // rungs of the ladder author markup from what you type, which is the
        // whole difference between them and this one. Only in the rendered view
        // (source view is for typing raw markup) and only where the format has a
        // literal spelling at all: escaping is a backslash before a byte from the
        // format's own alphabet, and a format with no such alphabet (HTML escapes
        // with entities, XML spells nothing) would have `\&` written into it,
        // which is two literal characters and not an escape. Marks (⌘b) still
        // format — that path returned above; and leaf's own structural inserts go
        // through `insert_raw`, never here, so a list marker or quote gutter is
        // written as the markup it is.
        if !self.markup_mode.authors()
            && self.view == View::Wysiwyg
            && !text.is_empty()
            && self.supports(Gesture::InsertLiteral)
        {
            self.insert_literal_typed(text);
            return;
        }
        self.insert_raw(text);
    }

    /// Insert `text` verbatim at the caret (replacing any selection) — the plain
    /// path with no Hidden-mode literal escaping. leaf's own structural inserts
    /// (a list marker, a quote gutter, an in-cell `<br>`) call this: they ARE
    /// markup by design and must not be escaped.
    fn insert_raw(&mut self, text: &str) {
        let (s, e) = self.selection().unwrap_or((self.caret, self.caret));
        self.splice(s, e, text, typed_edit_kind(text));
    }

    /// Open a paragraph for text about to be inserted at one of a block media's
    /// two caret stops, or at a table's trailing stop, and leave the caret
    /// standing in it.
    ///
    /// A block image is a paragraph whose entire content is the picture, and the
    /// caret's only homes on it are in front of it and just past it (see
    /// [`VisualMap::block_media_stop`]). Text inserted at either offset joins
    /// *that* paragraph — and a paragraph holding anything besides the image is
    /// no longer a block image but a line of text with an inline one in it. The
    /// frontend that was painting a photo there paints a text run instead; the
    /// picture is still in the file, and nothing said a word. Those two offsets
    /// are also exactly where a click on the picture lands, so the whole accident
    /// is one tap and one keystroke.
    ///
    /// So the break goes in first and the text lands in the new empty paragraph —
    /// what pressing Return before typing would have done, which is a habit no
    /// one should have to learn from losing a photo. A no-op everywhere else, and
    /// over a selection (which is replaced, not joined into).
    ///
    /// A picture inside a quote or a list leaves its container, because `\n\n`
    /// ends the block. The alternative is worse: the `\n> ` / next-item
    /// continuation [`newline`](Self::newline) writes stays in the same
    /// *paragraph*, which is the thing being prevented.
    ///
    /// A table's trailing stop ([`VisualMap::table_end_stop`]) is the same
    /// accident from the other side of a different block: the stop sits at the
    /// end of the table's last source line, and a line glued under a table is
    /// a row of it — `| 1 | 2 |x` is a three-cell row, not a paragraph. So the
    /// break goes in there too, and the text lands under the table.
    ///
    /// Only in the rendered view. Source view is for typing raw markup, where
    /// putting a character against an image is exactly what it looks like.
    fn open_paragraph_at_block_edge(&mut self, text: &str) {
        if self.view != View::Wysiwyg || text.is_empty() || text == "\n" {
            return;
        }
        if self.selection().is_some() {
            return;
        }
        // The map may be a revision behind (nothing has drawn since the last
        // edit), and this asks it about offsets — a stale answer would splice a
        // break into the wrong place. Free when it is already current, which it
        // is whenever a frontend drew a frame between keystrokes.
        self.rebuild_map();
        let at = self.caret;
        let side = match self.vmap.block_media_stop(at) {
            Some((side, _)) => side,
            None if self.vmap.table_end_stop(at) => MediaStop::After,
            None => return,
        };
        if !self.splice(at, at, "\n\n", EditKind::Other) {
            return;
        }
        // The break is part of the keystroke, not an edit of its own: leave the
        // run marked as typing so the character about to arrive folds into it and
        // one undo puts the document back the way it was found. (A paste, or a
        // multi-character insert, is `EditKind::Other` and stays its own step —
        // as it would have been anywhere else in the document.)
        self.last_edit_kind = Some(EditKind::Insert);
        if side == MediaStop::Before {
            // The break went in above the picture and the caret rode to the end
            // of it — which is still hard against the picture. Step back onto the
            // blank line it opened, so the text lands above rather than in front.
            self.caret = at;
        }
    }

    /// A delete key pressed at one of a block picture's two caret stops, handled
    /// as the picture being an *atom* rather than a run of bytes. Returns whether
    /// the key was consumed.
    ///
    /// The caret rests in front of a block image and just past it, never inside
    /// its markup — which the rendered view doesn't show. So the byte a delete
    /// key nominally takes there is one the writer cannot see, and taking it
    /// leaves the picture as broken markup rather than as anything anyone asked
    /// for: Backspace at the stop past `![](p.png)` removes the closing paren, and
    /// a photo becomes the literal text `![](p.png`. That is how a picture goes
    /// missing from a document with nobody having touched it — the same
    /// dissolution [`open_paragraph_at_block_edge`](Self::open_paragraph_at_block_edge)
    /// prevents from the typing side, and it cost this repository's own test vault
    /// a photo before it was found.
    ///
    /// So the key aimed *at* the picture deletes the picture, whole — Backspace
    /// when it is behind the caret, Delete when it is in front — which is what
    /// every editor does with an embed, and one undo away. The key aimed *away*
    /// from it would otherwise delete the paragraph break and merge a neighbour
    /// into the picture's own paragraph, which dissolves it just as surely; it
    /// steps the caret over the boundary instead and leaves the
    /// next press to delete in the block it has reached — the same "first press
    /// steps out of the atom, second press deletes" every delete key here gets,
    /// word-deletes included (⌥⌫ in front of a picture is aimed at the prose
    /// above, and reaches it on the second press rather than taking the break and
    /// the picture with it on the first).
    fn delete_around_block_media(&mut self, forward: bool) -> bool {
        // The map answers about offsets, so it has to be this revision's — see
        // the same call in `open_paragraph_at_block_edge`.
        self.rebuild_map();
        let Some((side, span)) = self.vmap.block_media_stop(self.caret) else {
            return false;
        };
        let aimed_at_it = side
            == if forward {
                MediaStop::Before
            } else {
                MediaStop::After
            };
        if !aimed_at_it {
            let over = if forward {
                self.vmap.stop_after(self.caret)
            } else {
                self.vmap.stop_before(self.caret)
            };
            if let Some(off) = over.filter(|&o| o >= self.caret_floor()) {
                self.caret = off;
                self.anchor = None;
                self.goal_col = None;
            }
            return true;
        }
        // Take the break that held the picture apart from its neighbour with it,
        // so the delete doesn't leave a blank paragraph standing where the
        // picture was. The last arm is a picture that is the whole document.
        let (from, to) = if self.source[..span.start].ends_with("\n\n") {
            (span.start - 2, span.end)
        } else if self.source[span.end..].starts_with("\n\n") {
            (span.start, span.end + 2)
        } else {
            (span.start, span.end)
        };
        self.splice(from.max(self.caret_floor()), to, "", EditKind::Other);
        true
    }

    /// The Hidden-mode typing path: replace any selection, then insert `text`
    /// escaped so it stays literal. When it replaces a selection the two edits
    /// fold into one undo step, so an overwrite undoes atomically (and restores
    /// the selection) exactly as a plain one does.
    fn insert_literal_typed(&mut self, text: &str) {
        let kind = typed_edit_kind(text);
        match self.selection() {
            Some((s, e)) => {
                if !self.splice(s, e, "", EditKind::Other) {
                    return;
                }
                // Typing over a whole marked run takes its delimiters with it
                // (the empty content couldn't hold them — see
                // `repair_mark_edges`) and leaves its marks armed at the caret.
                // The text taking the run's place inherits them, exactly as it
                // would have by landing inside a run that survived.
                let pending = self.pending_here();
                if !pending.is_empty() && !text.trim().is_empty() {
                    self.insert_with_marks(self.caret, text, pending);
                    return;
                }
                self.insert_literal_at(self.caret, text, kind, true);
            }
            None => {
                self.insert_literal_at(self.caret, text, kind, false);
            }
        }
    }

    /// The sticky-mark delta that is live right now: the marks armed by [`toggle`]
    /// at a collapsed caret, but only while the caret still stands where they
    /// were armed and nothing is selected. Empty otherwise, so a stale delta
    /// never styles text it wasn't meant for.
    fn pending_here(&self) -> InlineMarks {
        if self.anchor.is_none() && self.pending_at == Some(self.caret) {
            self.pending_marks
        } else {
            InlineMarks::empty()
        }
    }

    /// Drop the armed sticky marks — any caret motion, selection, or edit does
    /// this, so "start bold here" only ever applies at the exact spot it was
    /// asked for.
    fn clear_pending(&mut self) {
        self.pending_marks = InlineMarks::empty();
        self.pending_at = None;
    }

    /// Insert `text` at `at` carrying the armed sticky `marks`: a mark not yet in
    /// force is wrapped around the freshly typed text; a mark the caret already
    /// stands inside is *shed* — the text is inserted past the run's end so it
    /// lands unmarked ("type normally again"). The caret comes to rest inside any
    /// added runs, so continued typing inherits the marks with no re-wrapping,
    /// and the delta is cleared: the marks now live in the document, not here.
    fn insert_with_marks(&mut self, at: usize, text: &str, marks: InlineMarks) {
        let base = self.mark_spans_at(at);
        let base_set: InlineMarks = base.iter().map(|(k, _)| *k).collect();
        // Nothing to shed, and a run of exactly these marks standing just behind
        // the caret: carry on writing *that* run rather than opening a second
        // one beside it.
        if base_set.is_empty() && self.rejoin_run(at, text, marks) {
            return;
        }
        // Shed the marks we're turning off: step the insertion point past the
        // end of each run the caret sits in, so the new text falls outside it.
        let mut ins_at = at;
        for (kind, span) in &base {
            if marks.contains(*kind) {
                ins_at = ins_at.max(span.end);
            }
        }
        if !self.splice_exact(ins_at, ins_at, text, EditKind::Other) {
            return;
        }
        // The plain splice inserted exactly `text` at `ins_at`; that byte range
        // is the content every added mark wraps.
        let (mut cs, mut ce) = (ins_at, ins_at + text.len());
        for kind in marks.iter() {
            if !base_set.contains(kind) {
                let (ncs, nce) = self.wrap_span(cs, ce, kind);
                cs = ncs;
                ce = nce;
            }
        }
        self.caret = ce.min(self.source.len());
        self.anchor = None;
        self.last_edit_kind = None;
        // Realised: the marks are in the document now, and the caret sits inside
        // them, so there is no delta left to carry. Arm nothing, but remember the
        // spot so a *further* toggle before typing starts a clean delta here.
        self.pending_marks = InlineMarks::empty();
        self.pending_at = Some(self.caret);
        self.clamp_caret();
        self.record_caret();
    }

    /// Carry on the marked run just behind `at` — moving its closing delimiters
    /// out past the new text — instead of opening a second run of the same marks
    /// beside it. Returns whether it did.
    ///
    /// This is the far half of the mark-edge rule (see [`splice`](Self::splice)).
    /// A space typed after a bold word steps the caret out of the run, because
    /// `**bold **` is not bold; the next character has to step back *in*, or the
    /// writer who typed one bold phrase is left with `**bold** **and**` — two
    /// runs that read the same to a reader but spell the file in a way nobody
    /// wrote. Only whitespace may stand in the gap (a run doesn't reach across
    /// words it isn't marking), and the marks behind it must be exactly the ones
    /// armed — a run of *some* other kind is a neighbour, not this phrase.
    fn rejoin_run(&mut self, at: usize, text: &str, marks: InlineMarks) -> bool {
        if text.is_empty() || text.trim() != text {
            return false;
        }
        let gap_at = self.source[..at].trim_end_matches([' ', '\t']).len();
        // Walk in through the delimiters stacked at that point, innermost last:
        // `***both*** ` closes two runs with one `***`, and rejoining means
        // getting behind all of them.
        let (mut cut, mut kinds) = (gap_at, InlineMarks::empty());
        while let Some((kind, content_end)) = self
            .editor
            .ancestors_at(prev_boundary(&self.source, cut))
            .unwrap_or_default()
            .into_iter()
            .filter(|m| m.span.end == cut)
            .find_map(|m| Some((inline_kind(&m.kind)?, m.content_span.clone()?.end)))
        {
            if content_end >= cut {
                break; // a mark with no closing delimiter to step behind
            }
            kinds.insert(kind);
            cut = content_end;
        }
        if cut == gap_at || kinds != marks {
            return false;
        }
        // Re-spell the tail: the gap, then the new text, then the delimiters that
        // used to close in front of them — read out of the document rather than
        // written from a table, so whatever twig spells them with is what moves.
        let tail = format!(
            "{}{text}{}",
            &self.source[gap_at..at],
            &self.source[cut..gap_at]
        );
        if !self.splice_exact(cut, at, &tail, EditKind::Other) {
            return false;
        }
        self.caret = (cut + (at - gap_at) + text.len()).min(self.source.len());
        self.anchor = None;
        self.last_edit_kind = None;
        self.pending_marks = InlineMarks::empty();
        self.pending_at = Some(self.caret);
        self.clamp_caret();
        self.record_caret();
        true
    }

    /// Insert typed whitespace at a caret with sticky marks armed. Whitespace is
    /// never itself wrapped: a mark around a space draws nothing a reader can
    /// see, and in Markdown and Djot it draws its own delimiters instead
    /// (`** **`). So the space goes in unmarked — outside any run the armed
    /// marks are shedding — and the marks stay armed for the character after it,
    /// which rejoins the run (see [`rejoin_run`](Self::rejoin_run)).
    fn insert_space_with_marks(&mut self, at: usize, text: &str, marks: InlineMarks) {
        let base = self.mark_spans_at(at);
        // What the *next* character carries: the armed delta resolved against the
        // marks in force here, which the space must not quietly drop.
        let want = base
            .iter()
            .map(|(k, _)| *k)
            .collect::<InlineMarks>()
            .xor(marks);
        let mut ins_at = at;
        for (kind, span) in &base {
            if marks.contains(*kind) {
                ins_at = ins_at.max(span.end);
            }
        }
        if !self.splice(ins_at, ins_at, text, typed_edit_kind(text)) {
            return;
        }
        self.rearm(want);
        self.record_caret();
    }

    /// Wrap `[s, e)` in `kind` via twig and return the byte span the *content*
    /// (not the delimiters) occupies afterwards. Markdown/Djot inline delimiters
    /// are symmetric (`**`…`**`, `_`…`_`, `` ` ``…`` ` ``), so the bytes twig
    /// added split evenly around the content — half the growth on each side.
    fn wrap_span(&mut self, s: usize, e: usize, kind: InlineKind) -> (usize, usize) {
        // The read-only gate — this door reaches twig without the splice.
        if self.read_only {
            return (s, e);
        }
        match self.editor.toggle_inline(s, e, kind) {
            Ok(change) => {
                self.last_edit_kind = None;
                self.refresh();
                self.dirty = self.source != self.clean_source;
                let added = (change.new.end - change.new.start).saturating_sub(e - s);
                let half = added / 2;
                (change.new.start + half, change.new.end - half)
            }
            // Unsupported here (e.g. mark on Markdown): leave the text unwrapped
            // rather than lose the keystroke.
            Err(e2) => {
                self.status = Some(format!("{kind:?}: {e2}"));
                (s, e)
            }
        }
    }

    /// The safe offset to splice a block-level break at, given a caret that may
    /// sit exactly between an inline mark's content and its own closing
    /// delimiter (`content_span.end == off < span.end` for some enclosing mark
    /// — the WYSIWYG caret's natural resting place at the end of `**bold**`
    /// with nothing following it on the line: the closing `**` renders no
    /// glyph of its own, so the caret's "end of line" offset lands right
    /// before it). Splicing a paragraph/list/quote break at `off` itself would
    /// sever the delimiter from its content, stranding it alone on the new
    /// line. Walks out to the *outermost* such mark's `span.end` instead, so
    /// nested marks closing at the same point (`**_x_**`) all clear together.
    /// A no-op everywhere else — mid-run, or past real trailing content, no
    /// mark's `content_span` ends exactly at `off`.
    fn skip_trailing_close_delims(&mut self, off: usize) -> usize {
        let off = off.min(self.source.len());
        let runs = self.run_span_ids();
        self.editor
            .ancestors_at(off)
            .unwrap_or_default()
            .into_iter()
            .filter(|m| hides_delims(m, &runs))
            .filter(|m| off < m.span.end && m.content_span.as_ref().is_some_and(|c| c.end == off))
            .map(|m| m.span.end)
            .max()
            .unwrap_or(off)
    }

    /// The offset a *delete* aimed at the character before `off` should stop at,
    /// when `off` is the start of a run's text and the bytes behind it are that
    /// run's opening delimiter. The rich view draws no glyph for a `**`, so the
    /// byte behind the caret at the start of a bold word is not a character the
    /// writer can see, let alone one they aimed Backspace at: taking it leaves
    /// `a *bold** c` — the styling gone and a literal asterisk in its place. The
    /// delete steps over the whole delimiter to the visible character in front of
    /// it instead. Walks out to the *outermost* mark opening there, so
    /// `**_x_**` clears every delimiter at once, and is a no-op anywhere else.
    fn skip_leading_open_delims(&mut self, off: usize) -> usize {
        let off = off.min(self.source.len());
        let runs = self.run_span_ids();
        self.editor
            .ancestors_at(off)
            .unwrap_or_default()
            .into_iter()
            .filter(|m| hides_delims(m, &runs))
            .filter(|m| {
                m.span.start < off && m.content_span.as_ref().is_some_and(|c| c.start == off)
            })
            .map(|m| m.span.start)
            .min()
            .unwrap_or(off)
    }

    /// `off` moved *inside* the run whose closing delimiters end there — the
    /// other offset the rich view draws in the same place, since a `**` renders
    /// no glyph of its own. `**bold**` has a caret home on each side of its
    /// closing delimiter, one column apart on screen and eight bytes and a whole
    /// run apart in the file, and a plain ← lands on the outer one whenever a
    /// space follows the phrase. The inner one is what the writer is pointing at
    /// there: the end of their bold word. Walks in through every mark closing at
    /// that point, innermost last, so `***both***` lands inside both. A no-op
    /// anywhere else — mid-run, or in prose, no mark's span ends at `off`.
    fn step_inside_close_delims(&mut self, off: usize) -> usize {
        let mut off = off.min(self.source.len());
        let runs = self.run_span_ids();
        loop {
            let inner = self
                .editor
                .ancestors_at(prev_boundary(&self.source, off))
                .unwrap_or_default()
                .into_iter()
                .filter(|m| hides_delims(m, &runs) && m.span.end == off)
                .filter_map(|m| m.content_span.clone().map(|c| c.end))
                .filter(|&end| end < off)
                .max();
            match inner {
                Some(end) => off = end,
                None => return off,
            }
        }
    }

    /// The mirror at the opening edge: `off` moved inside the run whose
    /// delimiters *start* there, onto the first character of its text. See
    /// [`step_inside_close_delims`](Self::step_inside_close_delims).
    fn step_inside_open_delims(&mut self, off: usize) -> usize {
        let mut off = off.min(self.source.len());
        let runs = self.run_span_ids();
        loop {
            let inner = self
                .editor
                .ancestors_at(off)
                .unwrap_or_default()
                .into_iter()
                .filter(|m| hides_delims(m, &runs) && m.span.start == off)
                .filter_map(|m| m.content_span.clone().map(|c| c.start))
                .filter(|&start| start > off)
                .min();
            match inner {
                Some(start) => off = start,
                None => return off,
            }
        }
    }

    /// The ids of the document's attributed run spans — the inline
    /// `Container`s [`wysiwyg::is_run_span`] picks out — for [`hides_delims`],
    /// which sees an ancestor chain and so only a kind. Read once per gesture,
    /// not once per step of a walk.
    fn run_span_ids(&mut self) -> Vec<NodeId> {
        self.nodes()
            .iter()
            .filter(|n| wysiwyg::is_run_span(n))
            .map(|n| n.id)
            .collect()
    }

    /// The attributed span whose text is exactly `content` — the whole of
    /// `<span …>i</span>`'s `i`, or nothing at all when `content` is empty
    /// and sits between the tags of `<span …></span>` — as the whole range
    /// spelling the span: the node's span, widened to its attribute block
    /// where the format writes that outside the node, as djot's
    /// `[i]{data-size="large"}` does. `None` for any other range, including
    /// part of a span's text.
    ///
    /// An empty span has an interior of no bytes, or no known interior at
    /// all: twig gives Markdown's `<span …></span>` the first and djot's
    /// `[]{…}` the second, and the chain already says the offset is inside.
    fn run_span_of_content(&mut self, content: Range<usize>) -> Option<Range<usize>> {
        let runs = self.run_span_ids();
        let m = self
            .editor
            .ancestors_at(content.start)
            .unwrap_or_default()
            .into_iter()
            .filter(|m| runs.contains(&NodeId(m.node_id)))
            .find(|m| match &m.content_span {
                Some(c) => *c == content,
                None => content.is_empty(),
            })?;
        let mut range = m.span;
        if let Some(attrs) = self
            .editor
            .document()
            .ok()
            .and_then(|mut d| d.attrs_span(NodeId(m.node_id)).ok().flatten())
        {
            range.start = range.start.min(attrs.start);
            range.end = range.end.max(attrs.end);
        }
        Some(range)
    }

    /// The attributed block whose whole text is exactly `content` — the `T`
    /// of Markdown's `<div class="center">\n\nT\n\n</div>` or djot's
    /// `{.center}\nT` — as the range a delete that takes that text takes with
    /// it: the whole `<div>` when the block is all the div holds, or the
    /// `{…}` line down to the end of the text. The block version of
    /// [`run_span_of_content`](Self::run_span_of_content), for the same
    /// reason: a paragraph with no text is no block, so the div would stand
    /// around nothing and the `{…}` line above nothing, and a from-scratch
    /// map gives neither a caret home — the `T`'s row is gone with the `T`.
    /// `None` for a block with more text, a div holding more, a heading (an
    /// empty `# ` is still a heading), and a format whose attributes are the
    /// block's own tag (HTML's `<p class="center"></p>` is still a
    /// paragraph).
    fn attributed_block_of_content(&mut self, content: Range<usize>) -> Option<Range<usize>> {
        if content.is_empty() || !matches!(self.format, Format::Markdown | Format::Djot) {
            return None;
        }
        let nodes = self.nodes();
        let block = nodes
            .iter()
            .filter(|n| n.kind == Kind::Para)
            .find(|n| n.content_span.as_ref() == Some(&content))?;
        match self.format {
            Format::Djot => {
                let attrs = self
                    .editor
                    .document()
                    .ok()
                    .and_then(|mut d| d.attrs_span(block.id).ok().flatten())?;
                (attrs.end <= block.span.start).then_some(attrs.start..content.end)
            }
            _ => {
                let div = block
                    .parent
                    .and_then(|p| nodes.iter().find(|n| n.id == p))
                    .filter(|p| wysiwyg::element_tag(p) == Some("div"))?;
                let alone = nodes.iter().filter(|n| n.parent == Some(div.id)).count() == 1;
                alone.then(|| div.span.clone())
            }
        }
    }

    /// The inline mark kinds whose span covers `off`, each with that span — the
    /// span-carrying sibling of [`marks_at`](Self::marks_at), which reports node
    /// ids instead. Used to shed a mark by stepping past the end of its run.
    fn mark_spans_at(&mut self, off: usize) -> Vec<(InlineKind, std::ops::Range<usize>)> {
        let off = off.min(self.source.len());
        self.editor
            .ancestors_at(off)
            .unwrap_or_default()
            .into_iter()
            .filter(|m| off < m.span.end)
            .filter_map(|m| inline_kind(&m.kind).map(|k| (k, m.span.clone())))
            .collect()
    }

    /// Insert clipboard `text` at the caret, replacing the selection if there is
    /// one — always its own undo step, whatever its length.
    ///
    /// Provenance is the whole point, and only the caller has it. `insert` reads
    /// a lone character as a keystroke and folds it into the run around it,
    /// which is right for typing and wrong for a one-character paste: that paste
    /// would vanish mid-run on an undo it was never part of, and the characters
    /// the user actually typed would go with it. Length can't tell the two
    /// apart — `⌘V` of `x` and typing `x` are the same string — so the door the
    /// caller comes through is what says which happened.
    pub fn paste(&mut self, text: &str) {
        // Pasting against a block picture or a table's end joins the block
        // exactly as typing does, and for the same reason — see
        // `open_paragraph_at_block_edge`.
        self.open_paragraph_at_block_edge(text);
        let (s, e) = self.selection().unwrap_or((self.caret, self.caret));
        self.splice(s, e, text, EditKind::Other);
    }

    /// Replace `[start, end)` with `text` as one step of an IME composition —
    /// the same splice as [`edit`](Self::edit), but marked so the run of steps
    /// folds into a single undo.
    ///
    /// A composition is *one* act of writing. Typing `かんじ` and picking 感じ is a
    /// dozen calls here, each replacing the last one's provisional bytes, and an
    /// undo step per call means undoing a word means pressing ⌘Z until the reading
    /// unspools backwards through kana — the intermediate states were never text
    /// the user wrote. Only the frontend knows a call is provisional (the bytes
    /// look like any other edit), so the door the caller comes through is what
    /// says so, exactly as it is for [`paste`](Self::paste) versus
    /// [`insert`](Self::insert).
    ///
    /// Pair with [`end_composition`](Self::end_composition), or the *next*
    /// composition folds into this one.
    pub fn edit_composing(&mut self, start: usize, end: usize, text: &str) {
        self.splice(start, end, text, EditKind::Compose);
    }

    /// Close the open composition run, so the next one is its own undo step.
    /// Call when the IME commits or withdraws a composition.
    ///
    /// Only clears a *composition* run: a frontend that reports an end it never
    /// began (some IMEs unmark unprompted) would otherwise split the run of
    /// typing around it into two undo steps for no reason the user can see.
    pub fn end_composition(&mut self) {
        if self.last_edit_kind == Some(EditKind::Compose) {
            self.last_edit_kind = None;
        }
    }

    // ── the clipboard's rich flavor ──────────────────────────────────────────

    /// The selection rendered as HTML, for the clipboard's `text/html` flavor —
    /// what lets a paste into Docs/Mail/Slack keep its formatting. `None` when
    /// nothing is selected, or when the selection doesn't render (the caller
    /// still has [`selected_text`](Self::selected_text), which is what to publish
    /// as `text/plain` either way).
    ///
    /// **The fragment is a source substring, and that is the honest limit here.**
    /// It's parsed standalone, so a selection whose meaning depends on its
    /// surroundings converts as what it literally says rather than what it looks
    /// like on screen: half a list item is a paragraph, a row torn out of a table
    /// is the text of a row, the `**` of a bold run selected without its closing
    /// `**` is two asterisks. Every one of those still *renders* — there's no
    /// error to report — it just renders as the fragment and not as the document.
    /// Widening the range to whole blocks would publish text the user didn't
    /// select, which is a worse lie than a fragment being a fragment; the plain
    /// flavor has the same substring, so the two flavors at least agree.
    pub fn selection_html(&mut self) -> Option<String> {
        let (start, end) = self.selection()?;
        let inline = self.selection_is_inline(start, end);
        let html = html::render_fragment(&self.source[start..end], self.format)?;
        Some(match inline {
            true => html::strip_sole_paragraph(html),
            false => html,
        })
    }

    /// Paste the clipboard's `text/html` flavor, converting it to this document's
    /// format first. Its own undo step, like any [`paste`](Self::paste).
    ///
    /// Returns whether it landed. `false` means the HTML didn't convert to
    /// anything worth pasting — the caller should fall back to the plain flavor
    /// rather than treat it as an error. The `html` module has the full list of
    /// what that covers: a table twig won't build, markup it doesn't recognise,
    /// an empty result.
    pub fn paste_html(&mut self, html: &str) -> bool {
        match html::parse_fragment(html, self.format) {
            Some(source) => {
                self.paste(&source);
                true
            }
            None => false,
        }
    }

    /// Does the selection live *inside* a single top-level block?
    ///
    /// The question [`selection_html`](Self::selection_html) needs and the
    /// fragment can't answer: `**bold**` renders as `<p><strong>bold</strong></p>`
    /// whether the user selected one word of a sentence or a whole paragraph, and
    /// only the document knows which. Selecting a word and pasting into Docs
    /// should extend the line you paste into; selecting the paragraph should make
    /// a paragraph. So a selection strictly within one block is inline (its `<p>`
    /// is an artifact of standalone parsing), and one that covers a whole block —
    /// or spans two — keeps its structure.
    ///
    /// Reads the block from twig rather than guessing from the bytes:
    /// `ancestors_at` is `[doc, block, …inline]`, so index 1 is the top-level
    /// block containing an offset, and two ends inside the same one cannot have
    /// crossed a block boundary.
    fn selection_is_inline(&mut self, start: usize, end: usize) -> bool {
        // The last *character*, not `end - 1`: the selection's end is exclusive
        // and may sit mid-codepoint's-worth of bytes past the last char.
        let Some((off, _)) = self.source[start..end].char_indices().next_back() else {
            return false;
        };
        let (Some(head), Some(tail)) =
            (self.top_block_span(start), self.top_block_span(start + off))
        else {
            return false;
        };
        head == tail && !(start <= head.start && end >= head.end)
    }

    /// The byte span of the top-level block containing `offset`, or `None` at an
    /// offset that belongs to no block (the blank line between two of them).
    fn top_block_span(&mut self, offset: usize) -> Option<std::ops::Range<usize>> {
        self.editor
            .ancestors_at(offset)
            .ok()?
            .get(1)
            .map(|m| m.span.clone())
    }

    // ── indentation ──────────────────────────────────────────────────────────

    /// One indent level.
    ///
    /// Two spaces, not the four both frontends type for Tab today, because in a
    /// markdown document four columns isn't a width — it's a *meaning*. Four
    /// spaces at the head of a line is markdown's indented-code-block marker, so
    /// one Tab on a paragraph would reparse it into code and style it as such;
    /// two cannot, and the line stays the prose it was. Two is also exactly
    /// where a `- ` bullet's content starts, so an indented line lands under its
    /// parent item's text instead of beside it — the column a list-aware indent
    /// has to hit anyway, which keeps this width from being relitigated later.
    const INDENT: &'static str = "  ";

    /// Indent the selected lines — or the caret's line, with no selection — by
    /// one level (Tab).
    pub fn indent(&mut self) {
        self.reindent(true);
        // Nesting changes an ordered list's numbering (the nested item restarts,
        // its old siblings resume) — keep the source markers in step.
        self.renumber_here();
        // Nesting an empty `-` item under a text line reparses that text as a
        // setext heading; swap the dash for a `*` before it can (a no-op unless
        // the collapse actually happened).
        self.avoid_setext_collapse();
    }

    /// Take one indent level back off the selected lines, or the caret's line
    /// (Shift+Tab). A line with no indentation is left exactly as it is.
    ///
    /// A line with *less* than a full level gives back what it has rather than
    /// refusing: outdent's job is to walk a line left, and real documents — hand
    /// written, or reflowed by some other editor — are full of indentation that
    /// was never a clean multiple of anything. Refusing there would strand the
    /// line at a depth Shift+Tab couldn't undo.
    pub fn outdent(&mut self) {
        self.reindent(false);
        self.renumber_here();
    }

    /// The body of [`indent`](Self::indent) / [`outdent`](Self::outdent).
    ///
    /// One splice across the whole line range, never one per line: a Tab is one
    /// thing the user did, so it has to be one undo step and one reparse. Per
    /// line, twig would reparse the document once per line and leave a stack of
    /// steps that Shift+⌘Z walks back one line at a time.
    fn reindent(&mut self, add: bool) {
        let (sel_start, sel_end) = self.selection().unwrap_or((self.caret, self.caret));
        let start = source_line_range(&self.source, sel_start).start;
        let end = source_line_range(&self.source, sel_end).end;
        let region = self.source[start..end].to_string();
        let lines: Vec<&str> = region.split('\n').collect();
        // A blank line has no text to move, and padding it would leave nothing
        // but trailing whitespace — but Tab on a blank line *is* a request for
        // indentation to type into, so the skip only applies where the op has
        // other lines to do real work on.
        let skip_blank = add && lines.len() > 1;

        let mut out = String::with_capacity(region.len() + lines.len() * Self::INDENT.len());
        let mut deltas: Vec<isize> = Vec::with_capacity(lines.len());
        let mut line_off = start;
        for (i, full) in lines.iter().enumerate() {
            if i > 0 {
                out.push('\n');
            }
            // A list item moves by having its whole leading prefix *replaced*,
            // never by having spaces pushed in front of the line. twig spells
            // both prefixes, so the quote markers, the parent's indent and an
            // ordered marker's extra column all come out right without leaf
            // measuring any of them — and a line that only looks like an item
            // (a Djot continuation) reports no marker and is left to the plain
            // path, where a Tab is just a Tab.
            let marker = self.list_marker_on_line(line_off);
            let own = marker
                .as_ref()
                .map(|m| m.marker_start - m.line_start)
                .unwrap_or(0);
            let delta = if add {
                if skip_blank && full.trim().is_empty() {
                    out.push_str(full);
                    0
                } else if marker.is_some() && self.first_item_of_list(line_off) {
                    // The first item of a list has no preceding sibling to nest
                    // under, so a Tab here can't spell a sub-list — twig would
                    // reparse the shoved-over marker as the same list, only
                    // indented, which Shift+Tab then can't cleanly undo. Leave the
                    // item where it is, the way every list editor refuses to
                    // over-indent a list's first line.
                    out.push_str(full);
                    0
                } else if marker.is_some() {
                    // Nesting means standing where a *continuation* of this line
                    // would stand: past the parent's marker, inside its content
                    // column. That is `continuation_prefix`, less a checkbox.
                    let new = self.nesting_prefix_at(line_off);
                    let delta = new.len() as isize - own as isize;
                    out.push_str(&new);
                    out.push_str(&full[own..]);
                    delta
                } else {
                    out.push_str(Self::INDENT);
                    out.push_str(full);
                    Self::INDENT.len() as isize
                }
            } else if marker.is_some() {
                // Unnesting is the mirror: stand where the parent item's own
                // line starts, which drops exactly the level it contributed.
                let new = self.outdent_prefix_at(line_off);
                let delta = new.len() as isize - own as isize;
                out.push_str(&new);
                out.push_str(&full[own..]);
                delta
            } else {
                // A plain line gives back the ordinary step.
                let strip = outdent_width(full, Self::INDENT.len());
                out.push_str(&full[strip..]);
                -(strip as isize)
            };
            deltas.push(delta);
            line_off += full.len() + 1;
        }
        // Nothing to give back. Returning before the splice keeps an outdent at
        // column zero from spending an undo step on a document it never changed.
        if deltas.iter().all(|d| *d == 0) {
            return;
        }

        // Every line's text keeps its offset *within the line*, so the caret is
        // remapped by its column, not by its byte offset — which the prefixes on
        // the lines above it have already invalidated.
        let remap = |off: usize| -> usize {
            let (mut old_ls, mut new_ls) = (start, start);
            for (line, delta) in lines.iter().zip(&deltas) {
                let old_le = old_ls + line.len();
                let new_len = (line.len() as isize + delta) as usize;
                if off <= old_le {
                    let col = (off - old_ls) as isize;
                    return new_ls + ((col + delta).max(0) as usize).min(new_len);
                }
                old_ls = old_le + 1;
                new_ls += new_len + 1;
            }
            start + out.len()
        };
        let placed = match self.selection() {
            // Keep the rewritten region selected, the way a container toggle
            // keeps its own: it leaves a second Tab aimed at the same lines
            // rather than at whatever the shifted offsets now happen to cover.
            Some(_) => (start + out.len(), Some(start)),
            None => (remap(self.caret), None),
        };

        // A rolled-back splice leaves the old source in place, where every offset
        // computed above addresses text that was never written.
        if !self.splice(start, end, &out, EditKind::Other) {
            return;
        }
        // `splice` re-anchors to the end of the `Change`, which for a whole-region
        // rewrite is the last line's end — nowhere the caret was. Place it, then
        // re-record the caret so this is the state redo restores, not the one
        // `splice` left behind from the `Change`.
        self.caret = placed.0.min(self.source.len());
        self.anchor = placed.1;
        self.clamp_caret();
        self.record_caret();
    }

    /// The Enter key.
    ///
    /// In source view it's a literal newline. In WYSIWYG it's **AST-aware**: a
    /// bare `\n` is only a markdown soft break (same paragraph), so the block the
    /// caret is in decides what actually gets written.
    ///
    ///   - paragraph            → twig's [`Editor::split_block`], which parts the
    ///                            block at the caret and reopens its container
    ///   - list item            → likewise: the next item, its indent, quote
    ///                            prefix and `[ ]` box all reproduced by twig —
    ///                            except an *empty* item, which exits the list
    ///   - block quote          → likewise: a new paragraph inside the quote
    ///   - heading              → a new *paragraph*, not another heading
    ///   - code block           → a literal newline (stay in the block)
    ///   - thematic break       → the line under the rule, opened if it has
    ///                            none ([`stand_under_block`](Self::stand_under_block))
    ///   - blank line           → a literal newline (one Backspace undoes it)
    ///   - [`LineFlow::Preserve`] → a single soft break, which renders as a
    ///                            visible line
    ///
    /// Where `split_block` is used it replaces markup leaf used to spell by hand,
    /// and it is better at it: it drops the whitespace the caret was sitting in
    /// front of instead of stranding it at the head of the second half, and it
    /// knows continuations leaf's marker scan never covered — a checklist item
    /// continues as an *unchecked* checklist item rather than a plain bullet.
    ///
    /// The exceptions above are exceptions because `split_block` is either wrong
    /// there or refuses: parting a fence yields two fences with the code split
    /// between them, parting a heading yields a second heading where every editor
    /// gives a paragraph, and a blank line, an empty item, a setext heading and a
    /// table all report an error rather than a split.
    pub fn newline(&mut self) {
        if self.view == View::Source {
            self.insert_raw("\n");
            return;
        }
        // Enter over a selection replaces it with a paragraph break.
        if let Some((s, e)) = self.selection() {
            self.splice(s, e, "\n\n", EditKind::Other);
            return;
        }
        // On a blank page there is no block to push down, and a blank line
        // with nothing under it is not something Fold flow draws. Enter wrote
        // one anyway: an edit, and an undo step, that showed nothing.
        if self.line_flow == LineFlow::Fold && self.source[self.caret_floor()..].trim().is_empty() {
            return;
        }
        // A caret resting exactly between an inline mark's content and its own
        // closing delimiter (`**bold**` with nothing after it on the line —
        // the WYSIWYG caret's natural end-of-line position) must not splice a
        // block break there: every path below eventually does via
        // `insert_raw`/`self.caret`, and splicing before the hidden closing
        // delimiter would strand it alone on the new line.
        self.caret = self.skip_trailing_close_delims(self.caret);
        // The block the caret is in. `block_offset_for_caret` nudges off a line
        // end (where the caret sits at the doc level); on a bare line (e.g. an
        // empty list item) fall back to the caret so the enclosing list/quote is
        // still visible in the ancestors.
        let off = self.block_offset_for_caret().unwrap_or(self.caret);
        let kinds: Vec<Kind> = self
            .editor
            .ancestors_at(off)
            .map(|c| c.into_iter().map(|m| m.kind).collect())
            .unwrap_or_default();
        let has = |k: Kind| kinds.contains(&k);

        if has(Kind::CodeBlock) {
            self.insert_raw("\n");
            return;
        }
        // An *empty* list item exits the list — the standard double-Enter — which
        // `split_block` reports as an error rather than a split (there is no
        // content to part), so it stays leaf's. `list_marker_on_line` is itself
        // the AST gate — it answers from the tree, so a `- ` that reads as a
        // marker byte-for-byte but opens no item (a setext underline, a Djot
        // continuation line) never reaches here.
        if let Some(marker) = self.list_marker_on_line(self.caret)
            && self.item_is_empty(&marker)
        {
            self.exit_list(&marker);
            return;
        }
        // Beside a rule, at the home past it, the caret's source line is the
        // blank one under the rule, which the branch below would take for an
        // empty paragraph. Mid-document its lone newline left the caret on the
        // next block's first line, and what was typed there joined that block.
        if let Some(end) = self.rule_above_caret() {
            self.stand_under_block(end);
            return;
        }
        // On an *empty* paragraph line, a lone Enter should add a single blank line,
        // not another full paragraph break — so it moves down one line and one
        // Backspace undoes it, not two. (`split_block` errors here too.)
        let line_start = self.source[..self.caret].rfind('\n').map_or(0, |i| i + 1);
        let line_end = self.source[self.caret..]
            .find('\n')
            .map_or(self.source.len(), |i| self.caret + i);
        if self.source[line_start..line_end].trim().is_empty() {
            self.insert_raw("\n");
            return;
        }
        // At the start of a paragraph with a block above it, twig's split
        // writes a lone newline ahead of the paragraph, which Fold draws as one
        // more gap: the first Enter showed nothing. A heading's branch below
        // parted it after its `#`, leaving an empty heading over a paragraph
        // that had been the heading's text. Both open an empty paragraph above
        // instead, the caret staying with the text. In Preserve flow too: its
        // lone newline went inside a div, where nothing drew it, and between
        // a mark's delimiters, where it cut the mark.
        if let Some((at, text)) = self.paragraph_opening_at_caret() {
            let caret = self.caret;
            if self.splice(at, at, text, EditKind::Other) {
                self.caret = caret + text.len();
                self.record_caret();
            }
            return;
        }
        // In `Preserve` flow a soft break is a *visible* line the author means to
        // make, so Enter writes a single `\n` and typing continues the same
        // paragraph on the next line — the behaviour of an ordinary text editor.
        // A second Enter then lands on the blank line above and takes the
        // empty-line branch, so double-Enter still promotes to a full paragraph
        // break; and Backspace, which deletes a lone `\n` over a soft break,
        // undoes a single Enter symmetrically. In `Fold` flow a lone `\n` would
        // render as an invisible space, so Enter keeps making the paragraph break
        // that actually shows.
        //
        // Only in running prose. A list or a quote has a continuation of its own
        // to write, and a `\n` there is not a soft line but a lost container.
        // Nor in a heading, which a newline ends in Markdown and runs on
        // unseen in djot: it takes the heading's own branch below.
        let in_container = has(Kind::ListItem) || has(Kind::TaskListItem) || has(Kind::BlockQuote);
        if self.line_flow == LineFlow::Preserve && !in_container && !has(Kind::Heading) {
            self.insert_raw("\n");
            return;
        }
        // A heading gets a *paragraph*, never a second heading: Enter at the end
        // of a title is how every editor is asked for the body under it, and
        // `split_block` would repeat the `#` instead. Whitespace at the split
        // point goes with the break rather than opening the new paragraph, which
        // is what `split_block` does everywhere else.
        if has(Kind::Heading) {
            let mut end = self.caret;
            while self.source.as_bytes().get(end) == Some(&b' ') {
                end += 1;
            }
            self.splice(self.caret, end, "\n\n", EditKind::Other);
            return;
        }
        self.split_block_here();
    }

    /// Where Enter writes to open an empty paragraph above the paragraph or
    /// heading the caret starts, and what — `None` when the caret does not
    /// start one.
    ///
    /// The caret starts a paragraph that is in no list item, quote, fence or
    /// table when no text of the paragraph stands before it: at its first
    /// byte, or past only the opening delimiters of marks that begin with it
    /// (`**`, a colour's `<span …>`), which is where a tap before the first
    /// letter lands. A heading's `#` marker is such a delimiter. Parted there, twig's split cut the mark in two. The break
    /// goes in front of those delimiters, and in front of any `<div>` or
    /// fenced div the paragraph opens — the view draws no row for a blank line
    /// above a container's first block, so a centred paragraph's empty line
    /// has to open outside its div.
    ///
    /// Inside a list twig's split already writes a marked line, which draws,
    /// and so it does in a quote, except above the quote's first line.
    ///
    /// What is written is what it takes to draw one more empty row. Between
    /// two blocks the first and last blank lines are the gap, so a paragraph
    /// break goes where there are fewer than two and a newline where the gap
    /// is already there. Above the first block only the last blank line is
    /// the gap, so a newline goes where there is one already. Preserve flow
    /// draws every blank line, so a newline is always enough there, but under
    /// frontmatter, whose own blank line draws nothing.
    fn paragraph_opening_at_caret(&mut self) -> Option<(usize, &'static str)> {
        let caret = self.caret.min(self.source.len());
        let chain = self.editor.ancestors_at(caret).ok()?;
        if chain.iter().any(|m| {
            matches!(
                m.kind,
                Kind::ListItem | Kind::TaskListItem | Kind::CodeBlock | Kind::Table
            )
        }) {
            return None;
        }
        let para = chain
            .iter()
            .find(|m| matches!(m.kind, Kind::Para | Kind::Heading))?;
        // An empty heading has nothing to push down: Enter there is still
        // the body under it.
        let end = para.span.end.min(self.source.len());
        if self.source[caret.min(end)..end].trim().is_empty() {
            return None;
        }
        let mut at = para.span.start;
        if at != caret {
            let text_before = self
                .editor
                .subtree(twig::NodeId(para.node_id))
                .ok()?
                .iter()
                .any(|n| {
                    n.span.start < caret
                        && (n.kind == Kind::Image
                            || n.text.as_deref().is_some_and(|t| !t.trim().is_empty()))
                });
            if text_before {
                return None;
            }
        }
        // Out past each container this paragraph opens, innermost first: one
        // whose source before it is its opening line and blank lines. A quote
        // opens on its first paragraph's own line, and draws no line above
        // that paragraph: the break goes above the quote. Further down a
        // quote twig's split writes a quoted line, which draws.
        let mut outer: Vec<_> = chain
            .iter()
            .filter(|m| matches!(m.kind, Kind::Container | Kind::BlockQuote) && m.span.start < at)
            .collect();
        outer.sort_by_key(|m| std::cmp::Reverse(m.span.start));
        for c in outer {
            let before = &self.source[c.span.start..at];
            let opens = match c.kind {
                Kind::BlockQuote => !before.contains('\n'),
                _ => before.lines().skip(1).all(|l| l.trim().is_empty()),
            };
            if !opens {
                break;
            }
            at = c.span.start;
        }
        if chain
            .iter()
            .any(|m| m.kind == Kind::BlockQuote && m.span.start < at)
        {
            return None;
        }
        let content = self.source[..at].trim_end().len();
        let newlines = self.source[content..at].matches('\n').count();
        // Every newline starts a blank line but one that ends the line of
        // what stands above, a block or hidden frontmatter.
        let blank = if content == 0 {
            newlines
        } else {
            newlines.saturating_sub(1)
        };
        let first = self
            .top_blocks()
            .iter()
            .all(|m| m.kind == Kind::Metadata || m.span.start >= at);
        // Preserve flow draws every blank line, but the one under frontmatter.
        let gap = match (self.line_flow, first) {
            (LineFlow::Preserve, true) => usize::from(content > 0),
            (LineFlow::Preserve, false) => 0,
            (LineFlow::Fold, true) => 1,
            (LineFlow::Fold, false) => 2,
        };
        Some((at, if blank >= gap { "\n" } else { "\n\n" }))
    }

    /// Part the block at the caret with twig's [`Editor::split_block`], leaving
    /// the caret in the second half.
    ///
    /// twig reopens whatever the first half was inside of — the bullet with its
    /// indent, the quote's `>`, a checklist item's `[ ]` — which is the whole
    /// reason this replaced the markup leaf used to spell from the line's bytes.
    /// It renumbers nothing, though: a new item mid-list is written with its
    /// neighbour's number, so [`renumber_here`](Self::renumber_here) still runs
    /// behind it, folded into the same undo step.
    ///
    /// Falls back to a plain paragraph break if twig declines, so an unhandled
    /// shape still moves the caret down rather than swallowing the keystroke.
    fn split_block_here(&mut self) {
        // The read-only gate — this door reaches twig without the splice.
        if self.read_only {
            return;
        }
        match self.editor.split_block(self.caret) {
            Ok(change) => {
                self.last_edit_kind = None;
                self.refresh();
                self.anchor = None;
                self.caret = change.new.end;
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.clamp_caret();
                self.record_caret();
                // Aimed at the new block's *start*: the caret twig leaves is one
                // past the marker it wrote, where there is no list in reach.
                self.renumber_at(change.new.start);
            }
            Err(_) => self.insert_raw("\n\n"),
        }
    }

    /// Whether the item on the marker's line carries no content — the shape
    /// double-Enter reads as "I'm done with this list."
    fn item_is_empty(&self, line: &ListMarker) -> bool {
        let content_start = line.content_start().min(self.source.len());
        let line_end = self.source[self.caret..]
            .find('\n')
            .map(|i| self.caret + i)
            .unwrap_or(self.source.len());
        self.source[content_start..line_end.max(content_start)]
            .trim()
            .is_empty()
    }

    /// Leave the list: replace the empty item's marker with a blank line, so the
    /// caret lands in a fresh paragraph below it.
    ///
    /// Inside a quote the blank line has to stay quoted (a bare one would end the
    /// quote), and the caret's new line keeps the `> ` it was already behind —
    /// leaving the list without also leaving the quote.
    fn exit_list(&mut self, line: &ListMarker) {
        let prefix = self.quote_prefix_at(line.marker_start);
        let blank = prefix.trim_end();
        self.splice(
            line.line_start,
            self.caret,
            &format!("{blank}\n{prefix}"),
            EditKind::Other,
        );
    }

    /// What a line continuing the containers at `off` has to open with — the
    /// quote markers reproduced, each enclosing item's marker as its width in
    /// spaces. Also the column a nested item's marker stands in, which is what
    /// makes it Tab's answer.
    fn continuation_prefix_at(&mut self, off: usize) -> String {
        self.editor
            .document()
            .and_then(|mut d| d.continuation_prefix(off))
            .map(|p| p.text)
            .unwrap_or_default()
    }

    /// The column a *nested list* may open at inside the item at `off` — which
    /// is not always where the item's own text continues.
    ///
    /// twig counts a task item's `[ ] ` box as part of its marker, correctly:
    /// it is markup a rich view hides, and the item's own wrapped text does
    /// stand past it. But a nested list may only open at the *list* marker's
    /// column, and four columns further in is an indented continuation of the
    /// paragraph instead — `- [ ] a` + `      - [ ] b` is one item, not two.
    /// So the box's own width goes back.
    ///
    /// The one place leaf still reads a checkbox's spelling. It goes when twig
    /// reports the list marker's column apart from the box; `checked` is what
    /// says a box is there at all, so only its width is being measured here.
    fn nesting_prefix_at(&mut self, off: usize) -> String {
        let cont = self.continuation_prefix_at(off);
        let Some(item) = self.innermost_list_item(off) else {
            return cont;
        };
        if item.checked.is_none() {
            return cont;
        }
        let box_width = item
            .marker_span
            .and_then(|m| self.source.get(m))
            .and_then(|marker| marker.rfind('[').map(|i| marker.len() - i))
            .unwrap_or(0);
        // The trailing columns are the ones the item's own marker contributed,
        // so trimming from the end leaves any quote prefix standing.
        cont[..cont.len().saturating_sub(box_width)].to_string()
    }

    /// Where the line of the item *containing* the item at `off` begins — the
    /// prefix Shift+Tab moves back to, which gives up exactly the level the
    /// parent contributed. The quote prefix alone for a top-level item, which
    /// has no level left to give.
    fn outdent_prefix_at(&mut self, off: usize) -> String {
        let items: Vec<usize> = self
            .editor
            .document()
            .and_then(|mut d| d.ancestors_at_caret(off))
            .map(|c| {
                c.into_iter()
                    .filter(|m| m.kind == Kind::ListItem || m.kind == Kind::TaskListItem)
                    .map(|m| m.span.start)
                    .collect()
            })
            .unwrap_or_default();
        // The second-innermost item is the parent; its own line's indent is the
        // target. `list_marker_on_line` gives that line's prefix directly.
        let parent = items.len().checked_sub(2).map(|i| items[i]);
        match parent.and_then(|p| self.list_marker_on_line(p)) {
            Some(m) => self.source[m.line_start..m.marker_start].to_string(),
            None => self.quote_prefix_at(off),
        }
    }

    /// The block-quote prefix in force at `off` — `""` outside a quote, `"> "`
    /// inside one, `"> > "` inside two.
    ///
    /// Assembled from each enclosing quote's own [`FlatNode::marker_span`], so
    /// the `>` and the space after it are twig's spelling rather than leaf's.
    /// The whole line prefix can't answer this: it also carries the indent of
    /// whatever the quote holds, which a blank separator line must *not* repeat.
    fn quote_prefix_at(&mut self, off: usize) -> String {
        let Ok(chain) = self
            .editor
            .document()
            .and_then(|mut d| d.ancestors_at_caret(off))
        else {
            return String::new();
        };
        let quotes: Vec<usize> = chain
            .iter()
            .filter(|m| m.kind == Kind::BlockQuote)
            .map(|m| m.node_id as usize)
            .collect();
        let Ok(nodes) = self.editor.nodes() else {
            return String::new();
        };
        quotes
            .iter()
            .filter_map(|id| nodes.get(*id)?.marker_span.clone())
            .filter_map(|s| self.source.get(s))
            .collect()
    }

    /// Whether the item at `off` sits inside another one — the test Backspace
    /// uses to choose between outdenting and dropping the marker.
    ///
    /// Counted from the AST rather than from the line's leading whitespace,
    /// which is indentation in Markdown and, in Djot, may be nothing at all.
    fn item_is_nested(&mut self, off: usize) -> bool {
        self.editor
            .document()
            .and_then(|mut d| d.ancestors_at_caret(off))
            .map(|c| {
                c.into_iter()
                    .filter(|m| m.kind == Kind::ListItem || m.kind == Kind::TaskListItem)
                    .count()
                    > 1
            })
            .unwrap_or(false)
    }

    /// The innermost list item containing `probe`, under twig's **caret**
    /// containment rule — a block's end is inside it.
    ///
    /// Half-open containment can't answer this. An empty item's span is exactly
    /// its marker, so the caret sitting after `- ` is one past the end and the
    /// item it is plainly in tests as out of reach; that is the shape
    /// double-Enter has to recognise to leave the list.
    fn innermost_list_item(&mut self, probe: usize) -> Option<FlatNode> {
        let chain = self
            .editor
            .document()
            .and_then(|mut d| d.ancestors_at_caret(probe))
            .ok()?;
        let id = chain
            .iter()
            .rev()
            .find(|m| m.kind == Kind::ListItem || m.kind == Kind::TaskListItem)?
            .node_id as usize;
        self.editor.nodes().ok()?.get(id).cloned()
    }

    /// The list marker opening `off`'s line, per twig — `None` when that line
    /// opens no list item.
    ///
    /// [`Document::line_prefix`] is the whole hidden run from the line start:
    /// `>   1. ` is a quote's marker, an indent, and an item's marker together,
    /// and it is `None` on a *continuation* line, which opens nothing. That last
    /// case is the one leaf could never get right by reading bytes. `- a\n  - b`
    /// is two items in Markdown and one in Djot, where a marker cannot interrupt
    /// a paragraph and `  - b` is literal text — identical bytes, and only the
    /// parser knows which document it is looking at.
    ///
    /// The item's own marker is separated out via its
    /// [`FlatNode::marker_span`], so `marker_start` splits the prefix into what
    /// the containers around it contribute and what the item does.
    fn list_marker_on_line(&mut self, off: usize) -> Option<ListMarker> {
        let off = off.min(self.source.len());
        let prefix = self.editor.document().ok()?.line_prefix(off).ok()??;
        // The prefix belongs to a list only when an item's marker closes it —
        // a heading's `# ` or a bare quote's `> ` is a prefix too.
        let item = self.innermost_list_item(prefix.end.min(self.source.len()))?;
        let marker = item.marker_span.clone()?;
        if marker.end != prefix.end {
            return None;
        }
        Some(ListMarker {
            line_start: prefix.start,
            marker_start: marker.start,
            text: self.source.get(prefix)?.to_string(),
        })
    }

    /// Whether the list item on `line_start`'s line is the **first item** of its
    /// list — the one Tab must not nest, because nesting needs a preceding
    /// sibling to become the new parent and a first item has none. `false` for a
    /// line that isn't a list item, and for an item with a sibling above it (the
    /// one Tab *can* nest). Gated on the AST, not the marker bytes: `- ` reads
    /// the same in a setext underline that opens no list at all.
    fn first_item_of_list(&mut self, line_start: usize) -> bool {
        let Some(marker) = self.list_marker_on_line(line_start) else {
            return false;
        };
        // Probe just inside the marker, where the item's own node is in reach —
        // the marker offset itself can resolve to the enclosing list, not the
        // `list_item`, whose span starts at the marker.
        let probe = marker.content_start().min(self.source.len());
        let Some(item) = self.innermost_list_item(probe) else {
            return false;
        };
        let Ok(nodes) = self.editor.nodes() else {
            return false;
        };
        match item.parent {
            // First when the parent list opens with this very item.
            Some(pid) => nodes
                .get(pid.0 as usize)
                .is_some_and(|p| p.first_child == Some(item.id)),
            // A parentless item is trivially the first (and only) one.
            None => true,
        }
    }

    pub fn backspace(&mut self) {
        if let Some((s, e)) = self.selection() {
            self.splice(s, e, "", EditKind::Other);
            return;
        }
        // WYSIWYG: Backspace at the very start of a list item's content is a
        // structural key, not a character delete — it walks the "un-indent, then
        // un-list" ladder every list editor gives that keystroke (outdent a
        // nested item, strip a top-level one's marker to a paragraph). In source
        // view the `- ` is visible text the user is deleting a byte of, so it
        // keeps its literal meaning there, like Enter does.
        if self.view != View::Source && self.backspace_list_start() {
            return;
        }
        // WYSIWYG: and the same at the start of a heading's content — the `# `
        // there is markup the rich view hides, not text the user typed.
        if self.view != View::Source && self.backspace_heading_start() {
            return;
        }
        // WYSIWYG: and at the start of a block whose presentation is spelled
        // as hidden markup before it — djot's `{.center}` line, Markdown's
        // `<div class="center">` — Backspace takes that markup, the way it
        // takes a heading's `#`, rather than a byte out of it.
        if self.view != View::Source && self.backspace_attributed_block_start() {
            return;
        }
        // WYSIWYG: at a block picture's stops, a byte-at-a-time delete would take
        // the markup apart under a caret that cannot see it — see
        // `delete_around_block_media`.
        if self.view != View::Source && self.delete_around_block_media(false) {
            return;
        }
        // WYSIWYG: Backspace at a table's trailing stop steps back into its last
        // cell rather than taking the byte behind the caret — the row's closing
        // `|`, which the rich view never drew, so the key would have looked like
        // it did nothing. The stop before is the last cell's end.
        if self.view != View::Source && self.backspace_at_table_end() {
            return;
        }
        // WYSIWYG: a table cell's start is a wall. The bytes behind it are the
        // padding and the `|` that make the grid, and taking them merges two
        // cells — a structural edit nobody asked for, and one no table editor
        // gives the key. Tab and Shift+Tab are how the caret leaves a cell.
        if self.view != View::Source && self.at_cell_wall(BreakEdge::Backward) {
            return;
        }
        // WYSIWYG: at the start of a block's content, the byte behind the caret
        // is a block boundary, and Backspace over one is a join — twig's, so
        // that what a join is in each format is not this file's to know. After
        // the picture and table cases, which are block starts with their own
        // answers.
        if self.view != View::Source && self.backspace_joins_block() {
            return;
        }
        // WYSIWYG: Backspace on a *blank line* deletes back to the previous caret
        // stop, not a single newline. On a line with no text of its own, the byte
        // before the caret is a `\n` that spells part of a block boundary — the gap
        // between two blocks, drawn but never a caret home. Removing just it strands
        // the caret in that gap and leaves an odd blank line the eye reads as one
        // separator but the caret can't land on: the "extra newline" left behind
        // after leaving a list (Enter, Enter) or a paragraph and pressing Backspace.
        // Deleting to the previous stop instead collapses the whole break at once,
        // landing the caret at the end of the block above. Two blank lines in a row
        // are one stop apart, so this still removes exactly one — the lone-Enter /
        // lone-Backspace symmetry the empty-line case is built on is untouched.
        if self.view != View::Source
            && self.caret > self.caret_floor()
            && self.caret_on_blank_line()
            && let Some(stop) = self.vmap.stop_before(self.caret)
        {
            let stop = stop.max(self.caret_floor());
            if stop < self.caret {
                if self.source[stop..self.caret].trim().is_empty() {
                    self.splice(stop, self.caret, "", EditKind::Delete);
                } else {
                    // Hidden markup stands between the stop and the caret — a
                    // `</div>`, a comment, a link reference definition — and
                    // collapsing to the stop would delete it. Take the blank
                    // line alone, with the newline that opened it, and land
                    // the caret where the collapse would have.
                    self.delete_blank_line_to(stop);
                }
                return;
            }
        }
        if self.caret > self.caret_floor() {
            // An in-cell `<br>` draws as one newline glyph, so Backspace over it
            // takes the whole tag — a single-byte step would leave a broken `<br`
            // showing in the cell. Rich view only (source view edits the literal).
            if self.view != View::Source
                && let Some((start, end)) = self.cell_break_at(BreakEdge::Backward)
            {
                let start = start.max(self.caret_floor());
                if start < end {
                    self.splice(start, end, "", EditKind::Delete);
                    return;
                }
            }
            // Aim the delete at the character the writer can *see* behind the
            // caret, never at a delimiter the rich view drew nothing for. Two
            // steps, and either can apply: from the far side of a run's closing
            // `**` step back into the run (the caret is drawn at the end of its
            // word), and at the start of a run's text step out past its opening
            // `**` to the character in front of it, leaving the run standing.
            // Without them a plain Backspace unspells the phrase it is editing
            // and leaves a literal asterisk on screen.
            let end = if self.view == View::Source {
                self.caret
            } else {
                let inside = self.step_inside_close_delims(self.caret);
                // An attributed span with no text — `<span …></span>` as the
                // file was written — is hidden markup around nothing, and a
                // byte-step here would take its `>`. Backspace takes the span
                // whole, with the character before it: the character the key
                // looks aimed at, since the span draws nothing.
                if let Some(span) = self.run_span_of_content(inside..inside) {
                    let from = if self.source[..span.start].ends_with('\n') {
                        span.start
                    } else {
                        prev_boundary(&self.source, span.start)
                    };
                    let from = from.max(self.caret_floor());
                    self.splice(from, span.end, "", EditKind::Delete);
                    return;
                }
                self.skip_leading_open_delims(inside)
                    .max(self.caret_floor())
            };
            // Never delete back across the floor — that would eat hidden
            // frontmatter the WYSIWYG caret can't even see.
            let mut prev = prev_boundary(&self.source, end).max(self.caret_floor());
            // Take a hidden escape backslash with the char it escapes: the rich
            // view draws `\*` as a single `*`, so Backspace over it must delete
            // both bytes, never strand the `\` as a lone visible backslash (the
            // mirror of the Hidden-mode typing that wrote the escape). Source view
            // shows the `\`, so there it is an ordinary character.
            if self.view != View::Source
                && prev > self.caret_floor()
                && self.is_hidden_escape(prev - 1)
            {
                prev -= 1;
            }
            // The delete that takes the last of a span's text takes the span
            // with it, in the same edit: `<span …>i</span>` losing its `i`
            // would leave an empty span the map has no stop inside, so the
            // caret would draw at the next stop — a line away — until a
            // further key removed the span. Landing on the span's start is
            // where the letter was.
            if self.view != View::Source
                && let Some(span) = self.run_span_of_content(prev..end)
            {
                self.splice(span.start, span.end, "", EditKind::Delete);
                return;
            }
            // And the same for a block: the letter that was all of a centred
            // paragraph's text goes with the `<div>` around it, or the `{…}`
            // line above it, leaving a plain blank line where the letter was.
            // A paragraph with no text is no block, so the markup would stand
            // around nothing, the map would give it no caret home, and the
            // next key would take the tag apart.
            if self.view != View::Source
                && let Some(block) = self.attributed_block_of_content(prev..end)
            {
                self.splice(block.start, block.end, "", EditKind::Delete);
                return;
            }
            if prev < end {
                self.splice(prev, end, "", EditKind::Delete);
            }
        }
    }

    /// Remove the blank line the caret is on — its own newline and the one
    /// that ended the line before it — and put the caret on `stop`, the caret
    /// stop before it. The [`backspace`](Self::backspace) blank-line rule for a
    /// blank line that hidden markup separates from the block above: the
    /// navigable blank row after a `</div>` is always one of at least three
    /// newlines under the tag (the drawn separators either side of it), so
    /// taking two leaves the blank line the tag needs under it.
    fn delete_blank_line_to(&mut self, stop: usize) {
        let caret = self.caret;
        let line_start = self.source[..caret].rfind('\n').map_or(0, |i| i + 1);
        let line_end = self.source[caret..]
            .find('\n')
            .map_or(self.source.len(), |i| caret + i);
        let from = line_start.saturating_sub(1).max(stop);
        let to = (line_end + 1).min(self.source.len());
        self.splice(from, to, "", EditKind::Delete);
        self.caret = stop;
        self.anchor = None;
        self.goal_col = None;
        self.record_caret();
    }

    /// Move the caret to the stop before it and consume the key — what
    /// Backspace does where the byte behind the caret is hidden markup it
    /// has no structural answer for, rather than take that markup apart.
    fn step_back_to_stop(&mut self) {
        // The map answers about offsets, so it has to be this revision's — see
        // `open_paragraph_at_block_edge`.
        self.rebuild_map();
        if let Some(off) = self
            .vmap
            .stop_before(self.caret)
            .filter(|&o| o >= self.caret_floor())
        {
            self.caret = off;
            self.anchor = None;
            self.goal_col = None;
        }
    }

    /// Backspace's presentation behaviour: with the caret exactly at the start
    /// of a block's content, and that block's attributes spelled as hidden
    /// markup before it, strip the attributes. The peer of
    /// [`backspace_heading_start`](Self::backspace_heading_start), and the same
    /// reasoning: the `{.center}` line above a djot block and the
    /// `<div class="center">` around a Markdown one are what the byte behind
    /// the caret belongs to, and the rich view draws neither. The ordinary
    /// delete took the newline out of `{.center}\nhello` and left
    /// `{.center}hello` — the attribute line fused onto the text as prose —
    /// and out of `<div …>\n\nhello` it took the blank line the div needs.
    ///
    /// The whole attribute set goes, the way the whole `#` marker does — the
    /// press is over the line that spells it, not over one key of it — and
    /// twig's `set_block_attrs` with an empty list is the edit: it removes the
    /// djot line and unwraps the Markdown div. Where the block is the first of
    /// several in a div, twig has no sole child to unwrap and answers with a
    /// no-op, so the caret steps back to the stop before instead, as it does
    /// at a table's end. A later child of the div has an ordinary paragraph
    /// above it and is not this rule's.
    ///
    /// Returns whether it acted; `false` leaves Backspace its character delete.
    fn backspace_attributed_block_start(&mut self) -> bool {
        if !matches!(self.format, Format::Markdown | Format::Djot) {
            return false;
        }
        let caret = self.caret;
        let nodes = self.nodes();
        let Some(block) = nodes
            .iter()
            .filter(|n| matches!(n.kind, Kind::Para | Kind::Heading))
            .find(|n| n.content_span.as_ref().map_or(n.span.start, |c| c.start) == caret)
        else {
            return false;
        };
        match self.format {
            Format::Djot => {
                // twig records where the `{…}` block was written, so this is
                // the parser's own answer and not a scan for a `{` above the
                // block; `None` (a synthesized or merged set) is not a line
                // the caret is standing after.
                let spelled = self
                    .editor
                    .document()
                    .ok()
                    .and_then(|mut d| d.attrs_span(block.id).ok().flatten())
                    .is_some_and(|s| s.end <= caret);
                if !spelled {
                    return false;
                }
            }
            _ => {
                let Some(div) = block
                    .parent
                    .and_then(|p| nodes.iter().find(|n| n.id == p))
                    .filter(|p| wysiwyg::element_tag(p) == Some("div"))
                else {
                    return false;
                };
                let mut kids = nodes.iter().filter(|n| n.parent == Some(div.id));
                if kids.clone().any(|k| k.span.start < block.span.start) {
                    return false;
                }
                if kids.nth(1).is_some() {
                    self.step_back_to_stop();
                    return true;
                }
            }
        }
        self.write_block_attrs("block attributes", Vec::new());
        true
    }

    /// Backspace at the start of a block's content: join the block into the
    /// block before it, as one gesture — twig's `join_blocks`, the inverse of
    /// the split Enter makes, spelled the format's way. Two paragraphs join
    /// on a soft break; a paragraph under a marker heading joins onto the
    /// heading's line; a paragraph after a Markdown `<div>` moves inside it,
    /// the hidden `</div>` carried past the joined text; HTML's `</p><p>` is
    /// taken as one; a quote's or an item's continuation prefix is written.
    /// The joined text takes the block above's presentation and containers,
    /// which is the rule every editor with a centred paragraph follows.
    ///
    /// Leaf used to join by deleting the one newline behind the caret, which
    /// is the right bytes for two Markdown paragraphs and nothing else: under
    /// a heading it left two blocks, in HTML it took the `>` off a tag, and
    /// after a div it took the newline under the hidden `</div>`, which drew
    /// nothing different and took the tag apart on the next press. What a
    /// join is in each format is twig's to know, and now it does.
    ///
    /// Where twig refuses — the block above is a code block, a table or a
    /// rule with no text to join into, or the caret's block would have to
    /// leave a div that holds more after it — the caret steps back to the
    /// stop before instead, as it does at a table's end: the key moves the
    /// caret and takes no markup apart. Where nothing precedes the block, or
    /// the format cannot join at all, Backspace keeps its character delete.
    ///
    /// Returns whether it acted.
    fn backspace_joins_block(&mut self) -> bool {
        let caret = self.caret;
        if caret <= self.caret_floor() {
            return false;
        }
        let Some(text) = self.text_block_opening_at(caret) else {
            return false;
        };
        match self.join_blocks(caret) {
            Ok(change) => {
                // The caret keeps its place at the start of the text it stood
                // on, wherever the join put that text — after a soft break,
                // a space, or a quote's prefix. Found by the bytes, as
                // `block_content_in` finds a re-spelled block.
                let region = &self.source[change.new.clone()];
                let at = region
                    .find(&text)
                    .map_or(change.new.start, |i| change.new.start + i);
                self.land_after_join(at);
                true
            }
            Err(twig::Error::NotEditable) => {
                self.step_back_to_stop();
                true
            }
            Err(twig::Error::NotFound | twig::Error::UnsupportedFormat) => false,
            Err(e) => {
                self.status = Some(format!("join: {e}"));
                true
            }
        }
    }

    /// Delete at the end of a block's content: join the block after it into
    /// this one — [`backspace_joins_block`](Self::backspace_joins_block)'s
    /// mirror, and the same twig gesture aimed at the next block. The caret
    /// stays where it was, which is where the joined text now begins after
    /// the separator. Where twig refuses, the caret steps forward to the next
    /// stop instead; where no block follows, Delete keeps its character
    /// delete.
    fn delete_forward_joins_block(&mut self) -> bool {
        let caret = self.caret;
        let at_end = self
            .nodes()
            .iter()
            .filter(|n| matches!(n.kind, Kind::Para | Kind::Heading))
            .any(|n| n.content_span.as_ref().is_some_and(|c| c.end == caret));
        if !at_end {
            return false;
        }
        // The next stop, across a line end, in a block: what Delete at a
        // block's end points at. On the same line it is a hidden delimiter's
        // far side, which the ordinary delete handles; on a blank line it is
        // the empty paragraph the byte delete has always closed.
        self.rebuild_map();
        let Some(stop) = self.vmap.stop_after(caret) else {
            return false;
        };
        if !self.source[caret..stop].contains('\n') || !self.has_block_at(stop) {
            return false;
        }
        match self.join_blocks(stop) {
            Ok(change) => {
                self.land_after_join(change.old.start);
                true
            }
            Err(twig::Error::NotEditable | twig::Error::NotFound) => {
                self.caret = stop;
                self.anchor = None;
                self.goal_col = None;
                true
            }
            Err(twig::Error::UnsupportedFormat) => false,
            Err(e) => {
                self.status = Some(format!("join: {e}"));
                true
            }
        }
    }

    /// The content bytes of the paragraph or heading whose content opens
    /// exactly at `off` — the block a Backspace there is at the start of.
    fn text_block_opening_at(&mut self, off: usize) -> Option<String> {
        self.nodes()
            .into_iter()
            .filter(|n| matches!(n.kind, Kind::Para | Kind::Heading))
            .find_map(|n| {
                let c = n.content_span?;
                (c.start == off).then(|| self.source[c].to_string())
            })
    }

    /// Hand the block at `offset` to twig's `join_blocks`, with the undo
    /// plumbing every structural gesture has; the caret is the caller's to
    /// place from the change, via [`land_after_join`](Self::land_after_join).
    fn join_blocks(&mut self, offset: usize) -> Result<Change, twig::Error> {
        if self.read_only {
            return Err(twig::Error::NotEditable);
        }
        self.record_caret();
        let change = self.editor.join_blocks(offset)?;
        self.last_edit_kind = None; // structural edit is its own undo step
        self.refresh();
        Ok(change)
    }

    /// Finish a join: the caret at `at`, no selection, the map this
    /// revision's before the clamp — see `write_block_attrs` for why.
    fn land_after_join(&mut self, at: usize) {
        self.caret = at;
        self.anchor = None;
        self.goal_col = None;
        self.dirty = self.source != self.clean_source;
        self.status = None;
        self.rebuild_map();
        self.clamp_caret();
        self.record_caret();
    }

    // ── moving a block ─────────────────────────────────────────────────────────

    /// The block a move picks up at `offset` — the same one
    /// [`select_block_at`](Self::select_block_at) selects (the deepest node
    /// that is neither inline nor a multi-block container), widened to the
    /// list item when it is the item's first block, because that is what
    /// twig's `move_block` moves: a bullet's text is the bullet, and dragging
    /// it takes the item and everything under it. A block later in an item's
    /// tail moves alone. `None` on a blank line, and past the source.
    fn movable_block_at(&mut self, offset: usize) -> Option<FlatNode> {
        let off = offset.min(self.source.len());
        let chain = self.editor.ancestors_at(off).ok()?;
        let block = chain
            .iter()
            .rev()
            .find(|m| !wysiwyg::is_inline_kind(&m.kind) && !is_block_container(&m.kind))?;
        let nodes = self.nodes();
        let block = nodes.get(block.node_id as usize)?.clone();
        let item = block
            .parent
            .and_then(|p| nodes.get(p.0 as usize))
            .filter(|p| matches!(p.kind, Kind::ListItem | Kind::TaskListItem))
            .filter(|p| p.first_child == Some(block.id));
        Some(item.cloned().unwrap_or(block))
    }

    /// The source range of the block a move would pick up at `offset` — for
    /// the outline of the block being carried, without moving the caret. The
    /// range [`select_block_at`](Self::select_block_at) would select, except
    /// that it is the whole item for a bullet's text, as
    /// [`move_block`](Self::move_block) is.
    pub fn block_range_at(&mut self, offset: usize) -> Option<Range<usize>> {
        self.movable_block_at(offset).map(|b| b.span)
    }

    /// Move the block at `from` to the boundary `to` — twig's `move_block`,
    /// with the undo plumbing every structural gesture has and the caret
    /// riding the block to its new place. `from` is any offset inside the
    /// block (the one [`block_range_at`](Self::block_range_at) finds); `to`
    /// is a position *between* blocks — a block's first byte lands before it,
    /// its last after it, a blank line is itself a boundary, and the source's
    /// length is the document's end. The block takes the line prefixes of
    /// the container the boundary is inside — a `> ` on the way into a quote,
    /// none on the way out — and the blank lines a person would have typed
    /// are written and removed; twig spells all of that.
    ///
    /// A move that would move nothing — `to` inside the block, or on the
    /// boundary it already sits on — is a quiet no-op rather than an error,
    /// since it is what a block dropped back where it was asks for. Any other
    /// refusal reaches the status line: a `to` interior to a fence or a table,
    /// or a format with no blocks a caret could name.
    ///
    /// In WYSIWYG the frontmatter is hidden, and a boundary above it is not
    /// one a person can see: `to` is raised to the first rendered offset.
    ///
    /// `true` when the document changed. A move twig accepts that rewrites
    /// nothing — a one-item list sent past a paragraph is still a one-item
    /// list past that paragraph — is taken back rather than left as an undo
    /// step with no difference in it, and is `false` too.
    pub fn move_block(&mut self, from: usize, to: usize) -> bool {
        if self.read_only || self.refuse_unsupported("move block", Gesture::MoveBlock) {
            return false;
        }
        let len = self.source.len();
        let from = from.min(len);
        let to = to.clamp(self.caret_floor().min(len), len);
        let Some(block) = self.movable_block_at(from) else {
            self.status = Some("move block: no block here".into());
            return false;
        };
        // Where the caret stands in the block, as (line within the block,
        // bytes back from that line's end) — the shape that survives a
        // prefix being added or stripped on the way through a container.
        let caret = self.caret.clamp(block.span.start, block.span.end);
        let block_line = self.source[block.span.start..caret].matches('\n').count();
        let line_end = self.source[caret..block.span.end]
            .find('\n')
            .map_or(block.span.end, |i| caret + i);
        let tail = line_end - caret;
        let block_lines = self.source[block.span.start..block.span.end]
            .matches('\n')
            .count()
            + 1;
        let upward = to <= block.span.start;
        self.record_caret();
        match self.editor.move_block(from, to) {
            Ok(_) if self.editor.source_str().ok().as_deref() == Some(self.source.as_str()) => {
                let _ = self.editor.undo();
                self.status = None;
                false
            }
            Ok(change) => {
                self.last_edit_kind = None; // structural edit is its own undo step
                self.refresh();
                let at = self.moved_block_caret(&change.new, upward, block_lines, block_line, tail);
                self.caret = at;
                self.anchor = None;
                self.goal_col = None;
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.rebuild_map();
                self.clamp_caret();
                self.record_caret();
                true
            }
            // Nothing to move: the block dropped back onto its own boundary.
            Err(twig::Error::InvalidArgument) => {
                self.status = None;
                false
            }
            Err(e) => {
                self.status = Some(format!("move block: {e}"));
                false
            }
        }
    }

    /// The caret's place in the rewritten region after a move: the moved
    /// block's lines open the region when it went up (below the blank line it
    /// was dropped on, when it was) and close it (above the separator twig
    /// wrote after it) when it went down, and within them the
    /// caret keeps its line and its distance from that line's end. Clamped
    /// into the region rather than trusted, since a block can lose a line on
    /// the way — a quote it was the only content of goes with it.
    fn moved_block_caret(
        &self,
        new: &Range<usize>,
        upward: bool,
        block_lines: usize,
        block_line: usize,
        tail: usize,
    ) -> usize {
        let region = &self.source[new.start.min(self.source.len())..new.end.min(self.source.len())];
        let mut lines: Vec<(usize, usize)> = Vec::new();
        let mut start = 0;
        loop {
            match region[start..].find('\n') {
                Some(i) => {
                    lines.push((start, start + i));
                    start += i + 1;
                }
                None => {
                    lines.push((start, region.len()));
                    break;
                }
            }
        }
        // A separator line is blank, or a quote's bare `>`; the region can
        // open with one (the blank line the block was dropped on) or close
        // with one (the one twig wrote after it).
        let separator = |&(s, e): &(usize, usize)| {
            region[s..e]
                .trim()
                .trim_start_matches('>')
                .trim()
                .is_empty()
        };
        let first = if upward {
            lines.iter().position(|l| !separator(l)).unwrap_or(0)
        } else {
            let filled = lines
                .iter()
                .rposition(|l| !separator(l))
                .map_or(0, |i| i + 1);
            filled.saturating_sub(block_lines)
        };
        let (s, e) = lines[(first + block_line).min(lines.len() - 1)];
        new.start + e.saturating_sub(tail).max(s)
    }

    /// Move the caret's block one place up — Alt+↑: above the block before it,
    /// and out of its container, to just above it, when it is the first block
    /// there. Nothing above the document's first block, and nothing to do for
    /// a caret on a blank line; both say so in the status line.
    pub fn move_block_up(&mut self) {
        self.move_block_by(true);
    }

    /// Move the caret's block one place down — Alt+↓, the mirror of
    /// [`move_block_up`](Self::move_block_up): below the block after it, and
    /// out of its container when it is the last block there.
    pub fn move_block_down(&mut self) {
        self.move_block_by(false);
    }

    fn move_block_by(&mut self, up: bool) {
        if self.read_only || self.refuse_unsupported("move block", Gesture::MoveBlock) {
            return;
        }
        let Some(from) = self.block_offset_for_caret() else {
            self.status = Some("move block: no block here".into());
            return;
        };
        let Some(block) = self.movable_block_at(from) else {
            self.status = Some("move block: no block here".into());
            return;
        };
        let nodes = self.nodes();
        let to = if up {
            self.boundary_above(&nodes, &block)
        } else {
            self.boundary_below(&nodes, &block)
        };
        if to.is_none_or(|to| !self.move_block(from, to)) && self.status.is_none() {
            self.status = Some(if up {
                "move block: nothing above".into()
            } else {
                "move block: nothing below".into()
            });
        }
    }

    /// The boundary one step above `block`: before its previous sibling, or
    /// — for the first block in a container — before the container itself.
    /// `None` for the document's first block.
    fn boundary_above(&self, nodes: &[FlatNode], block: &FlatNode) -> Option<usize> {
        let parent = block.parent.and_then(|p| nodes.get(p.0 as usize))?;
        let previous = nodes
            .iter()
            .filter(|n| n.parent == Some(parent.id))
            .take_while(|n| n.id != block.id)
            .last();
        match previous {
            Some(p) => self.before(p),
            None if parent.kind == Kind::Doc => None,
            // An item leaving a nested list upward goes before the item
            // holding that list, as an item of the outer one; the list's own
            // first byte is inside the holding item's tail.
            None if matches!(
                parent.kind,
                Kind::BulletList | Kind::OrderedList | Kind::TaskList
            ) =>
            {
                match parent.parent.and_then(|p| nodes.get(p.0 as usize)) {
                    Some(item) if matches!(item.kind, Kind::ListItem | Kind::TaskListItem) => {
                        self.before(item)
                    }
                    _ => self.before(parent),
                }
            }
            None => self.before(parent),
        }
    }

    /// The boundary one step below `block`: after its next sibling, or — for
    /// the last block in a container — just past the container, at its
    /// parent's level ([`exit_below`](Self::exit_below)). `None` for the
    /// document's last block.
    fn boundary_below(&self, nodes: &[FlatNode], block: &FlatNode) -> Option<usize> {
        let parent = block.parent.and_then(|p| nodes.get(p.0 as usize))?;
        if let Some(n) = block.next_sibling.and_then(|n| nodes.get(n.0 as usize)) {
            return Some(self.after(n));
        }
        match parent.kind {
            Kind::Doc => None,
            _ => self.exit_below(nodes, parent),
        }
    }

    /// The boundary just past `container` at its parent's level: before its
    /// next sibling, or past its parent when it is the last thing there —
    /// the document's end at the top. Leaving a list item is the exception
    /// that keeps a block in the list: the next item's tail, which is what
    /// [`after`](Self::after) an item names.
    fn exit_below(&self, nodes: &[FlatNode], container: &FlatNode) -> Option<usize> {
        let item = matches!(container.kind, Kind::ListItem | Kind::TaskListItem);
        match container.next_sibling.and_then(|n| nodes.get(n.0 as usize)) {
            Some(n) if item => Some(self.after(n)),
            Some(n) => self.before(n),
            None => {
                let parent = container.parent.and_then(|p| nodes.get(p.0 as usize))?;
                match parent.kind {
                    Kind::Doc => Some(self.source.len()),
                    _ => self.exit_below(nodes, parent),
                }
            }
        }
    }

    /// The boundary before `node`: its first byte. twig reads a container's
    /// opening as before it — a quote's marker, a fence's first byte — so the
    /// span's start is the boundary at the parent's level for every kind.
    fn before(&self, node: &FlatNode) -> Option<usize> {
        Some(node.span.start)
    }

    /// The boundary after `node`: its own end, which for a container proper
    /// (a quote, a list, a fenced or tagged container) is the end of its last
    /// line — after its last block, still inside it, where a block arriving
    /// from outside joins it. Past the container is the next line's start: a
    /// blank line, the next block, or the document's end.
    fn after(&self, node: &FlatNode) -> usize {
        let container = is_block_container(&node.kind)
            && !matches!(node.kind, Kind::ListItem | Kind::TaskListItem);
        let end = node.span.end.min(self.source.len());
        if container && self.source.as_bytes().get(end) == Some(&b'\n') {
            end + 1
        } else {
            end
        }
    }

    /// Where a block dragged over rendered row `row` would land — the
    /// boundary before the row's block when the row is in its upper half, the
    /// boundary after it otherwise, and the document's end for a row below
    /// everything. `None` for a row that holds no block (a decoration row is
    /// resolved to the block under it, so this is a table's bottom rule with
    /// nothing after it, or a map with no rows). The frontend draws its
    /// indicator above [`DropTarget::row`] and hands
    /// [`DropTarget::offset`] to [`move_block`](Self::move_block).
    ///
    /// The block is the deepest one, not the widened item: a drop on a
    /// bullet's text lands before or after that bullet, and its nested
    /// children — rows of their own — answer for themselves.
    pub fn drop_target_at(&mut self, row: usize) -> Option<DropTarget> {
        let rows = self.vmap.rows.len();
        if row >= rows {
            return Some(DropTarget {
                offset: self.source.len(),
                row: rows,
            });
        }
        // A gap or a rule is not a block; the block under it is the one meant.
        let row = (row..rows).find(|&r| self.vmap.row_is_navigable(r))?;
        let start = self.vmap.row_start(row)?;
        let chain = self.editor.ancestors_at(start).ok()?;
        let block = chain
            .iter()
            .rev()
            .find(|m| !wysiwyg::is_inline_kind(&m.kind) && !is_block_container(&m.kind))?;
        let span = block.span.clone();
        let (first, last) = self.vmap.row_range_for(span.clone());
        if row <= usize::midpoint(first, last) {
            Some(DropTarget {
                offset: span.start,
                row: first,
            })
        } else {
            Some(DropTarget {
                offset: span.end.min(self.source.len()),
                row: last + 1,
            })
        }
    }

    /// Backspace at a table's trailing stop: move onto the stop before it (the
    /// last cell's end) and consume the key. `false` anywhere else. See
    /// [`VisualMap::table_end_stop`] for why the byte behind the caret there is
    /// not one to delete.
    fn backspace_at_table_end(&mut self) -> bool {
        // The map answers about offsets, so it has to be this revision's — see
        // `open_paragraph_at_block_edge`.
        self.rebuild_map();
        if !self.vmap.table_end_stop(self.caret) {
            return false;
        }
        if let Some(off) = self
            .vmap
            .stop_before(self.caret)
            .filter(|&o| o >= self.caret_floor())
        {
            self.caret = off;
            self.anchor = None;
            self.goal_col = None;
        }
        true
    }

    /// Whether the caret stands at a table cell's wall on the `edge` side — at
    /// or before its first caret stop for Backspace, at or past its last for
    /// Delete — where the only bytes between it and the neighbouring cell are
    /// the padding and the `|` the rich view draws as the grid. The padding
    /// counts as the cell's: a caret placed between `| ` and the text is at the
    /// same wall, so the first press cannot eat the space either. An in-cell
    /// `<br>` at the edge is not a wall; the delete takes it whole, as ever.
    fn at_cell_wall(&mut self, edge: BreakEdge) -> bool {
        // The map answers about offsets, so it has to be this revision's.
        self.rebuild_map();
        if self.cell_break_at(edge).is_some() {
            return false;
        }
        let caret = self.caret;
        let src = self.source.as_bytes();
        let pad = |b: &u8| *b == b' ' || *b == b'\t';
        self.vmap
            .tables
            .iter()
            .flat_map(|t| &t.grid)
            .flat_map(|row| &row.cells)
            .any(|cell| {
                let lo = cell.start
                    - src[..cell.start]
                        .iter()
                        .rev()
                        .take_while(|b| pad(b))
                        .count();
                let hi = cell.end + src[cell.end..].iter().take_while(|b| pad(b)).count();
                (lo..=hi).contains(&caret)
                    && match edge {
                        BreakEdge::Backward => {
                            self.vmap.stop_before(caret).is_none_or(|s| s < cell.start)
                        }
                        BreakEdge::Forward => {
                            self.vmap.stop_after(caret).is_none_or(|s| s > cell.end)
                        }
                    }
            })
    }

    /// Whether the caret's own source line holds nothing but whitespace — an
    /// empty paragraph, or the blank line a block boundary is spelled with. The
    /// test for [`backspace`](Self::backspace)'s stop-wise delete: such a line has
    /// no text of its own, so the newline before the caret belongs to the gap
    /// between blocks rather than to any word the caret is editing.
    fn caret_on_blank_line(&self) -> bool {
        let line_start = self.source[..self.caret].rfind('\n').map_or(0, |i| i + 1);
        let line_end = self.source[self.caret..]
            .find('\n')
            .map_or(self.source.len(), |i| self.caret + i);
        self.source[line_start..line_end].trim().is_empty()
    }

    /// The source span of an in-cell hard break (`<br>`) touching the caret on the
    /// `edge` side — the byte range to delete whole. A table row is one source
    /// line, so its break is spelled `<br>` yet drawn as a single newline glyph
    /// (see `wysiwyg.rs`); a delete over it must take every byte, or a one-byte
    /// step strands a broken `<br` in the cell. `Backward` matches a break ending
    /// at the caret (Backspace), `Forward` one starting at it (Delete). `None`
    /// when no such break is adjacent. Only the in-cell break is spelled `<br>`
    /// (an ordinary hard break is `  \n`), so the leading `<` alone tells them
    /// apart — no ancestor walk needed. Rich view only; source view shows the
    /// literal tag and deletes it a byte at a time.
    fn cell_break_at(&mut self, edge: BreakEdge) -> Option<(usize, usize)> {
        let caret = self.caret;
        let nodes = self.nodes();
        let src = self.source.as_bytes();
        nodes
            .iter()
            .find(|n| {
                n.kind == Kind::HardBreak
                    && n.span.start < n.span.end
                    && src.get(n.span.start) == Some(&b'<')
                    && match edge {
                        BreakEdge::Backward => n.span.end == caret,
                        BreakEdge::Forward => n.span.start == caret,
                    }
            })
            .map(|n| (n.span.start, n.span.end))
    }

    /// Whether the source byte at `off` is a backslash twig consumed as an escape
    /// (hidden in the rich view), as against a literal backslash (drawn). A
    /// backslash escapes exactly an ASCII-punctuation character (the CommonMark /
    /// Djot rule twig follows), so `\` + punctuation is the whole test — no AST
    /// round-trip needed.
    fn is_hidden_escape(&self, off: usize) -> bool {
        let b = self.source.as_bytes();
        b.get(off) == Some(&b'\\') && b.get(off + 1).is_some_and(u8::is_ascii_punctuation)
    }

    /// Backspace's list behaviour: when the caret sits exactly at the start of a
    /// list item's content (right after its marker), outdent the item if it's
    /// nested, else strip the marker so it becomes a paragraph. Returns whether
    /// it acted — `false` leaves Backspace its ordinary character delete.
    fn backspace_list_start(&mut self) -> bool {
        let Some(marker) = self.list_marker_on_line(self.caret) else {
            return false;
        };
        // Only right after the marker. That the line opens a real item is
        // already settled: `list_marker_on_line` answers from the tree.
        if self.caret != marker.content_start() {
            return false;
        }
        if self.item_is_nested(marker.marker_start) {
            // Nested: give back one level, keeping the marker and carrying the
            // caret with it.
            self.outdent();
        } else {
            // Top level: drop the marker, leaving a paragraph, then renumber the
            // siblings the removed item was counted among. Only the marker goes —
            // a quote prefix in front of it still has a quote to hold up.
            self.splice(marker.marker_start, self.caret, "", EditKind::Other);
            self.renumber_here();
        }
        true
    }

    /// Backspace's heading behaviour: with the caret exactly at the start of an
    /// ATX heading's content — right after the `#` marker the rich view hides —
    /// strip the marker so the line becomes a paragraph. The peer of
    /// [`backspace_list_start`](Self::backspace_list_start)'s ladder, and the same
    /// reasoning: hidden block markup is structure, so the keystroke over it is
    /// structural.
    ///
    /// Without this the ordinary delete takes the space out of `# Title` and
    /// leaves `#Title`, which is no longer a heading at all — the hash the view
    /// had been hiding surfaces as literal text the user has to delete a second
    /// time, having never typed it. A closing sequence (`# Title #`, hidden at the
    /// other end) goes with the marker for the same reason.
    ///
    /// Returns whether it acted; `false` leaves Backspace its character delete.
    fn backspace_heading_start(&mut self) -> bool {
        let caret = self.caret;
        // The heading whose content opens exactly at the caret. A bare `#` has no
        // content span at all — its content starts (and ends) where the line does.
        let Some((span, content_end, marker)) = self.nodes().iter().find_map(|n| {
            let (start, end) = match &n.content_span {
                Some(c) => (c.start, c.end),
                None => (n.span.end, n.span.end),
            };
            (n.kind == Kind::Heading && start == caret)
                .then(|| (n.span.clone(), end, n.marker_span.clone()))
        }) else {
            return false;
        };
        // twig reports the marker's own extent, so there is nothing to walk back
        // over and no `#` in this file. A setext heading has no marker — its
        // content opens the line — so it falls through to the ordinary delete,
        // as does anything else sitting at a content start.
        // `m.end == caret` is what excludes a setext heading, whose marker is the
        // underline *after* the content rather than a prefix before it.
        let Some(marker) = marker.filter(|m| m.end == caret) else {
            return false;
        };
        let start = marker.start;
        // A closing `#` sequence is hidden too, so it can't be left behind. Only
        // when the tail really is one: trailing spaces alone are nothing to strip.
        let tail = &self.source[content_end..span.end];
        if tail.contains('#') && tail.chars().all(|c| c == '#' || c.is_whitespace()) {
            let kept = self.source[caret..content_end].to_string();
            self.splice(start, span.end, &kept, EditKind::Other);
            // The splice leaves the caret past the text it re-wrote; the caret
            // belongs where the content now starts, which is where it already was.
            self.caret = start;
            self.record_caret();
        } else {
            self.splice(start, caret, "", EditKind::Other);
        }
        true
    }

    pub fn delete_forward(&mut self) {
        if let Some((s, e)) = self.selection() {
            self.splice(s, e, "", EditKind::Other);
        } else if self.caret < self.source.len() {
            // The mirror of Backspace's: forward-delete in front of a picture
            // would eat the `!` off its markup and leave a link where a photo was.
            if self.view != View::Source && self.delete_around_block_media(true) {
                return;
            }
            // And of Backspace's cell wall: Delete at a cell's end would take
            // the padding and the `|` after it.
            if self.view != View::Source && self.at_cell_wall(BreakEdge::Forward) {
                return;
            }
            // And of Backspace's join: at the end of a block's content, Delete
            // joins the next block into this one.
            if self.view != View::Source && self.delete_forward_joins_block() {
                return;
            }
            // Delete forward over an in-cell `<br>` takes the whole tag, the mirror
            // of Backspace's swallow (see `cell_break_at`) — else a byte-step
            // strands a broken `<br` in the cell.
            if self.view != View::Source
                && let Some((start, end)) = self.cell_break_at(BreakEdge::Forward)
            {
                self.splice(start, end, "", EditKind::Delete);
                return;
            }
            // The mirror of Backspace's two steps: from in front of a run's
            // opening `**` step into it, onto the first letter of its text, and
            // at the end of a run's text step out past its closing `**` to the
            // character beyond. Either way Delete takes the character it looks
            // like it is pointing at, and never a delimiter drawn as nothing.
            // The caret then settles back inside the run it was standing in —
            // see `settle_inside_close_delims`.
            let from = if self.view == View::Source {
                self.caret
            } else {
                let inside = self.step_inside_open_delims(self.caret);
                // The mirror of Backspace's empty-span rule: an attributed
                // span with no text goes whole, with the character after it.
                if let Some(span) = self.run_span_of_content(inside..inside) {
                    let to = if self.source[span.end..].starts_with('\n') {
                        span.end
                    } else {
                        next_boundary(&self.source, span.end)
                    };
                    self.splice(span.start, to, "", EditKind::Delete);
                    return;
                }
                self.skip_trailing_close_delims(inside)
            };
            let next = next_boundary(&self.source, from);
            // And of its emptying rules: the span goes with its last letter,
            // and so does the block's div or `{…}` line.
            if self.view != View::Source
                && let Some(span) = self.run_span_of_content(from..next)
            {
                self.splice(span.start, span.end, "", EditKind::Delete);
                return;
            }
            if self.view != View::Source
                && let Some(block) = self.attributed_block_of_content(from..next)
            {
                self.splice(block.start, block.end, "", EditKind::Delete);
                return;
            }
            if from < next {
                self.splice(from, next, "", EditKind::Delete);
            }
        }
    }

    /// Delete from the caret back to the start of the previous word (⌥⌫ /
    /// Ctrl+⌫). Deletes the selection instead when one is active.
    pub fn delete_word_back(&mut self) {
        if let Some((s, e)) = self.selection() {
            self.splice(s, e, "", EditKind::Other);
        } else {
            // A word back from just past a picture is a word *of its markup*, and
            // a word back from in front of one runs through the paragraph break
            // into the prose above — dissolving the picture either way. See
            // `delete_around_block_media`.
            if self.view != View::Source && self.delete_around_block_media(false) {
                return;
            }
            let start = self.word_left_from(self.caret).max(self.caret_floor());
            if start < self.caret {
                let (s, e) = self.widen_over_emptied_inlines(start, self.caret);
                self.splice(s, e, "", EditKind::Delete);
            }
        }
    }

    /// Delete from the caret forward to the end of the next word (⌥⌦ /
    /// Ctrl+Del). Deletes the selection instead when one is active.
    pub fn delete_word_forward(&mut self) {
        if let Some((s, e)) = self.selection() {
            self.splice(s, e, "", EditKind::Other);
        } else {
            // The mirror: a word forward from in front of a picture is its markup.
            if self.view != View::Source && self.delete_around_block_media(true) {
                return;
            }
            let end = self.word_right_from(self.caret);
            if end > self.caret {
                let (s, e) = self.widen_over_emptied_inlines(self.caret, end);
                self.splice(s, e, "", EditKind::Delete);
            }
        }
    }

    /// Delete from the caret back to the start of its line (⌘⌫). Deletes the
    /// selection instead when one is active, as every other delete here does.
    ///
    /// The line is the view's own — the one Home and End work on, so in WYSIWYG
    /// a soft-wrapped row is a line. It is not Home's *target*, though: Home
    /// stops at the first character and this takes the indentation with it, the
    /// way Cocoa's `deleteToBeginningOfLine:` does. Stopping at the text would
    /// leave an indent behind that nothing can then ask to delete, where a caret
    /// left at column 0 is one press of Home away from either.
    pub fn delete_to_line_start(&mut self) {
        if let Some((s, e)) = self.selection() {
            self.splice(s, e, "", EditKind::Other);
            return;
        }
        // Never back across the floor: hidden frontmatter isn't on this line, or
        // on any line the WYSIWYG caret can see.
        let (start, _) = self.line_span();
        let start = start.max(self.caret_floor());
        if start < self.caret {
            let (s, e) = self.widen_over_emptied_inlines(start, self.caret);
            self.splice(s, e, "", EditKind::Delete);
        }
    }

    /// Kill from the caret to the end of its line (^K). Deletes the selection
    /// instead when one is active.
    ///
    /// At the end of the line it does nothing, rather than pulling the line
    /// below up into this one. Joining has no meaning to give it in both views
    /// at once: a WYSIWYG line ends at a soft wrap as often as at a newline, and
    /// there is nothing there to delete, while the newline a *source* line ends
    /// with is only half of the blank line that separates two paragraphs —
    /// deleting one leaves a soft break, which is not the join it looks like.
    /// The views agreeing is worth more than emacs' second press, and Delete is
    /// already the key that joins.
    pub fn delete_to_line_end(&mut self) {
        if let Some((s, e)) = self.selection() {
            self.splice(s, e, "", EditKind::Other);
            return;
        }
        let (_, end) = self.line_span();
        if end > self.caret {
            let (s, e) = self.widen_over_emptied_inlines(self.caret, end);
            self.splice(s, e, "", EditKind::Delete);
        }
    }

    /// Grow a WYSIWYG word-delete to swallow any inline node it empties.
    ///
    /// A glyph-space range covers what the user can see, which for `**bold**` is
    /// the word and never the delimiters around it — so deleting the word on its
    /// own leaves `a **** c`, markup wrapped around nothing. They asked for the
    /// word, and the styling was the word's; the two go together. Only the
    /// node's delimiters are taken, and those are hidden here anyway, so nothing
    /// visible outside the range is lost.
    ///
    /// Repeated to a fixed point: emptying `***bold***` empties the emph inside
    /// the strong, and only then is the strong empty too.
    fn widen_over_emptied_inlines(&mut self, start: usize, end: usize) -> (usize, usize) {
        if self.view == View::Source {
            return (start, end);
        }
        let nodes = self.nodes();
        let (mut s, mut e) = (start, end);
        loop {
            let mut grew = false;
            for n in nodes.iter().filter(|n| wysiwyg::is_inline(n)) {
                let Some(text) = inline_content_span(n, &self.source) else {
                    continue;
                };
                // Some of its text survives, so the node still has a job.
                if text.start < s || text.end > e {
                    continue;
                }
                if n.span.start < s || n.span.end > e {
                    s = s.min(n.span.start);
                    e = e.max(n.span.end);
                    grew = true;
                }
            }
            if !grew {
                return (s, e);
            }
        }
    }

    /// One splice of document text, keeping the **mark-edge rule**: an inline
    /// mark's content never begins or ends with whitespace. In Markdown and Djot
    /// a delimiter standing against a space is not a delimiter at all — `**bold **`
    /// is four literal asterisks around a word, and a rich view drawing the
    /// document faithfully has no choice but to show them. That is correct
    /// rendering of what the file says, and nobody typing a space after a bold
    /// word meant to say it.
    ///
    /// So the space goes *outside* the run instead — `**bold** ` — which is the
    /// same document to a reader and a live one to a parser. The caret follows it
    /// out and keeps the marks armed (see [`rearm`](Self::rearm)), so the next
    /// character rejoins the run (see [`rejoin_run`](Self::rejoin_run)) and the
    /// writer sees one unbroken bold phrase, never a flash of raw syntax.
    ///
    /// Every ordinary edit — typing, deleting, pasting, an IME step — comes
    /// through here, so the rule holds however the whitespace arrives at the
    /// edge. The repair is decided *after* the plain edit, by asking whether the
    /// mark actually died: a code span's backticks aren't whitespace-sensitive
    /// (`` `code ` `` is still code), and nothing is re-spelled when nothing broke.
    fn splice(&mut self, start: usize, end: usize, text: &str, kind: EditKind) -> bool {
        let fix = self.mark_edge_fix(start, end, text);
        if !self.splice_exact(start, end, text, kind) {
            return false;
        }
        if let Some(fix) = fix {
            self.repair_mark_edges(fix);
        }
        if text.is_empty() && end > start {
            self.settle_inside_close_delims();
        }
        true
    }

    /// After a delete, take a caret left standing past a run's closing delimiters
    /// back inside the run.
    ///
    /// A delete leaves the caret where the deleted bytes began, and when those
    /// bytes were the last thing after a marked phrase — the space the mark-edge
    /// rule pushed out of `**bold** `, say — that spot is the far side of the
    /// closing `**`. The rich view has nothing to draw there: the delimiters are
    /// hidden, so the caret shows at the end of the word either way, and the two
    /// offsets are one place on screen with two different meanings. Typing at the
    /// outer one lands past the run, so the writer who backspaced a space out of
    /// their bold phrase watches the next character come out plain, and the
    /// toolbar button go dark, with the caret never appearing to move.
    ///
    /// The end of the run's text is the caret's home there — a delete that took
    /// away everything after a phrase leaves the caret at the end of that phrase,
    /// which is inside it — so it settles onto that
    /// ([`step_inside_close_delims`](Self::step_inside_close_delims) does the
    /// walk, through every mark closing at the point): the word stays bold, the
    /// button stays lit, and the next character carries on the phrase.
    ///
    /// Rich view only, and only where a mark really closes at the caret — mid-run
    /// or in plain prose no span ends there and the caret stays put. The opening
    /// edge is left alone on purpose: a caret in front of a run inherits from the
    /// text on its left, which is the plain text outside.
    fn settle_inside_close_delims(&mut self) {
        if self.view != View::Wysiwyg {
            return;
        }
        let at = self.step_inside_close_delims(self.caret);
        if at != self.caret {
            self.caret = at;
            self.clear_pending();
            self.record_caret();
        }
    }

    /// The splice exactly as asked, with no mark-edge repair — for the callers
    /// that are *writing* the delimiters themselves ([`insert_with_marks`](Self::insert_with_marks)
    /// and [`rejoin_run`](Self::rejoin_run)) and place their own offsets around
    /// the bytes they inserted.
    ///
    /// One `edit_range` through twig, then re-anchor the caret from the returned
    /// `Change` and refresh the cached source. A reparse-breaking edit (rare for
    /// Markdown/Djot) leaves the document untouched and reports.
    ///
    /// Returns whether the edit landed — for a caller that has offsets of its
    /// own to place afterwards, which a rolled-back splice would leave pointing
    /// into text that never came to exist.
    fn splice_exact(&mut self, start: usize, end: usize, text: &str, kind: EditKind) -> bool {
        // The read-only gate, for every edit at once — see the field.
        if self.read_only {
            return false;
        }
        // twig records an undo step for every edit; when this one continues a
        // run of the same kind (typing, deleting), tell twig to fold it into the
        // step before it so the whole run undoes at once.
        let coalesce = kind != EditKind::Other && self.last_edit_kind == Some(kind);
        // Hand twig the pre-edit caret before the splice, so the undo step it
        // retires carries where the caret was standing.
        self.record_caret();
        match self.editor.edit_range(start, end, text) {
            Ok(change) => {
                self.last_edit_kind = Some(kind);
                // Counted first, so the fold has the step it folds.
                self.refresh();
                if coalesce {
                    self.coalesce_last_undo();
                }
                self.caret = change.new.end;
                self.anchor = None;
                self.goal_col = None;
                self.clear_pending();
                self.dirty = self.source != self.clean_source;
                self.status = None;
                // And the post-edit caret, so a later redo restores it.
                self.record_caret();
                true
            }
            // The edit was rolled back, so twig's history did not move and
            // neither may ours: pushing here would leave a step with no edit
            // under it and shift every later undo onto the wrong caret.
            Err(e) => {
                self.status = Some(format!("edit: {e}"));
                false
            }
        }
    }

    /// The re-spelling that would keep the mark-edge rule for the edit
    /// `[start, end)` → `text`, or `None` when the edit leaves no whitespace
    /// against a delimiter and the plain splice is already right. Computed
    /// *before* the edit, while the run's spans and delimiters can still be read
    /// off the document; applied afterwards, and only if the mark really died —
    /// see [`repair_mark_edges`](Self::repair_mark_edges).
    ///
    /// Rich view only. Source view is for typing raw markup, where a space put
    /// against a `**` is exactly the character it looks like.
    fn mark_edge_fix(&mut self, start: usize, end: usize, text: &str) -> Option<MarkEdgeFix> {
        if self.view != View::Wysiwyg || start > end || end > self.source.len() {
            return None;
        }
        // Every inline mark standing over the edit, outermost first, with the
        // content span that says where its delimiters are.
        let chain: Vec<(InlineKind, std::ops::Range<usize>, std::ops::Range<usize>)> = self
            .editor
            .ancestors_at(start)
            .unwrap_or_default()
            .into_iter()
            .filter_map(|m| {
                let kind = inline_kind(&m.kind)?;
                let content = m.content_span.clone()?;
                Some((kind, m.span.clone(), content))
            })
            .collect();
        // The innermost run whose *content* holds the whole edit: the one whose
        // text is being changed, rather than one the edit merely sits under.
        let (kind, span, content) = chain
            .iter()
            .rev()
            .find(|(_, _, c)| c.start <= start && end <= c.end)?
            .clone();
        // What that content becomes. Whitespace at either end of it is what
        // would put out the mark.
        let body = format!(
            "{}{text}{}",
            &self.source[content.start..start],
            &self.source[end..content.end]
        );
        let (lead, trail) = if body.trim().is_empty() {
            // Nothing but whitespace left: there is no content to mark at all,
            // and the delimiters go with it rather than closing on a space.
            (body.len(), 0)
        } else {
            (
                body.len() - body.trim_start().len(),
                body.len() - body.trim_end().len(),
            )
        };
        // Nothing against a delimiter, and something still between them: the
        // plain edit stands. An emptied run is broken just as surely (`**b**`
        // with the `b` deleted is the literal `****`) and is re-spelt as the
        // nothing it now says.
        if lead == 0 && trail == 0 && !body.is_empty() {
            return None;
        }
        // Marks that open or close exactly where this one does — `***both***` is
        // two runs sharing an edge — spell their delimiters as one run of bytes,
        // so the whitespace has to clear all of them together.
        let (mut open_at, mut close_at) = (span.start, span.end);
        for _ in 0..chain.len() {
            match chain.iter().find(|(_, _, c)| c.start == open_at) {
                Some((_, s, _)) => open_at = s.start,
                None => break,
            }
        }
        for _ in 0..chain.len() {
            match chain.iter().find(|(_, _, c)| c.end == close_at) {
                Some((_, s, _)) => close_at = s.end,
                None => break,
            }
        }
        let open = &self.source[open_at..content.start];
        let close = &self.source[content.end..close_at];
        let core = &body[lead..body.len() - trail];
        let respelt = if core.is_empty() {
            body.clone()
        } else {
            format!(
                "{}{open}{core}{close}{}",
                &body[..lead],
                &body[body.len() - trail..]
            )
        };
        // The caret sits just past the inserted text within the new content —
        // which, when that lands in the whitespace, is now outside the delimiters.
        let pos = (start - content.start) + text.len();
        let caret = if core.is_empty() || pos <= lead {
            open_at + pos
        } else if pos >= lead + core.len() {
            open_at + lead + open.len() + core.len() + close.len() + (pos - lead - core.len())
        } else {
            open_at + lead + open.len() + (pos - lead)
        };
        Some(MarkEdgeFix {
            kind,
            probe: content.start,
            start: open_at,
            end: close_at + text.len() - (end - start),
            text: respelt,
            caret,
            // The marks in force here, resolved against any armed sticky delta —
            // what the writer is typing in, and so what has to still be true on
            // the far side of the delimiter the caret just stepped over.
            want: chain
                .iter()
                .filter(|(_, s, _)| start < s.end)
                .map(|(k, _, _)| *k)
                .collect::<InlineMarks>()
                .xor(self.pending_here()),
        })
    }

    /// Apply a [`MarkEdgeFix`] — but only if the edit it was computed for really
    /// did break the mark. Whether whitespace at a delimiter is fatal is the
    /// format's business, not leaf's: `**bold **` is no longer strong, while
    /// `` `code ` `` is still perfectly good verbatim, and Djot's braced spellings
    /// don't care either. Asking the parser afterwards settles it for every kind
    /// and format at once, and costs a re-spelling only where one is due.
    ///
    /// The repair rides along with the edit that caused it — one undo step puts
    /// back what the writer typed, not a delimiter shuffle they never saw.
    fn repair_mark_edges(&mut self, fix: MarkEdgeFix) {
        if fix.end > self.source.len() {
            return;
        }
        if self.marks_at(fix.probe).iter().any(|(k, _)| *k == fix.kind) {
            return; // still a mark: these delimiters don't mind the whitespace
        }
        let resumed = self.last_edit_kind;
        if !self.splice_exact(fix.start, fix.end, &fix.text, EditKind::Other) {
            return;
        }
        self.coalesce_last_undo();
        // The keystroke owns the undo step, so the run of typing it belongs to
        // keeps coalescing over the repair rather than breaking in two here.
        self.last_edit_kind = resumed;
        self.caret = fix.caret.min(self.source.len());
        self.anchor = None;
        self.goal_col = None;
        self.rearm(fix.want);
        self.clamp_caret();
        self.record_caret();
    }

    /// Arm whatever sticky delta reproduces `want` at the caret — the marks the
    /// writer is typing in, carried across an edit that moved the caret out of
    /// the run holding them. Arms nothing when the caret already stands in
    /// exactly those marks, but still remembers the spot, so a further ⌘b starts
    /// a clean delta here (see [`toggle`](Self::toggle)).
    fn rearm(&mut self, want: InlineMarks) {
        let here: InlineMarks = self
            .marks_at(self.caret)
            .into_iter()
            .map(|(k, _)| k)
            .collect();
        self.pending_marks = want.xor(here);
        self.pending_at = Some(self.caret);
    }

    /// Insert `text` at `at` as a *literal* run via twig's `insert_literal`,
    /// which backslash-escapes any character that would otherwise open markup in
    /// this format and position (`*` → `\*`, a line-start `#` → `\#`). The mirror
    /// of [`splice`](Self::splice) for the Hidden reveal mode's typing path, with
    /// the same caret re-anchor, coalescing, and rollback contract. `at` must be
    /// a collapsed point — a selection is deleted by the caller first, since
    /// `insert_literal` inserts rather than replaces.
    fn insert_literal_at(
        &mut self,
        at: usize,
        text: &str,
        kind: EditKind,
        force_coalesce: bool,
    ) -> bool {
        // The read-only gate: this door goes to twig directly, not through
        // `splice_exact`, so it guards itself — see the field.
        if self.read_only {
            return false;
        }
        // `force_coalesce` folds this into the immediately preceding edit (the
        // selection-delete of an overwrite) so the pair is one undo step; else it
        // coalesces only when it continues a run of the same-kind typing.
        let coalesce =
            force_coalesce || (kind != EditKind::Other && self.last_edit_kind == Some(kind));
        // The mark-edge rule holds for typed text however it is spelled — see
        // `splice`. Only an insert twig passed through unchanged can use it,
        // since a fix is measured in the bytes that actually land, and an escape
        // adds bytes this couldn't have counted.
        let fix = self.mark_edge_fix(at, at, text);
        #[cfg(test)]
        if std::mem::take(&mut self.refuse_next_literal) {
            self.status = Some("edit: refused".into());
            return false;
        }
        self.record_caret();
        match self.editor.insert_literal(at, text) {
            Ok(change) => {
                self.last_edit_kind = Some(kind);
                // Counted first, so the fold has the step it folds.
                self.refresh();
                if coalesce {
                    self.coalesce_last_undo();
                }
                self.caret = change.new.end;
                self.anchor = None;
                self.goal_col = None;
                self.clear_pending();
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.record_caret();
                if let Some(fix) = fix.filter(|_| change.new.end - change.new.start == text.len()) {
                    self.repair_mark_edges(fix);
                }
                true
            }
            Err(e) => {
                self.status = Some(format!("edit: {e}"));
                false
            }
        }
    }

    /// After a structural list edit (a new item, a nest/unnest), renumber the
    /// ordered list the caret sits in so its source markers run `1, 2, 3, …`
    /// again — a raw splice leaves them stale (`1. 2. 2. 3.`). twig does the
    /// renumber as its own edit; fold it into the edit that triggered it so the
    /// two undo as one, and only when it actually changed the source (a no-op or
    /// a caret outside any ordered list must not coalesce the real edit into the
    /// step before it).
    fn renumber_here(&mut self) {
        self.renumber_at(self.caret);
    }

    /// [`renumber_here`](Self::renumber_here) aimed somewhere other than the
    /// caret — for an edit that leaves the caret one past the item it just wrote,
    /// where twig resolves no list to renumber.
    fn renumber_at(&mut self, off: usize) {
        // The read-only gate — this door reaches twig without the splice.
        if self.read_only {
            return;
        }
        let before = self.source.clone();
        if self.editor.renumber_ordered_lists(off).is_err() {
            return; // not inside an ordered list — nothing to renumber
        }
        self.refresh();
        if self.source != before {
            self.coalesce_last_undo();
            self.dirty = self.source != self.clean_source;
            self.clamp_caret();
            self.record_caret();
        }
    }

    /// Repair the one trap a list edit can spring on itself. An *empty* `-`
    /// sub-item written directly beneath a text line reparses that text as a
    /// setext heading — `- hello\n  - ` is `<h2>hello</h2>`, because a lone `-`
    /// is also a setext-H2 underline (twig is right; pandoc agrees). `*` and `+`
    /// bullets can't underline anything, so swap the dash for a `*`: the item
    /// stays an empty nested bullet, the parent stays prose, and the source
    /// round-trips instead of hiding a heading the user never asked for. Folded
    /// into the triggering edit's undo step, the way renumbering is.
    ///
    /// Gated on the collapse having actually happened (the swapped dash was
    /// swallowed into a `heading`), so a real setext heading the author wrote —
    /// or a `- x` with content, which can't underline anything — is never
    /// touched. This has to live in the *edit*, not the renderer: leaving the
    /// hazardous bytes on disk and only painting over them would ship a file
    /// every other CommonMark tool reads as a heading.
    ///
    /// This one keeps its own byte scan, and has to: the hazard is precisely
    /// that the dash stopped being a list marker, so [`list_marker_on_line`] —
    /// which asks twig which lines open an item — reports nothing here. There is
    /// no node to ask about. It is also the last Markdown spelling leaf writes on
    /// purpose rather than for want of an answer; once twig spells continuations
    /// itself, avoiding the trap becomes twig's, and this goes.
    ///
    /// [`list_marker_on_line`]: Self::list_marker_on_line
    fn avoid_setext_collapse(&mut self) {
        let caret = self.caret.min(self.source.len());
        let line_start = self.source[..caret].rfind('\n').map_or(0, |i| i + 1);
        let bytes = self.source.as_bytes();
        let mut dash = line_start;
        while matches!(bytes.get(dash), Some(b' ' | b'\t')) {
            dash += 1;
        }
        // A dash bullet is the only marker that doubles as a setext underline.
        if bytes.get(dash) != Some(&b'-') {
            return;
        }
        // Only an *empty* item is a bare underline; `- x` carries content and
        // can't fold the line above into a heading.
        let line_end = self.source[dash..]
            .find('\n')
            .map_or(self.source.len(), |i| dash + i);
        if !self.source[dash + 1..line_end].trim().is_empty() {
            return;
        }
        // The tell: that dash was swallowed into a `heading`. A properly nested
        // empty item sits under a `list_item`, with no heading in reach. Probe
        // the dash byte itself (well inside the heading), not the caret, whose
        // end-of-line offset can fall on the half-open span boundary.
        let collapsed = self
            .editor
            .ancestors_at(dash)
            .map(|c| c.into_iter().any(|m| m.kind == Kind::Heading))
            .unwrap_or(false);
        if !collapsed {
            return;
        }
        let caret = self.caret;
        if self.splice(dash, dash + 1, "*", EditKind::Other) {
            // Same width, so the caret keeps its column; fold into the edit that
            // triggered this so Tab stays one undo step.
            self.coalesce_last_undo();
            self.caret = caret.min(self.source.len());
            self.clamp_caret();
            self.record_caret();
        }
    }

    fn snapshot(&self) -> CaretState {
        CaretState {
            caret: self.caret,
            anchor: self.anchor,
        }
    }

    /// Hand twig the current caret and selection as the blob for the live
    /// document state. Called before an edit — so the step twig retires records
    /// where the caret was, and undo can restore it — and again once the op has
    /// placed the caret, so redo restores where the edit left it.
    ///
    /// This is the whole of leaf's undo-caret bookkeeping now. twig carries the
    /// caret through its own history, so coalescing falls out for free (folding
    /// two twig steps into one drops the intermediate blob, keeping the run's
    /// first) and the parallel stacks that had to march in lockstep — and could
    /// silently drift out of it — are gone.
    fn record_caret(&mut self) {
        let _ = self.editor.set_caret_blob(&self.snapshot().to_blob());
    }

    /// Toggle an inline mark over the selection (Bold / Italic / Code / …). Keeps
    /// the toggled region selected so a second press cleanly reverses it.
    pub fn toggle(&mut self, kind: InlineKind) {
        // The read-only gate — this door reaches twig without the splice.
        if self.read_only {
            return;
        }
        // Ahead of the no-selection branch below: arming a mark for text not yet
        // typed is a promise `insert` cannot keep in a format with no delimiters
        // to spell it with. Per *kind*, not per format — Markdown spells five
        // of the eight marks (highlight among them, under the `highlight`
        // extension leaf parses with), djot all eight, HTML seven.
        if self.refuse_unsupported(&format!("{kind:?}"), Gesture::ToggleInline(kind)) {
            return;
        }
        let Some((s, e)) = self.selection() else {
            // No selection: arm the mark for the next text typed here, the way a
            // word processor does. `⌘b`, type, `⌘b` again toggles bold on and off
            // in the flow of typing without ever selecting anything — the delta
            // is realised onto the freshly typed text by `insert`. A fresh caret
            // position starts the delta over from the marks actually in force.
            if self.pending_at != Some(self.caret) {
                self.pending_marks = InlineMarks::empty();
                self.pending_at = Some(self.caret);
            }
            self.pending_marks.flip(kind);
            self.status = None;
            return;
        };
        // Whitespace at the edge of a selection is not part of what was chosen —
        // a double-click takes the space after the word with it — and a mark
        // cannot close against one anyway: `**word **` is four literal asterisks
        // (the mark-edge rule, see `splice`). Mark the words, leave the spaces.
        let picked = &self.source[s..e];
        let (s, e) = (
            s + (picked.len() - picked.trim_start().len()),
            e - (picked.len() - picked.trim_end().len()),
        );
        if s >= e {
            self.status = Some(format!("{kind:?}: nothing selected to mark"));
            return;
        }
        // Styling a selection is a one-shot act, not a sticky mode.
        self.clear_pending();
        self.record_caret();
        match self.editor.toggle_inline(s, e, kind) {
            Ok(change) => {
                self.last_edit_kind = None; // structural edit is its own undo step
                self.refresh();
                self.anchor = Some(change.new.start);
                self.caret = change.new.end;
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.record_caret();
            }
            Err(e) => self.status = Some(format!("{kind:?}: {e}")),
        }
    }

    /// Whether the caret stands in a highlight — what a frontend asks to enable
    /// or disable its highlight-colour controls, the way
    /// [`caret_in_table`](Self::caret_in_table) gates the grid ones.
    ///
    /// A fact about the *caret*, and the other half of
    /// [`Capabilities::mark_color`], which is the fact about the format. A
    /// frontend needs both: djot spells a highlight and no colour for it, so a
    /// caret standing in `{=word=}` answers `true` here and still has no palette
    /// to offer.
    ///
    /// The rule is [`active_inline_marks`](Self::active_inline_marks)' rule, so
    /// the palette appears exactly where the Highlight button is lit — with one
    /// deliberate exception: a mark *armed* at a bare caret and not yet typed
    /// into lights the button and answers `false` here, because there is no node
    /// to colour until the text exists.
    pub fn caret_in_mark(&mut self) -> bool {
        self.mark_offset().is_some()
    }

    /// The offset [`set_mark_color`](Self::set_mark_color) speaks for — the one
    /// standing in the highlight the gesture means — or `None` when neither end
    /// of what is selected is in one.
    ///
    /// The caret first, and the selection's *start* after it, because of what
    /// [`toggle`](Self::toggle) leaves behind: a fresh `==word==` is selected
    /// whole, with the caret at its far edge, one past the closing `==` and so
    /// (by `marks_at`' half-open rule) not in the mark at all. Highlight a word
    /// and colour it — the two presses a coloured highlight is made of — would
    /// otherwise refuse on the second, having just written the highlight the
    /// author is pointing at.
    fn mark_offset(&mut self) -> Option<usize> {
        let in_mark = |d: &mut Self, off: usize| {
            d.marks_at(off)
                .into_iter()
                .any(|(k, _)| k == InlineKind::Mark)
                .then_some(off)
        };
        let caret = self.caret.min(self.source.len());
        in_mark(self, caret).or_else(|| {
            let start = self.selection()?.0;
            in_mark(self, start)
        })
    }

    /// The colour of the highlight at the caret — `None` both when the caret is
    /// in no highlight and when the highlight it is in names no colour, which
    /// are the same answer to "which swatch is lit".
    ///
    /// The innermost mark, by span, for the same reason
    /// [`current_heading_level`](Self::current_heading_level) walks the tree:
    /// what the caret is *in* is the deepest node containing it. A `data-color`
    /// naming a colour this build has no variant for reads as `None` — the
    /// renderer already draws that as a plain highlight rather than guessing,
    /// and the toolbar agrees with the renderer.
    pub fn mark_color_at_caret(&mut self) -> Option<MarkColor> {
        let at = self.mark_offset()?;
        self.mark_color_at(at)
    }

    /// [`mark_color_at_caret`](Self::mark_color_at_caret) at a given offset —
    /// the innermost `mark` covering it, and the colour it names.
    fn mark_color_at(&mut self, off: usize) -> Option<MarkColor> {
        self.nodes()
            .into_iter()
            .filter(|n| n.kind == Kind::Mark)
            .filter(|n| n.span.start <= off && off < n.span.end)
            .min_by_key(|n| n.span.end - n.span.start)
            .and_then(|n| MarkColor::from_attrs(&n.attrs))
    }

    /// Colour the highlight at the caret, or clear its colour with `None` — the
    /// palette behind a toolbar's Highlight button.
    ///
    /// Markdown only, and the one gesture whose availability is a fact about the
    /// *parse extensions* rather than about the format alone: the colour is
    /// spelled `==🔴 text==`, an emoji twig reads back out of the content and
    /// records as the mark's `data-color`, and only an editor parsing with
    /// `highlight_colors` (which [`parse_extensions`] turns on for every leaf
    /// document) reads it back that way. Djot spells the highlight and no colour
    /// for it, so this refuses there — see [`Capabilities::mark_color`].
    ///
    /// **A colour is a property of a highlight that already exists.** There is
    /// no "highlight this in red" here, because that is two splices and would be
    /// two undo steps under one press; a frontend that wants it calls
    /// [`toggle`](Self::toggle) with [`InlineKind::Mark`] first, which is the
    /// order the two buttons already sit in. With no highlight at the caret this
    /// says so in the status line and writes nothing.
    ///
    /// The caret keeps its place in the *text*: the splice is entirely in the
    /// prefix between the opening `==` and the first word, so an offset past it
    /// rides the emoji's width, and one standing on the prefix itself lands
    /// where the prefix now ends.
    pub fn set_mark_color(&mut self, color: Option<MarkColor>) {
        // The read-only gate — this door reaches twig without the splice.
        if self.read_only {
            return;
        }
        if self.refuse_unsupported("highlight colour", Gesture::SetMarkColor) {
            return;
        }
        let Some(at) = self.mark_offset() else {
            self.status = Some("highlight colour: no highlight at the caret".into());
            return;
        };
        // Clearing a colour a highlight hasn't got is twig's one *successful*
        // no-op, and the `Change` it hands back then describes whatever edit came
        // before it — a stale span that would drag the caret somewhere it never
        // was. Answer it here, where the question is cheap, rather than trusting
        // a change that isn't one.
        if color.is_none() && self.mark_color_at(at).is_none() {
            self.status = None;
            return;
        }
        self.record_caret();
        match self.editor.set_mark_color(at, color.map(twig_mark_color)) {
            Ok(change) => {
                // Re-anchored from the offsets as they were, *before* `refresh`
                // sees the new bytes: the caret it clamps is one standing inside
                // a prefix that didn't exist a moment ago, and walking it back to
                // a char boundary of the emoji loses the place this is restoring.
                let caret = reanchor(self.caret, &change);
                let anchor = self.anchor.map(|a| reanchor(a, &change));
                self.last_edit_kind = None; // structural edit is its own undo step
                self.refresh();
                self.caret = caret;
                self.anchor = anchor;
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.clamp_caret();
                self.record_caret();
            }
            Err(e) => self.status = Some(format!("highlight colour: {e}")),
        }
    }

    /// One press of a colour swatch: colour the highlight at the caret, or —
    /// over a selection that isn't highlighted yet — highlight it and colour it,
    /// as **one** undo step.
    ///
    /// [`set_mark_color`](Self::set_mark_color) is the exact gesture and stays
    /// one splice; this is the compound every toolbar actually presses, and it
    /// lives here rather than in each frontend because the rule it encodes —
    /// what a swatch means when there is no highlight under it yet — is one
    /// answer, not one per frontend. The two splices are folded into a single
    /// history step, so the press that made a red highlight is taken back by a
    /// single undo rather than leaving an uncoloured one behind.
    ///
    /// `None` clears the colour, and over an unhighlighted selection means
    /// simply "highlight this" — the same thing the Highlight button does.
    /// A bare caret in no highlight is left alone with a status line, because
    /// [`toggle`](Self::toggle) there arms a mark for text not yet typed and a
    /// colour cannot be armed with it.
    pub fn highlight(&mut self, color: Option<MarkColor>) {
        if self.caret_in_mark() || self.selection().is_none() {
            self.set_mark_color(color);
            return;
        }
        self.toggle(InlineKind::Mark);
        // The format may not spell a highlight at all (`toggle` said so), and
        // there is nothing to colour if it doesn't.
        if self.status.is_some() {
            return;
        }
        let before = self.revision;
        self.set_mark_color(color);
        // Only fold when the colour really spliced. `highlight(None)` over a
        // fresh highlight is a no-op by design, and coalescing there would eat
        // the *previous* edit into the toggle instead.
        if self.revision != before {
            self.coalesce_last_undo();
        }
    }

    // ── the presentation vocabulary ─────────────────────────────────────────
    //
    // Six gestures and five queries over twig's two attribute ops. Each gesture
    // edits **one key and keeps the rest**: it reads the node's attributes,
    // removes its own key (and, for alignment, its own tokens out of `class`),
    // adds the new value or nothing, and passes the list back whole — twig's
    // contract is replace-not-merge, so the read is the caller's job. A
    // paragraph that came in as `class="lead center" id="intro"
    // data-line-height="1.5"` and is right-aligned goes out as `class="lead
    // right" id="intro" data-line-height="1.5"`. Nothing leaf did not write is
    // touched, which is what lets a document from elsewhere pass through the
    // editor unharmed.
    //
    // Clearing is the same gesture with `None`: the key goes, and an empty list
    // at the end unwraps the span or the Markdown div, which twig does.

    /// Set — or with `None` clear — the alignment of the block the caret is in.
    ///
    /// A block property, so the gesture is `set_block_attrs` on the caret's
    /// block **whatever is selected**: a line is a block's, and "centre this"
    /// with three words selected means the paragraph, not the words. The
    /// vocabulary is [`Align`], written as `class` tokens; other tokens on the
    /// same `class` are kept.
    ///
    /// In Markdown the attributes live on a `<div>` around the block — twig has
    /// no paragraph attribute syntax to write — and this reads them back off
    /// that div when the block is its sole child, so a second press rewrites
    /// the div rather than nesting a second one.
    pub fn set_alignment(&mut self, align: Option<Align>) {
        let attrs = self.block_attrs_at_caret();
        if align.is_none()
            && self.refuse_clear_from_div("alignment", &attrs, |a| Align::from_attrs(a).is_some())
        {
            return;
        }
        let attrs = with_class_token(
            &attrs,
            |t| Align::from_token(t).is_some(),
            align.map(Align::name),
        );
        self.write_block_attrs("alignment", attrs);
    }

    /// Set — or with `None` clear — the line spacing of the block the caret is
    /// in. [`set_alignment`](Self::set_alignment)'s peer in every respect but
    /// the key: [`LineHeight`] under `data-line-height`, one of the menu's
    /// three names or an exact ratio, written in its canonical spelling.
    pub fn set_line_spacing(&mut self, spacing: Option<LineHeight>) {
        let attrs = self.block_attrs_at_caret();
        if spacing.is_none()
            && self.refuse_clear_from_div("line spacing", &attrs, |a| {
                LineHeight::from_attrs(a).is_some()
            })
        {
            return;
        }
        let spelling = spacing.map(LineHeight::name);
        let attrs = with_attr(&attrs, "data-line-height", spelling.as_deref());
        self.write_block_attrs("line spacing", attrs);
    }

    /// Set — or with `None` clear — the size of the selected run, or of the
    /// caret's whole block when nothing is selected.
    ///
    /// Size, face and colour are the *run's*, and the block's when no run is
    /// chosen. With a selection the gesture is `wrap_range_attrs`, which wraps
    /// the range in an attributed span or re-styles the span it already lies in
    /// (never nesting a second, and unwrapping it when the last key goes). With
    /// no selection it is `set_block_attrs` on the caret's block, so that "make
    /// this paragraph larger" is a click with the caret in it rather than a
    /// select-all first.
    ///
    /// The walker reads the key at both levels with the nearer winning, so a
    /// span's `data-size` inside a block carrying its own applies to the span.
    ///
    /// The vocabulary is [`FontSize`]: one of CSS's seven keywords, which is
    /// what a menu offers first because a step reads as a step up under every
    /// theme, or the point size an author asked for, which is exact and is all
    /// it is. Either is written in its canonical spelling, so a size set twice
    /// from the same field writes the same bytes both times.
    pub fn set_font_size(&mut self, size: Option<FontSize>) {
        let spelling = size.map(FontSize::name);
        self.set_run_attr("size", "data-size", spelling.as_deref());
    }

    /// Set — or with `None` clear — the face of the selected run, or of the
    /// caret's whole block. [`set_font_size`](Self::set_font_size)'s peer, with
    /// [`FontFace`] under `data-font` — one of the four generics, or the family
    /// the author named, which the frontends resolve through the platform's
    /// font registry and fall back to the body face without.
    pub fn set_font_family(&mut self, font: Option<FontFace>) {
        let spelling = font.as_ref().map(FontFace::name);
        self.set_run_attr("font", "data-font", spelling.as_deref());
    }

    /// Set — or with `None` clear — the *text* colour of the selected run, or of
    /// the caret's whole block. [`set_font_size`](Self::set_font_size)'s peer,
    /// with [`TextColor`] under `data-color` — one of the seven names, whose
    /// two inks the theme owns, or the triple the author picked, which is
    /// painted as written in both appearances.
    ///
    /// The same key and the same seven names [`set_mark_color`](Self::set_mark_color)
    /// writes, and a different thing: that one colours a highlight's
    /// *background* and rides the `mark` node twig owns the spelling of, this
    /// one colours the letters and rides an attributed span. The two never
    /// collide, because a `mark` is a `mark` and a span is a span — and they
    /// share a vocabulary on purpose, so that a frontend with a red for a
    /// highlight has a red for text and both are *that* red.
    pub fn set_text_color(&mut self, color: Option<TextColor>) {
        let spelling = color.map(TextColor::name);
        self.set_run_attr("text colour", "data-color", spelling.as_deref());
    }

    /// Insert a page break at the caret — `::page-break`, a leaf directive with
    /// no label and no attributes, which twig spells in every format that names
    /// a leaf container (Markdown under the `directives` extension
    /// [`parse_extensions`] turns on, and djot, where it is an empty `:::
    /// page-break` fence).
    ///
    /// Placed exactly as [`insert_thematic_break`](Self::insert_thematic_break)
    /// places a rule, and for the same reason: a directive is a block, so twig
    /// alone has nowhere to put one mid-paragraph and lands it after the
    /// caret's whole block. A bare paragraph is therefore parted at the caret
    /// first and the break aimed at the *first* half. See that method for the
    /// whole of the rule, including why a code block, a list item, a table and
    /// a setext heading are left unsplit.
    ///
    /// The frontends that paginate read the row's
    /// [`DirectiveMark`](crate::wysiwyg::DirectiveMark) and open a page there;
    /// the ones that do not draw the `⧉ page-break` placeholder every leaf
    /// directive gets.
    pub fn insert_page_break(&mut self) {
        if self.read_only || self.refuse_unsupported("page break", Gesture::InsertDirective) {
            return;
        }
        self.caret = self.skip_trailing_close_delims(self.caret);
        // A selection is replaced by the break, as a rule replaces one.
        if let Some((s, e)) = self.selection() {
            self.splice(s, e, "", EditKind::Other);
        }
        self.anchor = None;
        self.record_caret();
        let at = self.caret;
        // A failure here is not fatal: the break still lands after the
        // block, which is what this call was trying to improve on.
        let parted = self.part_for_block(at);
        match self.editor.insert_directive(at, PAGE_BREAK, None, &[]) {
            Ok(change) => {
                self.last_edit_kind = None;
                self.refresh();
                if parted {
                    self.coalesce_last_undo();
                }
                self.anchor = None;
                self.caret = change.new.end;
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.clamp_caret();
                self.record_caret();
                self.caret_under_written_block(change.new, parted);
            }
            Err(e) => self.status = Some(format!("page break: {e}")),
        }
    }

    /// The alignment in force at the caret, or `None` for the theme's default —
    /// which swatch of an alignment control is lit.
    ///
    /// Read off the nearest node that names one: the block the caret is in, and
    /// the `div`s around it after that. [`mark_color_at_caret`](Self::mark_color_at_caret)'s
    /// shape, one property along.
    pub fn alignment_at_caret(&mut self) -> Option<Align> {
        self.presentation_chain()
            .iter()
            .find_map(|attrs| Align::from_attrs(attrs))
    }

    /// The line spacing in force at the caret, or `None` for the theme's own.
    /// [`alignment_at_caret`](Self::alignment_at_caret)'s peer.
    pub fn line_spacing_at_caret(&mut self) -> Option<LineHeight> {
        self.presentation_chain()
            .iter()
            .find_map(|attrs| LineHeight::from_attrs(attrs))
    }

    /// The size in force at the caret, or `None` for the theme's own — the
    /// entry a size menu shows ticked.
    ///
    /// Run-level, so the chain starts one node deeper: the attributed span the
    /// caret stands in, then its block, then the `div`s around it. The nearest
    /// wins, which is the rule the walker draws by.
    ///
    /// A name or a value, whichever the nearest node wrote. A `data-size` the
    /// grammar does not cover — a `huge` from elsewhere — is not a size this
    /// can answer, so the answer is `None` and the menu ticks *Default*, the
    /// same thing it did before the vocabulary opened.
    pub fn font_size_at_caret(&mut self) -> Option<FontSize> {
        self.presentation_chain()
            .iter()
            .find_map(|attrs| FontSize::from_attrs(attrs))
    }

    /// The face in force at the caret, or `None` for the theme's body face.
    /// [`font_size_at_caret`](Self::font_size_at_caret)'s peer.
    pub fn font_family_at_caret(&mut self) -> Option<FontFace> {
        self.presentation_chain()
            .iter()
            .find_map(|attrs| FontFace::from_attrs(attrs))
    }

    /// The *text* colour in force at the caret, or `None` for the theme's.
    /// [`font_size_at_caret`](Self::font_size_at_caret)'s peer, and not
    /// [`mark_color_at_caret`](Self::mark_color_at_caret) — that one reads a
    /// highlight's background off a `mark`, and a `mark` is never in this chain.
    pub fn text_color_at_caret(&mut self) -> Option<TextColor> {
        self.presentation_chain()
            .iter()
            .find_map(|attrs| TextColor::from_attrs(attrs))
    }

    /// The selection-or-caret half of the three run-level gestures: a span over
    /// a real selection, the caret's block over none.
    fn set_run_attr(&mut self, what: &str, key: &str, value: Option<&str>) {
        match self.selection() {
            Some((start, end)) => {
                let attrs = with_attr(&self.run_attrs_over(start, end), key, value);
                self.write_run_attrs(what, start, end, attrs);
            }
            None => {
                let own = self.block_attrs_at_caret();
                if value.is_none()
                    && self.refuse_clear_from_div(what, &own, |a| a.iter().any(|(k, _)| k == key))
                {
                    return;
                }
                let attrs = with_attr(&own, key, value);
                self.write_block_attrs(what, attrs);
            }
        }
    }

    /// A clear this gesture cannot carry out, said out loud instead of written:
    /// the node it rewrites — the caret's block, or the `<div>` around it that
    /// [`block_attrs_at_caret`](Self::block_attrs_at_caret) folds to in Markdown
    /// — does not name the property at all, and a `div` further out does.
    ///
    /// Handing twig the block's attributes with the key already absent changes
    /// no byte, and the query goes on answering `Some` off the div: the menu
    /// entry the author pressed stays unticked, and nothing says why. Twig's
    /// `set_block_attrs` reaches one node, so leaf cannot clear a key it did not
    /// write on a node it is not rewriting — the honest answer is the status
    /// line, in the voice the other refusals use.
    ///
    /// `names` is the property's own reading of an attribute list, because
    /// alignment lives in a `class` token rather than a key of its own. Spans
    /// are skipped: one inside the block is not what a *block* gesture writes
    /// either, but neither is it "the div around the block", and the run-level
    /// gestures reach it through a selection.
    fn refuse_clear_from_div(
        &mut self,
        what: &str,
        own: &Attrs,
        names: impl Fn(&Attrs) -> bool,
    ) -> bool {
        if names(own) {
            return false;
        }
        let caret = self.caret.min(self.source.len());
        if !self
            .attr_chain_at(caret)
            .iter()
            .any(|(span, attrs)| !span && names(attrs))
        {
            return false;
        }
        self.status = Some(format!("{what}: set on the div around the block"));
        true
    }

    /// Hand `attrs` to twig as the caret's block's whole attribute set, with the
    /// status, undo and caret plumbing [`set_mark_color`](Self::set_mark_color)
    /// has.
    ///
    /// **The caret keeps its place in the text, not its byte offset.** How a
    /// format spells a block's attributes is markup written *around* the block
    /// — djot's `{…}` line above it, a `<div …>` and two blank lines in front of
    /// it in Markdown, a longer opening tag in HTML — and every one of those
    /// grows or shrinks above the author's own bytes. Where twig's change
    /// rewrites the block whole (Markdown's div is spliced as one region, block
    /// included) the plain arithmetic of [`reanchor`] has nothing to shift by
    /// and parks the caret at the end of the splice, past the closing `</div>`:
    /// the caret is then in no block at all, so a second press of the same menu
    /// answers "no block at the caret" and the toolbar's queries read nothing.
    /// [`reanchor_in_block`] is what carries it across instead — the block's
    /// content span before and after, which is the one thing the respelling
    /// leaves alone.
    ///
    /// Read *before* the splice and applied *after* `refresh`, because both
    /// halves of that mapping are facts about a tree twig is between: the
    /// block's old bytes are gone once the edit lands, and its new ones are not
    /// in `self.source` until the refresh puts them there.
    fn write_block_attrs(&mut self, what: &str, attrs: Attrs) {
        if self.read_only || self.refuse_unsupported(what, Gesture::SetBlockAttrs) {
            return;
        }
        // A blank line has no block to carry an attribute, and twig answers
        // `NotFound` there — say so in leaf's own words instead.
        let Some(at) = self.block_offset_for_caret() else {
            self.status = Some(format!("{what}: no block at the caret"));
            return;
        };
        self.record_caret();
        let pairs = attr_pairs(&attrs);
        let was = self.block_content_at(at);
        let text = was.clone().map(|s| self.source[s].to_string());
        match self.editor.set_block_attrs(at, &pairs) {
            Ok(change) => {
                let (caret, anchor) = (self.caret, self.anchor);
                self.last_edit_kind = None; // structural edit is its own undo step
                self.refresh();
                let now = self.block_content_in(&change.new, text.as_deref());
                // A block the two halves cannot both name — a code block, a
                // caret in a list's marker — takes the plain arithmetic, which
                // is what it had before.
                let block = was.as_ref().zip(now.as_ref());
                self.caret = reanchor_in_block(caret, &change, block);
                self.anchor = anchor.map(|a| reanchor_in_block(a, &change, block));
                self.dirty = self.source != self.clean_source;
                self.status = None;
                // The clamp reads the caret floor off the map, and this edit
                // can move the floor: taking the `{…}` line off a djot
                // document's first block moves the first rendered offset to 0,
                // and a floor read from the old map stood the caret past the
                // block's text. So the map is this revision's before the clamp
                // — see `open_paragraph_at_block_edge`.
                self.rebuild_map();
                self.clamp_caret();
                self.record_caret();
            }
            Err(e) => self.status = Some(format!("{what}: {e}")),
        }
    }

    /// The content span of the innermost paragraph or heading covering `off` —
    /// the author's own bytes, without the `# ` or the `<p>` that spells the
    /// block around them.
    ///
    /// The same two kinds [`block_attrs_at_caret`](Self::block_attrs_at_caret)
    /// reads, so that what a gesture re-anchors by is the block it wrote to.
    fn block_content_at(&mut self, off: usize) -> Option<Range<usize>> {
        self.nodes()
            .into_iter()
            .filter(|n| matches!(n.kind, Kind::Para | Kind::Heading))
            .filter(|n| n.span.start <= off && off <= n.span.end)
            .min_by_key(|n| n.span.end - n.span.start)
            .map(|n| n.content_span.unwrap_or(n.span))
    }

    /// [`block_content_at`](Self::block_content_at)'s other half: the content
    /// span of the block `region` holds now, found by the bytes it held before.
    ///
    /// Matched on the text rather than taken as the first block in the region,
    /// because a rewritten region is markup and all — `<div class="center">`
    /// carries words of its own — and because the block this gesture moved is
    /// the one whose content the respelling did not touch. `None` where the
    /// region holds no block at all, which is djot's every case: the `{…}` line
    /// is spliced above the block and the block itself never moves through the
    /// change at all, only past it.
    fn block_content_in(
        &mut self,
        region: &Range<usize>,
        text: Option<&str>,
    ) -> Option<Range<usize>> {
        let text = text?;
        let spans: Vec<Range<usize>> = self
            .nodes()
            .into_iter()
            .filter(|n| matches!(n.kind, Kind::Para | Kind::Heading))
            .filter(|n| region.start <= n.span.start && n.span.end <= region.end)
            .map(|n| n.content_span.unwrap_or(n.span))
            .collect();
        spans
            .into_iter()
            .find(|s| self.source.get(s.clone()) == Some(text))
    }

    /// Hand `attrs` to twig as the attribute set of the span over `[start,
    /// end)` — wrapping one, or re-styling the one the range already lies in,
    /// or unwrapping it when `attrs` is empty.
    ///
    /// What the splice leaves selected is the span's **content** — the author's
    /// words — and not the whole of `change.new`, which is markup and all:
    /// `[big]{data-size="large"}` in djot, `<span …>big</span>` in Markdown. A
    /// selection reaching past the node's own span lies in no span at all, so a
    /// second press of the menu would nest a fresh one instead of re-styling
    /// the one just written.
    fn write_run_attrs(&mut self, what: &str, start: usize, end: usize, attrs: Attrs) {
        if self.read_only || self.refuse_unsupported(what, Gesture::WrapRangeAttrs) {
            return;
        }
        self.record_caret();
        let pairs = attr_pairs(&attrs);
        match self.editor.wrap_range_attrs(start, end, &pairs) {
            Ok(change) => {
                self.last_edit_kind = None;
                self.refresh();
                let content = self.span_content_in(&change.new);
                self.anchor = Some(content.start);
                self.caret = content.end;
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.clamp_caret();
                self.record_caret();
            }
            Err(e) => self.status = Some(format!("{what}: {e}")),
        }
    }

    /// The content range of the attributed span `spliced` now holds — the
    /// outermost one inside it, since that is the one just written — or
    /// `spliced` itself where the splice left no span, which is what an unwrap
    /// leaves behind.
    fn span_content_in(&mut self, spliced: &Range<usize>) -> Range<usize> {
        self.nodes()
            .into_iter()
            .filter(wysiwyg::is_run_span)
            .filter(|n| spliced.start <= n.span.start && n.span.end <= spliced.end)
            .max_by_key(|n| n.span.end - n.span.start)
            .and_then(|n| n.content_span)
            .unwrap_or_else(|| spliced.clone())
    }

    /// The attribute set `set_block_attrs` is about to **replace** at the caret
    /// — which is the block's own, except in Markdown, where twig writes a
    /// block's attributes onto a `<div>` around it and rewrites that div when
    /// the block is its sole child. Reading the paragraph there would hand back
    /// an empty list and quietly drop everything the div said.
    ///
    /// Empty when the caret is in no block at all, which is the same list a
    /// block carrying no attributes gives — and the right one either way, since
    /// the gesture then refuses on its own.
    fn block_attrs_at_caret(&mut self) -> Attrs {
        let Some(off) = self.block_offset_for_caret() else {
            return Vec::new();
        };
        let nodes = self.nodes();
        let Some(block) = nodes
            .iter()
            .filter(|n| matches!(n.kind, Kind::Para | Kind::Heading))
            .filter(|n| n.span.start <= off && off <= n.span.end)
            .min_by_key(|n| n.span.end - n.span.start)
        else {
            return Vec::new();
        };
        if self.format == Format::Markdown
            && let Some(parent) = block.parent.and_then(|p| nodes.iter().find(|n| n.id == p))
            && wysiwyg::element_tag(parent) == Some("div")
            && nodes.iter().filter(|n| n.parent == Some(parent.id)).count() == 1
        {
            return parent.attrs.clone();
        }
        block.attrs.clone()
    }

    /// The attribute set `wrap_range_attrs` is about to **replace** over
    /// `[start, end)` — the innermost attributed span the range lies inside,
    /// which twig re-styles rather than nesting a second one in. Empty when the
    /// range lies in no span, where the gesture mints a fresh one.
    fn run_attrs_over(&mut self, start: usize, end: usize) -> Attrs {
        self.nodes()
            .into_iter()
            .filter(wysiwyg::is_run_span)
            .filter(|n| n.span.start <= start && end <= n.span.end)
            .min_by_key(|n| n.span.end - n.span.start)
            .map(|n| n.attrs)
            .unwrap_or_default()
    }

    /// The attribute lists that bear on a presentation query, **nearest first**:
    /// the attributed spans the caret stands in (innermost first), then its
    /// block, then the `div`s around it. A `find_map` down this is the whole of
    /// each query, and the order is the rule the walker draws by.
    ///
    /// Read at the caret, and at the selection's *start* when the caret stands
    /// in no span there. [`write_run_attrs`](Self::write_run_attrs) leaves the
    /// caret one past the span it just wrote — `toggle`'s convention — so
    /// asking the menu which entry that press just ticked must not answer
    /// `None`. Exactly the reason [`mark_offset`](Self::mark_offset) tries both.
    fn presentation_chain(&mut self) -> Vec<Attrs> {
        let caret = self.caret.min(self.source.len());
        let mut chain = self.attr_chain_at(caret);
        if !chain.iter().any(|(span, _)| *span)
            && let Some((start, _)) = self.selection()
        {
            let alt = self.attr_chain_at(start);
            if alt.iter().any(|(span, _)| *span) {
                chain = alt;
            }
        }
        chain.into_iter().map(|(_, attrs)| attrs).collect()
    }

    /// [`presentation_chain`](Self::presentation_chain) at one offset — every
    /// node bearing the vocabulary that covers it, innermost first, each paired
    /// with whether it is an attributed span (which is what tells the caller
    /// its run-level answer came from a run).
    ///
    /// Sorted by span length, which *is* the nesting order: a span lies inside
    /// its block and a block inside its div, so shortest-first is
    /// nearest-first without a second tree walk.
    fn attr_chain_at(&mut self, off: usize) -> Vec<(bool, Attrs)> {
        let off = off.min(self.source.len());
        let mut hits: Vec<(usize, bool, Attrs)> = Vec::new();
        for n in self.nodes() {
            let span = wysiwyg::is_run_span(&n);
            let block = matches!(n.kind, Kind::Para | Kind::Heading);
            let div = wysiwyg::element_tag(&n) == Some("div");
            if !(span || block || div) {
                continue;
            }
            // A span is half-open, the way a mark is: the offset one past it is
            // the text after it. A block and a div claim their end too, so a
            // caret resting at the end of a line still reads its paragraph.
            let inside = if span {
                n.span.start <= off && off < n.span.end
            } else {
                n.span.start <= off && off <= n.span.end
            };
            if !inside {
                continue;
            }
            hits.push((n.span.end - n.span.start, span, n.attrs));
        }
        hits.sort_by_key(|(len, _, _)| *len);
        hits.into_iter()
            .map(|(_, span, attrs)| (span, attrs))
            .collect()
    }

    /// Convert the block at the caret to a heading level or paragraph.
    pub fn set_block(&mut self, kind: BlockKind) {
        // The read-only gate — this door reaches twig without the splice.
        if self.read_only {
            return;
        }
        if self.refuse_unsupported(&format!("{kind:?}"), Gesture::SetBlock) {
            return;
        }
        self.record_caret();
        // A blank line has no node to convert, and twig opens a block there
        // rather than declining — so the caret's own offset is the right thing
        // to hand it when `block_offset_for_caret` finds nothing.
        let offset = self.block_offset_for_caret().unwrap_or(self.caret);
        match self.editor.set_block(offset, kind) {
            Ok(change) => {
                self.last_edit_kind = None;
                self.refresh();
                // Opening a block on a blank line writes a marker the caret
                // belongs *after*; converting an existing one moves nothing.
                self.caret = self.caret.max(change.new.end);
                self.clamp_caret();
                self.anchor = None;
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.record_caret();
            }
            Err(e) => self.status = Some(format!("{kind:?}: {e}")),
        }
    }

    /// Whether `off` is inside a text block (paragraph, heading, code block…).
    fn has_block_at(&mut self, off: usize) -> bool {
        self.editor.ancestors_at(off).ok().is_some_and(|chain| {
            chain
                .iter()
                .any(|m| !wysiwyg::is_inline_kind(&m.kind) && !is_block_container(&m.kind))
        })
    }

    /// The offset to hand twig's `set_block`: the caret when it is already inside
    /// a block, otherwise nudged onto the previous character (a caret at a line
    /// end sits at the doc level, outside the block). `None` when the caret is on
    /// a blank line — a new paragraph with no block node to convert.
    fn block_offset_for_caret(&mut self) -> Option<usize> {
        let caret = self.caret.min(self.source.len());
        if self.has_block_at(caret) {
            return Some(caret);
        }
        // Nudge to the previous character — but never across a newline: that would
        // target the previous block, and a blank line genuinely has no block.
        if let Some((i, ch)) = self.source[..caret].char_indices().next_back()
            && ch != '\n'
            && self.has_block_at(i)
        {
            return Some(i);
        }
        None
    }

    /// The heading level of the text block at the caret, or `None` when that
    /// block is not a heading.
    pub fn current_heading_level(&mut self) -> Option<u32> {
        let caret = self.caret;
        self.nodes()
            .into_iter()
            .filter(|n| n.kind == Kind::Heading)
            .find(|n| n.span.start <= caret && caret <= n.span.end)
            .and_then(|n| n.level)
    }

    /// The inline marks in force at the caret (or over the selection) — what a
    /// toolbar draws lit, and the block-level [`Doc::current_heading_level`]'s
    /// inline counterpart. Cheap enough to call every frame: one twig
    /// `ancestors_at` query per caret (two with a selection), each walking root
    /// → deepest node at one offset. It never snapshots the tree the way
    /// `current_heading_level` does, and the returned set is a `Copy` bitset, so
    /// the only allocation is twig's own small ancestor `Vec`.
    ///
    /// **A selection reports a mark only when the mark covers *all* of it.**
    /// That's what every real toolbar means by an active button — Bold lit over
    /// a half-bold selection would claim a press turns bold *off*, when
    /// [`Doc::toggle`] hands the range to twig and gets the whole thing bolded.
    /// Whole-coverage is asked as "is the same mark node standing over both the
    /// first and the last character?": inline nodes are contiguous, so one node
    /// covering both ends covers every byte between them. Two touching runs
    /// (`**a****b**`) are two nodes, and correctly light nothing.
    ///
    /// At a bare caret a mark is active when the caret stands inside the mark's
    /// span — `span.start <= caret < span.end`, delimiters included, which is
    /// what makes the boundaries behave. In `a **bold** b` the offsets from the
    /// opening `*` (2) through the last byte of the closing `**` (9) are all
    /// bold, so the WYSIWYG caret both before `b` and after `d` (the delimiters
    /// are hidden, and those offsets are 4 and 8) reports bold — matching where
    /// typing would actually land inside the marked run. The offset one past the
    /// mark (10) is the text after it and reports nothing, at the end of the
    /// buffer exactly as in the middle.
    pub fn active_inline_marks(&mut self) -> InlineMarks {
        let Some((start, end)) = self.selection() else {
            // The marks actually in force at the caret, flipped by any armed
            // sticky delta — so `⌘b` at a bare caret lights the Bold button
            // immediately, before a single character is typed.
            let base: InlineMarks = self
                .marks_at(self.caret)
                .into_iter()
                .map(|(k, _)| k)
                .collect();
            return base.xor(self.pending_here());
        };
        // The selection's *last character*, not its exclusive end: `end` is the
        // offset one past the selection, which for a selection ending exactly at
        // a mark's close is already outside it (`[4,10)` of `a **bold** b` is
        // entirely bold, but offset 10 is the space after).
        let last = prev_boundary(&self.source, end);
        let head = self.marks_at(start);
        let tail = self.marks_at(last);
        head.into_iter()
            .filter(|m| tail.contains(m))
            .map(|(k, _)| k)
            .collect()
    }

    /// The inline marks whose span covers `off`, each with the id of the node
    /// carrying it — the id is what lets a selection tell one mark node from
    /// another of the same kind.
    fn marks_at(&mut self, off: usize) -> Vec<(InlineKind, u32)> {
        let off = off.min(self.source.len());
        self.editor
            .ancestors_at(off)
            .unwrap_or_default()
            .into_iter()
            // `span.end` is the offset one *past* the mark, so it isn't in it.
            // twig already resolves a boundary to whatever starts there — in
            // `**bold** x` offset 8 is the following text, not the strong — but
            // when nothing follows, the tie has nobody to break for and the
            // chain still ends at the mark. That would make the answer at the
            // last offset of the document depend on whether the file happens to
            // end in a newline; the rule is `span.start <= off < span.end`, and
            // it's the same rule at the end of a buffer as in the middle.
            .filter(|m| off < m.span.end)
            .filter_map(|m| inline_kind(&m.kind).map(|k| (k, m.node_id)))
            .collect()
    }

    /// Toggle a heading at the caret: if the block is already this heading level,
    /// revert it to a paragraph; otherwise convert it to this heading level.
    /// This gives the heading commands the same toggle feel as bold/italic/code —
    /// re-applying a heading a line already has turns it back into body text.
    pub fn toggle_heading(&mut self, level: u32) {
        if self.current_heading_level() == Some(level) {
            self.set_block(BlockKind::Paragraph);
        } else {
            self.set_block(BlockKind::Heading(level));
        }
    }

    /// Toggle a block quote around the selection, or around the block at the
    /// caret — the toolbar's Quote button.
    pub fn toggle_blockquote(&mut self) {
        self.toggle_container(BlockContainerKind::BlockQuote);
    }

    /// Toggle a numbered (`ordered`) or bulleted list over the selection, or
    /// over the block at the caret — one op with the kind as a flag, the way
    /// `toggle_heading` takes its level, so a frontend needs no twig type to
    /// name the two buttons.
    ///
    /// Pressing the *other* list's button while in a list converts in place
    /// rather than nesting, so the pair reads as one three-state control
    /// (bulleted / numbered / neither) rather than two independent wrappers.
    pub fn toggle_list(&mut self, ordered: bool) {
        self.toggle_container(if ordered {
            BlockContainerKind::OrderedList
        } else {
            BlockContainerKind::BulletList
        });
    }

    /// Toggle a fenced code block over the selection, or over the block at the
    /// caret — the toolbar's Code Block button, and the only way the rich view
    /// offers to open one: a typed backtick is escaped there, since it is a
    /// character the author wrote and not markup they meant.
    ///
    /// Fencing is twig's ([`Editor::toggle_code_block`]): it measures the fence
    /// against the body, keeps a quote's `> ` on every line, and peels a fence
    /// (or dedents an indented block) on the way back. What is leaf's is the
    /// shape twig declines: a blank line holds no block to fence, and the
    /// gesture nobody should have to learn is "type something first" — so leaf
    /// spells the empty fence itself, with the caret on the empty line inside it
    /// and a blank line either side, which is what typing into a new block and
    /// then Backspacing out of it would have left.
    ///
    /// Inside a list item twig fences at the item's content column and keeps
    /// the marker (a task item's box too) on the opening fence's line, so the
    /// block stays the item's; unfencing puts the marker back on the text.
    /// What twig still refuses — an AsciiDoc item's own first line — is
    /// reported rather than worked around.
    pub fn toggle_code_block(&mut self) {
        // The read-only gate — this door reaches twig without the splice.
        if self.read_only {
            return;
        }
        if self.refuse_unsupported("code block", Gesture::ToggleCodeBlock) {
            return;
        }
        let selected = self.selection();
        // The caret at a code block's rows resolves to its *content*, and twig
        // finds the block from any offset inside its span — but a caret parked
        // on the closing fence's own line end is past it, so the block's start
        // is handed over whenever the caret is in one at all.
        let (start, end) = match selected {
            Some(range) => range,
            None => match self.code_block_start_at_caret() {
                Some(start) => (start, start),
                None => match self.block_offset_for_caret() {
                    Some(off) => (off, off),
                    None => return self.open_empty_code_block(),
                },
            },
        };
        self.record_caret();
        match self.editor.toggle_code_block(start, end, None) {
            Ok(change) => {
                // Read the caret's place out of the *pre-edit* source, before
                // `refresh` swaps it out — and how many lines the region held,
                // which is what says whether a fence line went in above the
                // caret's line or came off it.
                let place = selected
                    .is_none()
                    .then(|| self.caret_line_tail(&change.old));
                let old_lines = self.source[change.old.clone()].matches('\n').count();
                self.last_edit_kind = None; // structural edit is its own undo step
                self.refresh();
                match place {
                    // From a selection: keep the block selected, so a second
                    // press reverses the first (twig unfences from any offset in
                    // the fence's span, the region's start included).
                    None => {
                        self.anchor = Some(change.new.start);
                        self.caret = change.new.end;
                    }
                    // From a caret: the same line, the same distance from its
                    // end, shifted by the opening fence that was written above
                    // it (or peeled off). Dedenting an indented block keeps the
                    // lines one-to-one, and shifts nothing.
                    Some((line, tail)) => {
                        let new_lines = self.source[change.new.start.min(self.source.len())
                            ..change.new.end.min(self.source.len())]
                            .matches('\n')
                            .count();
                        let line = match new_lines.cmp(&old_lines) {
                            std::cmp::Ordering::Greater => line + 1,
                            std::cmp::Ordering::Less => line.saturating_sub(1),
                            std::cmp::Ordering::Equal => line,
                        };
                        self.anchor = None;
                        self.caret = self.line_tail_offset(&change.new, (line, tail));
                    }
                }
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.clamp_caret();
                self.record_caret();
            }
            Err(e) => self.status = Some(format!("code block: {e}")),
        }
    }

    /// Write an empty fenced block on the blank line the caret stands on, and
    /// put the caret on the empty line inside it — the half of
    /// [`toggle_code_block`](Self::toggle_code_block) that is leaf's, because
    /// twig reports `NotFound` for a range no block covers.
    ///
    /// Inside a quote the fence lines and the empty line keep the quote's
    /// prefix, as twig's own fencing does. A blank line goes in on whichever
    /// side has text against it, since a fence may interrupt a paragraph in
    /// Markdown but a block standing tight against its neighbours is not the
    /// document any editor writes.
    fn open_empty_code_block(&mut self) {
        let caret = self.caret.min(self.source.len());
        let prefix = self.quote_prefix_at(caret);
        let line_start = self.source[..caret].rfind('\n').map_or(0, |i| i + 1);
        let line_end = self.source[caret..]
            .find('\n')
            .map_or(self.source.len(), |i| caret + i);
        let prev_blank = line_start == 0
            || self.source[..line_start - 1]
                .rsplit('\n')
                .next()
                .is_some_and(|l| l.trim_start_matches(['>', ' ']).is_empty());
        let next_blank = line_end >= self.source.len()
            || self.source[line_end + 1..]
                .split('\n')
                .next()
                .is_some_and(|l| l.trim_start_matches(['>', ' ']).is_empty());
        // A blank line inside a quote is a bare `>`: the space after it is
        // the content's, and there is none.
        let blank = prefix.trim_end();
        let mut text = String::new();
        if !prev_blank {
            text.push_str(blank);
            text.push('\n');
        }
        text.push_str(&prefix);
        text.push_str("```\n");
        text.push_str(&prefix);
        let inside = text.len();
        text.push('\n');
        text.push_str(&prefix);
        text.push_str("```");
        if !next_blank {
            text.push('\n');
            text.push_str(blank);
        }
        // Over whatever the blank line held (its quote prefix, trailing
        // spaces), so the block's own prefix is the one twig will read back.
        let at = line_start;
        if !self.splice(at, line_end, &text, EditKind::Other) {
            return;
        }
        self.caret = at + inside;
        self.anchor = None;
        self.clamp_caret();
        self.record_caret();
    }

    /// Whether the caret stands in a code block, fenced or indented — what
    /// lights the Code Block button. Wider than
    /// [`caret_in_fenced_code`](Self::caret_in_fenced_code), which asks the
    /// narrower question a language prompt needs.
    pub fn caret_in_code_block(&mut self) -> bool {
        self.code_block_start_at_caret().is_some()
    }

    /// Whether the caret stands anywhere inside a block quote, however deep —
    /// what lights the Block Quote button. A quote is a container rather than
    /// a block kind, so this is true alongside a heading or a code block, not
    /// instead of one.
    pub fn caret_in_blockquote(&mut self) -> bool {
        let off = self.caret.min(self.source.len());
        // A quote's span stops short of its closing newline, so the end of its
        // last line is `span.end` itself, and has to count.
        self.nodes()
            .into_iter()
            .any(|n| n.kind == Kind::BlockQuote && n.span.start <= off && off <= n.span.end)
    }

    // ── Task list items ──────────────────────────────────────────────────────
    // The checkbox in `- [x] done`. twig owns all three gestures: the box is
    // inline content of the item's first paragraph rather than part of its
    // marker, so adding or removing one must leave the item's continuation
    // indentation alone, and an item inside a quote is found past the quote
    // markers. leaf names the gesture and the offset; the spelling is twig's.

    /// Whether the list item at the caret carries a checkbox, and which way it
    /// faces — `Some(true)` ticked, `Some(false)` empty, `None` for a plain list
    /// item or no item at all. What a toolbar reads to light its checkbox button.
    pub fn task_checked_at_caret(&mut self) -> Option<bool> {
        self.task_checked_at(self.caret)
    }

    /// [`task_checked_at_caret`](Self::task_checked_at_caret) for an arbitrary
    /// offset — what a frontend asks before deciding a click landed on a box.
    pub fn task_checked_at(&mut self, offset: usize) -> Option<bool> {
        self.innermost_list_item(offset.min(self.source.len()))?
            .checked
    }

    /// Tick or untick the task item at the caret (the checkbox's keyboard half).
    /// A no-op with a reported reason when the caret is in no task item — minting
    /// a box here is [`toggle_task_item`](Self::toggle_task_item)'s job.
    pub fn toggle_task_checked(&mut self) {
        self.toggle_task_at(self.caret);
    }

    /// Tick or untick the task item covering `offset` — what a *click* on a
    /// rendered checkbox is. Separate from the caret form because a click carries
    /// its own offset and must not first move the caret there: ticking a box
    /// three paragraphs away should not take the cursor with it.
    pub fn toggle_task_at(&mut self, offset: usize) {
        // The read-only gate — this door reaches twig without the splice.
        if self.read_only {
            return;
        }
        if self.refuse_unsupported("task", Gesture::ToggleTaskChecked) {
            return;
        }
        let offset = offset.min(self.source.len());
        self.record_caret();
        match self.editor.toggle_task_checked(offset) {
            Ok(_) => self.after_task_edit(),
            Err(e) => self.status = Some(format!("task: {e}")),
        }
    }

    /// Give the list item at the caret a checkbox, or take its checkbox away —
    /// the gesture that converts between a plain bullet and a task. A new box
    /// arrives unticked.
    ///
    /// Outside a list the caret's block becomes a bullet first, and the box
    /// goes on the new item — one undo step. twig's gesture only converts an
    /// item that is already there, and a Checklist button that did nothing on
    /// a paragraph read as broken.
    pub fn toggle_task_item(&mut self) {
        // The read-only gate — this door reaches twig without the splice.
        if self.read_only {
            return;
        }
        if self.refuse_unsupported("task", Gesture::ToggleTaskItem) {
            return;
        }
        if self
            .innermost_list_item(self.caret.min(self.source.len()))
            .is_none()
        {
            if self.refuse_unsupported(
                "task",
                Gesture::ToggleBlockContainer(BlockContainerKind::BulletList),
            ) {
                return;
            }
            self.begin_undo_group();
            self.toggle_list(false);
            if self
                .innermost_list_item(self.caret.min(self.source.len()))
                .is_some()
            {
                self.box_item_at_caret();
            }
            self.end_undo_group();
            return;
        }
        self.box_item_at_caret();
    }

    /// twig's half of [`toggle_task_item`](Self::toggle_task_item): the box on
    /// the item the caret is already in.
    fn box_item_at_caret(&mut self) {
        let caret = self.caret.min(self.source.len());
        self.record_caret();
        match self.editor.toggle_task_item(caret) {
            Ok(_) => self.after_task_edit(),
            Err(e) => self.status = Some(format!("task: {e}")),
        }
    }

    /// Settle after a task gesture. The caret rides its old byte offset and is
    /// clamped back in: a box is three or four bytes on the item's first line, so
    /// text after it shifts by that much at most, and `clamp_caret` lands it on a
    /// real stop either way.
    fn after_task_edit(&mut self) {
        self.last_edit_kind = None;
        self.refresh();
        self.anchor = None;
        self.dirty = self.source != self.clean_source;
        self.status = None;
        self.clamp_caret();
        self.record_caret();
    }

    // ── Tables ───────────────────────────────────────────────────────────────
    // A table is a grid, and twig edits it as one — add/remove/move a row or
    // column, set a column's alignment — re-spelling the whole table in a single
    // splice. Every gesture is anchored at the caret's cell. leaf just names the
    // gesture and re-reads the result; the whole table's numbering, borders, and
    // delimiter are twig's to keep straight.

    /// Whether the caret is inside a table — what a frontend asks to enable or
    /// disable its table controls.
    ///
    /// An HTML `<table>` still answers `true`: the caret really is in a table,
    /// and the reason the grid controls stay dark there is
    /// [`Capabilities::table`], which is a fact about the document's format
    /// rather than about the caret. A frontend needs both.
    pub fn caret_in_table(&mut self) -> bool {
        let caret = self.caret.min(self.source.len());
        self.editor
            .ancestors_at(caret)
            .map(|c| c.into_iter().any(|m| m.kind == Kind::Table))
            .unwrap_or(false)
    }

    /// One grid op, guarded and settled — the shared body of the seven below.
    ///
    /// The guard is why this exists rather than seven copies of the same three
    /// lines, and it is the one guard leaf cannot delegate to twig. The table
    /// editor is the gesture family that consults no `Syntax` table (it spells a
    /// grid, not a delimiter) and therefore the one twig's `Format::supports`
    /// deliberately has no variant for: handed an HTML `<table>` it rebuilds the
    /// grid as a *pipe table* and reports success, swapping the element out for
    /// `| a | b |` and taking the rest of the document's markup with it. Nothing
    /// downstream could tell that from a successful edit — the splice is real,
    /// the reparse succeeds, `dirty` is honest — which is what makes it worth
    /// stopping at the door rather than detecting after the fact. See
    /// [`spells_pipe_tables`].
    fn table_op(
        &mut self,
        what: &str,
        op: impl FnOnce(&mut Editor, usize) -> Result<(), twig::Error>,
    ) {
        if self.refuse_unless(what, spells_pipe_tables(self.format)) {
            return;
        }
        self.record_caret();
        let at = self.caret;
        let r = op(&mut self.editor, at);
        self.apply_table(r, what);
    }

    /// Insert an empty row below (`below`) or above the caret's row.
    pub fn table_insert_row(&mut self, below: bool) {
        self.table_op("table row", |e, at| e.table_insert_row(at, below));
    }

    /// Delete the caret's row (not the header, not the last body row).
    pub fn table_delete_row(&mut self) {
        self.table_op("table row", |e, at| e.table_delete_row(at));
    }

    /// Insert an empty column right (`right`) or left of the caret's column.
    pub fn table_insert_column(&mut self, right: bool) {
        self.table_op("table column", |e, at| e.table_insert_column(at, right));
    }

    /// Delete the caret's column (unless it is the only one).
    pub fn table_delete_column(&mut self) {
        self.table_op("table column", |e, at| e.table_delete_column(at));
    }

    /// Set the caret's column to `alignment`.
    pub fn table_set_alignment(&mut self, alignment: Alignment) {
        self.table_op("table alignment", |e, at| {
            e.table_set_alignment(at, alignment)
        });
    }

    /// Move the caret's row one place down (`down`) or up, within the body rows.
    pub fn table_move_row(&mut self, down: bool) {
        self.table_op("table row", |e, at| e.table_move_row(at, down));
    }

    /// Move the caret's column one place right (`right`) or left.
    pub fn table_move_column(&mut self, right: bool) {
        self.table_op("table column", |e, at| e.table_move_column(at, right));
    }

    /// Settle the caret and document flags after a table op (or report its
    /// error). twig re-spells the whole table, so the caret rides its old byte
    /// offset and is clamped back into the rebuilt bytes — near enough to where
    /// it was, since the op preserves the cells' content and order around it.
    fn apply_table(&mut self, result: Result<(), twig::Error>, what: &str) {
        match result {
            Ok(()) => {
                self.last_edit_kind = None;
                self.refresh();
                self.anchor = None;
                self.clamp_caret();
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.record_caret();
            }
            Err(e) => self.status = Some(format!("{what}: {e}")),
        }
    }

    /// One `toggle_block_container` over the block-level target.
    ///
    /// leaf says *where*; twig decides everything else — which blocks the range
    /// covers, whether that means wrapping, unwrapping, nesting or converting,
    /// and how this document's format spells the prefix. The rule that a
    /// container only comes off when the range covers every block it holds is
    /// what the re-anchoring below is built around.
    fn toggle_container(&mut self, kind: BlockContainerKind) {
        // The read-only gate — this door reaches twig without the splice.
        if self.read_only {
            return;
        }
        if self.refuse_unsupported(&format!("{kind:?}"), Gesture::ToggleBlockContainer(kind)) {
            return;
        }
        let selected = self.selection();
        // A blank line holds no block, and twig opens an *empty* container on one
        // — since 3.2.0; it used to decline the range with `NotFound`, which is
        // why this used to lend it a scratch paragraph to wrap. Worth knowing
        // here because the line-for-line caret mapping below cannot describe it:
        // opening one under a paragraph writes the blank line the format needs
        // above the marker too, so the rewritten region has a line the old one
        // didn't, and "the same line, the same distance from its end" lands on
        // that new blank instead of in the container.
        let opened_empty = selected.is_none() && self.block_offset_for_caret().is_none();
        // Without a selection the target is the caret's own block, resolved the
        // way `set_block` resolves it — a caret at a line end sits at the doc
        // level and has to be nudged back onto the block it looks like it's in.
        // An empty range is enough: twig widens to the whole lines it touches.
        let (start, end) = match selected {
            Some(range) => range,
            None => {
                let off = self.block_offset_for_caret().unwrap_or(self.caret);
                (off, off)
            }
        };
        self.record_caret();
        match self.editor.toggle_block_container(start, end, kind) {
            Ok(change) => {
                // Read the caret's place out of the *pre-edit* source, before
                // `refresh` swaps that source out from under it.
                let place = (selected.is_none() && !opened_empty)
                    .then(|| self.caret_line_tail(&change.old));
                self.last_edit_kind = None; // structural edit is its own undo step
                self.refresh();
                match place {
                    // Both land the caret at the far end of what twig wrote, and
                    // differ only in what they leave selected.
                    //
                    // From a selection: select what the container now holds, the
                    // way `toggle` keeps its marked region selected — and for a
                    // stronger reason than symmetry: a container comes *off* only
                    // a range covering every block it holds, so a selection left
                    // on its old bytes (now short by a prefix per line) would nest
                    // on the second press instead of reversing the first.
                    //
                    // From a blank line: nothing to select, and the end of the
                    // region is exactly past the bare `> ` / `- ` twig wrote —
                    // the caret standing inside the container that was asked for.
                    None => {
                        self.anchor = (!opened_empty).then_some(change.new.start);
                        self.caret = change.new.end;
                    }
                    Some(place) => {
                        self.anchor = None;
                        self.caret = self.line_tail_offset(&change.new, place);
                    }
                }
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.clamp_caret();
                self.record_caret();
            }
            Err(e) => self.status = Some(format!("{kind:?}: {e}")),
        }
    }

    /// The caret's place inside the region a container toggle is rewriting, in
    /// the only terms the rewrite preserves: which of the region's lines it sits
    /// on, and how many bytes of that line lie ahead of it.
    ///
    /// A container's markup goes in at column 0 and never touches what follows
    /// on the line, so that pair survives the edit exactly where a byte offset
    /// does not — a caret left on its old offset slides back by one prefix per
    /// line above it, which on a hard-wrapped paragraph parks it *inside* the
    /// `> ` it just asked for.
    fn caret_line_tail(&self, old: &std::ops::Range<usize>) -> (usize, usize) {
        let caret = self.caret.clamp(old.start, old.end);
        let line = self.source[old.start..caret].matches('\n').count();
        let end = self.source[caret..old.end]
            .find('\n')
            .map_or(old.end, |i| caret + i);
        (line, end - caret)
    }

    /// [`caret_line_tail`](Self::caret_line_tail) undone against the rewritten
    /// region: the offset `tail` bytes back from the end of the region's `line`.
    ///
    /// Both walks are clamped rather than trusted, because the one op that does
    /// *not* keep a region's lines one-to-one is stripping a list — twig blows
    /// the items back apart with blank lines between them — and a caret landing
    /// on the nearest line of the right item beats one landing out of the region
    /// entirely.
    fn line_tail_offset(
        &self,
        new: &std::ops::Range<usize>,
        (line, tail): (usize, usize),
    ) -> usize {
        let region = &self.source[new.start.min(self.source.len())..new.end.min(self.source.len())];
        let mut start = 0;
        for _ in 0..line {
            match region[start..].find('\n') {
                Some(i) => start += i + 1,
                None => break,
            }
        }
        let end = region[start..]
            .find('\n')
            .map_or(region.len(), |i| start + i);
        new.start + end.saturating_sub(tail).max(start)
    }

    /// Link the selection to `destination` — the toolbar's Link button. With no
    /// selection it acts at the caret, which re-points a link the caret is
    /// already standing in (twig replaces an existing link's destination and
    /// keeps its text) and otherwise spells a link that has no text of its own:
    /// an autolink (`<https://x.dev>`) where the destination is one, and
    /// `[destination](destination)` where it isn't.
    ///
    /// `destination` reaches twig raw. Escaping it is format knowledge and the
    /// two formats genuinely disagree — Markdown ends a destination at the first
    /// space and moves it into `<…>`, djot reads that `<…>` as part of the URL
    /// itself — so the side holding the document is the side that gets to spell
    /// it. A destination twig can't carry at all (one with a newline) comes back
    /// as an error rather than a quietly rewritten URL.
    pub fn insert_link(&mut self, destination: &str) {
        if self.read_only || self.refuse_unsupported("link", Gesture::InsertLink) {
            return;
        }
        let (start, end) = self.selection().unwrap_or((self.caret, self.caret));
        self.record_caret();
        match self.editor.insert_link(start, end, destination) {
            Ok(change) => {
                self.last_edit_kind = None;
                self.refresh();
                match self.link_text_span(change.new.start) {
                    // A link with text of its own: select it, so typing replaces
                    // a `[dest](dest)`'s stand-in label and a second press
                    // re-points what the first one linked.
                    Some(text) => {
                        self.anchor = (text.start != text.end).then_some(text.start);
                        self.caret = text.end;
                    }
                    // An autolink is finished the moment it's written — its text
                    // *is* the URL. Leaving it selected would aim the next press
                    // at the one shape twig still wraps instead of re-points.
                    None => {
                        self.anchor = None;
                        self.caret = change.new.end;
                    }
                }
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.clamp_caret();
                self.record_caret();
            }
            Err(e) => self.status = Some(format!("link: {e}")),
        }
    }

    /// Insert a block-level image at the caret: `![alt](destination)`. Any
    /// selection becomes the alt text (so "select a caption, insert image" labels
    /// it); with no selection, `alt` is used — empty for none. The caret lands
    /// just past the inserted image.
    ///
    /// Both halves go through twig (`insert_literal` for the alt text,
    /// `insert_image` for the image), so neither is spelled here. That used to be a
    /// `format!`, and it was wrong the first time an app inserted a real filename:
    /// Markdown ends a destination at the first space, so `![](my photo.png)` is
    /// not an image at all — and the fix is per-format, since moving into the
    /// `<…>` form is exactly wrong for Djot, where `<…>` becomes the URL itself.
    pub fn insert_image(&mut self, destination: &str, alt: &str) {
        if self.read_only || self.refuse_unsupported("image", Gesture::InsertImage) {
            return;
        }
        let (start, end) = self.selection().unwrap_or((self.caret, self.caret));
        self.record_caret();
        // With no selection and an explicit `alt`, the alt text has to exist in the
        // document before it can be the image's — and it is raw caller input, so
        // it goes in through `insert_literal`, which escapes it for the format
        // rather than letting a `]` in someone's caption close the image early.
        let (start, end) = if start == end && !alt.is_empty() {
            match self.editor.insert_literal(start, alt) {
                Ok(change) => (change.new.start, change.new.end),
                Err(e) => {
                    self.status = Some(format!("image: {e}"));
                    return;
                }
            }
        } else {
            (start, end)
        };
        match self.editor.insert_image(start, end, destination) {
            Ok(change) => {
                self.last_edit_kind = None;
                self.refresh();
                // Just past the image, nothing selected — where a caret belongs
                // after inserting one.
                self.anchor = None;
                self.caret = change.new.end;
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.clamp_caret();
                self.record_caret();
            }
            Err(e) => self.status = Some(format!("image: {e}")),
        }
    }

    /// Insert a block-level image, video, or audio at the caret. The image case
    /// is [`insert_image`](Self::insert_image); video and audio are spelled as
    /// HTML elements, which is the only spelling Markdown and Djot have for them:
    ///
    /// ```text
    /// <video src="clip.mp4" controls>alt</video>
    /// <audio src="take.mp3" controls>alt</audio>
    /// ```
    ///
    /// HTML rather than a `::video{…}` directive deliberately. A directive means
    /// something only to an app that knows the vocabulary, so the document would
    /// read as literal punctuation everywhere else; `<video>` is what every other
    /// renderer already understands, and what leaf's own reader picks back up
    /// through `html_elements` promotion (see [`parse_extensions`]).
    ///
    /// The one-line spelling needs twig ≥ 2.5.1, which widened CommonMark's
    /// HTML-block tag list to cover `<video>`/`<audio>`/`<picture>` under
    /// `html_elements`. Before that only the multi-line form parsed as a block at
    /// all, and this wrote three lines to work around it.
    ///
    /// `controls` is always written: a player with no transport is a still frame
    /// the reader can't do anything with. Any selection becomes the element's
    /// fallback text, exactly as it becomes an image's alt.
    ///
    /// The same verbatim-insertion caveat as [`insert_image`](Self::insert_image)
    /// applies, and bites harder here: a `"` in `destination` closes the
    /// attribute. A frontend taking these from a file picker is fine; one taking
    /// them from free text should keep them tame.
    ///
    /// [`MediaInfo`]: crate::MediaInfo
    pub fn insert_media(&mut self, kind: MediaKind, destination: &str, alt: &str) {
        if kind == MediaKind::Image {
            return self.insert_image(destination, alt);
        }
        // Gated on the *image* gesture, not on one of its own — there isn't one,
        // since the bytes below are spelled here rather than by twig, and an HTML
        // document would in fact parse them. The button is one control with three
        // kinds behind it, and two of them working in a format where the third
        // cannot is a worse surface than three that agree — especially as
        // `insert_image` is the kind anyone reaches for first.
        if self.refuse_unsupported("media", Gesture::InsertImage) {
            return;
        }
        let (start, end) = self.selection().unwrap_or((self.caret, self.caret));
        let alt_text = self
            .selected_text()
            .map(str::to_string)
            .unwrap_or_else(|| alt.to_string());
        let tag = match kind {
            MediaKind::Audio => "audio",
            _ => "video",
        };
        let markup = format!("<{tag} src=\"{destination}\" controls>{alt_text}</{tag}>");
        self.edit(start, end, &markup);
    }

    /// Append media at the **end** of the document, as a block of its own —
    /// the verb for a picture that *arrives* rather than one the writer
    /// places: an attachment imported while the caret was wherever it last
    /// was, a drawing placed from a tray under the body. At the caret it
    /// would land inline in front of whatever word the caret happened to be
    /// beside, which for an editor nobody has tapped yet is the first word
    /// of the document.
    ///
    /// The document is ended with a blank line first, where it does not
    /// already end with one, so the media is a paragraph of its own rather
    /// than a lazy continuation of the last one; an empty document needs no
    /// separator. Then [`insert_media`](Self::insert_media) at the new end,
    /// with everything that means: the same markup per kind, the same
    /// refusal on a format without images. The selection is dropped — the
    /// verb is about the end of the document, not about what was selected —
    /// and the caret is left past the media, as `insert_media` leaves it.
    pub fn append_media(&mut self, kind: MediaKind, destination: &str, alt: &str) {
        if self.read_only || self.refuse_unsupported("media", Gesture::InsertImage) {
            return;
        }
        let separator = if self.source.is_empty() || self.source.ends_with("\n\n") {
            ""
        } else if self.source.ends_with('\n') {
            "\n"
        } else {
            "\n\n"
        };
        let end = self.source.len();
        self.anchor = None;
        self.caret = end;
        if !separator.is_empty() {
            self.edit(end, end, separator);
        }
        self.anchor = None;
        self.caret = self.source.len();
        self.insert_media(kind, destination, alt);
    }

    /// Insert a thematic break at the caret — the toolbar's Horizontal Rule
    /// button. Spelling and placement are both twig's; leaf used to write `---`
    /// itself, which was the Markdown spelling in a djot document too.
    ///
    /// A rule is a block, so `insert_thematic_break` alone has nowhere to put one
    /// mid-paragraph and lands it after the caret's whole block. To get a rule
    /// *at* the caret — the paragraph parted in two around it, which is what a
    /// rule button is understood to do — the paragraph is first divided with
    /// `split_block` and the rule then aimed at the **first** half. Aiming it at
    /// the offset `split_block` returns puts the rule after the *second* half
    /// instead, which is a rule in the right document and the wrong place.
    ///
    /// Only a plain paragraph is split, and only where there is something to
    /// part: at the paragraph's end the split has no second half to mint and
    /// would write the separator anyway — a blank line and the empty slot Enter
    /// leaves for the next paragraph, which the rule then lands above and
    /// nothing fills — so there the rule goes straight after the paragraph,
    /// which is where the split-and-aim was sending it regardless. At the
    /// paragraph's *start* the split is kept, though it parts nothing either:
    /// `|para` becomes `\npara` with the caret on the new blank line, and a
    /// rule aimed at a blank line is written on it (twig ≥ 3.5.2), which is how
    /// "before the paragraph" is said through a gesture that only knows
    /// "after" — `---\n\npara`, and `prev\n\n---\n\npara` mid-document. Everywhere
    /// else the rule simply lands after the block, which is both twig's own
    /// answer and the better one: splitting a fenced code block would leave two
    /// fences with a rule between them, and splitting a list item would mint an
    /// item nobody asked for on the way to a rule that lands after the list
    /// regardless. A table and a setext heading refuse the split outright, so
    /// they take the same path by themselves.
    ///
    /// The caret ends on the line under the rule, which is where the writer
    /// goes on: at the start of the paragraph's second half when the rule
    /// parted one, and otherwise on an empty line opened under the rule — see
    /// [`stand_under_block`](Self::stand_under_block). It used to be left at
    /// the end of what twig wrote, which is the rule's own row: the caret was
    /// drawn beside the rule, and mid-document the next keystroke joined the
    /// block below.
    pub fn insert_thematic_break(&mut self) {
        if self.read_only || self.refuse_unsupported("thematic break", Gesture::InsertThematicBreak)
        {
            return;
        }
        self.caret = self.skip_trailing_close_delims(self.caret);
        // A selection is replaced by the rule, so collapse it first and let the
        // split-and-rule below run from the caret it leaves behind.
        if let Some((s, e)) = self.selection() {
            self.splice(s, e, "", EditKind::Other);
        }
        self.anchor = None;
        self.record_caret();
        let at = self.caret;
        // A failure here is not fatal: the rule still lands after the block,
        // which is exactly what this call was trying to improve on.
        let parted = self.part_for_block(at);
        match self.editor.insert_thematic_break(at) {
            Ok(change) => {
                self.last_edit_kind = None;
                self.refresh();
                if parted {
                    self.coalesce_last_undo();
                }
                self.anchor = None;
                self.caret = change.new.end;
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.clamp_caret();
                self.record_caret();
                self.caret_under_written_block(change.new, parted);
            }
            Err(e) => self.status = Some(format!("thematic break: {e}")),
        }
    }

    /// Part the bare paragraph at `at` for a block gesture, where
    /// [`caret_parts_bare_paragraph`](Self::caret_parts_bare_paragraph) says
    /// to, and say whether it was parted.
    ///
    /// The split is counted as a step of its own, so the gesture can fold the
    /// block it writes next into it. It used to go to twig uncounted: twig
    /// kept it as a step and leaf's count did not, so one Undo took back the
    /// block and left the paragraph parted, with Undo then reporting nothing
    /// left to undo.
    fn part_for_block(&mut self, at: usize) -> bool {
        if !self.caret_parts_bare_paragraph() || self.editor.split_block(at).is_err() {
            return false;
        }
        self.refresh();
        true
    }

    /// Where a block gesture leaves the caret once twig has written the block
    /// `new` spans — a rule, a page break: on the line under it. That is the
    /// start of the paragraph's second half when the gesture `parted` one,
    /// and otherwise the line [`stand_under_block`](Self::stand_under_block)
    /// finds or writes, folded into the gesture's step.
    ///
    /// The block is the outermost node twig's change opens, whatever its
    /// kind, since the kind a leaf directive comes back as is the format's.
    fn caret_under_written_block(&mut self, new: Range<usize>, parted: bool) {
        let block = self
            .nodes()
            .into_iter()
            .filter(|n| n.kind != Kind::Doc && new.contains(&n.span.start))
            .min_by_key(|n| (n.span.start, std::cmp::Reverse(n.span.end)));
        let Some(block) = block else {
            return;
        };
        let (end, _) = wysiwyg::block_line(&self.source, block.span.end);
        if parted {
            // The second half starts at the first text under the block; the
            // split left it no leading whitespace.
            let rest = &self.source[end..];
            self.caret = end + (rest.len() - rest.trim_start().len());
            self.record_caret();
        } else if self.stand_under_block(end) {
            self.coalesce_last_undo();
        }
    }

    /// Stand the caret on the line under the block whose last line's text ends
    /// at `end` — a thematic break, a page break — first writing that line if
    /// the block has none: where the rule and page-break buttons leave the
    /// caret, and where Enter on a rule takes it. Whether anything was
    /// written, so a caller can fold it into its own step.
    ///
    /// The caret's line is the first under the block that the view gives a
    /// home: the second blank line in [`LineFlow::Fold`], which draws the
    /// first as the gap closing the block, and the first in
    /// [`LineFlow::Preserve`], where every blank line is somewhere to type.
    /// When anything follows, one more blank line has to stand between the
    /// caret's line and it, or what is typed there runs on into the next
    /// block. Only what that shape is missing is written. A block twig wrote on
    /// a blank line still has the blank lines that were under the caret and
    /// needs one line more, or none, where a rule written under a paragraph
    /// needs two.
    ///
    /// Blank means blank inside the block's container: a quote's blank lines
    /// keep their `>`, and the caret's line wears the quote's whole prefix, as
    /// the line Enter opens in a quote does. The document's last line, when no
    /// newline ends it, counts only once a blank line stands above it, since
    /// only then does the view draw it as a row. Fold always writes the gap
    /// above it if it isn't there, so there a block ending the document takes
    /// one newline more and the caret its end.
    fn stand_under_block(&mut self, end: usize) -> bool {
        let prefix = self.continuation_prefix_at(end);
        let blank = prefix.trim_end();
        let gap = usize::from(self.line_flow == LineFlow::Fold);
        // The blank lines directly under the block's own, by where each starts,
        // and whether the line that ends the run has anything on it.
        let mut blanks = Vec::new();
        let mut follows = false;
        let mut at = end;
        while let Some(nl) = self.source[at..].find('\n') {
            let start = at + nl + 1;
            let len = self.source[start..].find('\n');
            let line = &self.source[start..len.map_or(self.source.len(), |len| start + len)];
            let line = line.trim_end();
            if line != blank {
                follows = !line.trim().is_empty();
                break;
            }
            let Some(len) = len else {
                if gap > 0 || !blanks.is_empty() {
                    blanks.push(start);
                }
                break;
            };
            blanks.push(start);
            at = start + len;
        }
        let missing = (gap + 1 + usize::from(follows)).saturating_sub(blanks.len());
        // What is missing goes in at the rule's end, above the blank lines
        // already there, so the caret's line is the same one counted from
        // the rule either way: written here, or one of those, moved down.
        let mut text = String::new();
        let mut caret = None;
        for line in 1..=missing {
            text.push('\n');
            if line == gap + 1 {
                text.push_str(&prefix);
                caret = Some(end + text.len());
            } else {
                text.push_str(blank);
            }
        }
        let caret = caret.unwrap_or_else(|| {
            let start = blanks[gap - missing];
            let line = self.source[start..].split('\n').next().unwrap_or("");
            start + text.len() + line.trim_end_matches('\r').len().min(prefix.len())
        });
        if !text.is_empty() && !self.splice(end, end, &text, EditKind::Other) {
            return false;
        }
        self.caret = caret.min(self.source.len());
        self.anchor = None;
        self.goal_col = None;
        self.record_caret();
        !text.is_empty()
    }

    /// The thematic break the caret stands under, as its text's end, when
    /// Enter there belongs to the rule: the caret at the rule's home past it
    /// (the start of the line under it), that line blank, and the caret drawn
    /// on the rule's own row.
    ///
    /// Drawn on the rule's row, because the home past a rule is also the start
    /// of the line under it, and which of the two the view draws it on is the
    /// flow's. [`LineFlow::Fold`] draws that line as the gap under the rule,
    /// no caret's home, so the caret is beside the rule. [`LineFlow::Preserve`]
    /// makes every blank line a home, so it draws the caret there and Enter is
    /// an ordinary blank line's — unless it is the document's unterminated
    /// last line, which is no row in either flow.
    fn rule_above_caret(&mut self) -> Option<usize> {
        let caret = self.caret.min(self.source.len());
        let above = self.source[..caret].strip_suffix('\n')?;
        let above_start = above.rfind('\n').map_or(0, |i| i + 1);
        let text = above[above_start..].trim_end();
        let (last, _) = text.char_indices().next_back()?;
        let rule = self
            .editor
            .ancestors_at(above_start + last)
            .ok()?
            .into_iter()
            .find(|m| m.kind == Kind::ThematicBreak)?;
        let (end, home) = wysiwyg::block_line(&self.source, rule.span.end);
        if home != caret {
            return None;
        }
        let blank = self.continuation_prefix_at(end);
        let line_end = self.source[caret..].find('\n').map(|i| caret + i);
        let line = &self.source[caret..line_end.unwrap_or(self.source.len())];
        if line.trim_end() != blank.trim_end() && !line.trim().is_empty() {
            return None;
        }
        (self.line_flow == LineFlow::Fold || line_end.is_none()).then_some(end)
    }

    /// Insert a fresh table at the caret — the toolbar's Table button. One
    /// header row, `rows` empty body rows, `cols` columns, spelled by twig in
    /// the document's own dialect and placed the way its thematic break is:
    /// after the caret's block, blank-separated. A bare paragraph is parted
    /// around the caret first, exactly as
    /// [`insert_thematic_break`](Self::insert_thematic_break) parts it, so the
    /// table lands *at* the caret rather than after everything the caret's
    /// paragraph says.
    ///
    /// The caret ends in the first header cell, selected the way Tab selects
    /// a cell — the natural next act is to type the heading, and Tab then
    /// walks the grid. That cell is read back from the rebuilt table map
    /// rather than computed from the splice, because twig's blank line and
    /// quote prefix put the first bar at an offset only the reparse knows.
    ///
    /// The shape is the caller's: a menu offers a few, a dialog asks. Zero
    /// rows or columns is twig's refusal (a header with nothing under it is
    /// what its row delete refuses to leave), reported through `status` —
    /// and refused here before the paragraph is parted, which would otherwise
    /// be left parted around a table never written.
    pub fn insert_table(&mut self, rows: usize, cols: usize) {
        if self.read_only || self.refuse_unsupported("table", Gesture::InsertTable) {
            return;
        }
        // twig's one refusal of a shape, asked before the paragraph is parted
        // for a table that would never be written into it.
        if rows == 0 || cols == 0 {
            self.status = Some("table: needs at least one row and one column".into());
            return;
        }
        self.caret = self.skip_trailing_close_delims(self.caret);
        if let Some((s, e)) = self.selection() {
            self.splice(s, e, "", EditKind::Other);
        }
        self.anchor = None;
        self.record_caret();
        let at = self.caret;
        let parted = self.part_for_block(at);
        match self.editor.insert_table(at, rows, cols) {
            Ok(change) => {
                self.last_edit_kind = None;
                self.refresh();
                if parted {
                    self.coalesce_last_undo();
                }
                self.anchor = None;
                self.caret = change.new.end;
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.clamp_caret();
                // Into the first header cell of the table just written: the
                // first table whose grid begins inside the splice.
                self.rebuild_map();
                let first_cell = self
                    .vmap
                    .tables
                    .iter()
                    .filter_map(|t| t.grid.first().and_then(|row| row.cells.first()))
                    .find(|cell| cell.start >= change.new.start && cell.start < change.new.end)
                    .map(|cell| (cell.start, cell.end));
                if let Some((start, end)) = first_cell {
                    self.select_cell(start, end);
                }
                self.record_caret();
            }
            Err(e) => self.status = Some(format!("table: {e}")),
        }
    }

    /// Whether the caret sits in a paragraph and nothing else — no list item, no
    /// quote, no fence, no table — with paragraph text still ahead of it. The
    /// one shape where parting the block around the caret is unambiguously what
    /// a rule button means; see
    /// [`insert_thematic_break`](Self::insert_thematic_break) for why every other
    /// container is left to take the rule after itself.
    ///
    /// The "text ahead" half is what keeps `split_block` from running at the
    /// one edge where its output composes badly. At a paragraph's end twig
    /// cannot mint the empty second half (no format spells an empty
    /// paragraph), so it writes only the separator — a blank line and the
    /// slot Enter leaves for the paragraph to come — and a block then aimed at
    /// the first half lands above a slot that nothing fills: `para\n` with the
    /// caret at 4 came out as `para\n\n* * *\n\n\n`. Trailing whitespace counts
    /// as nothing ahead, since the split would shed it as the second half's
    /// leading indent and leave the same slot. Which end of the newline a
    /// paragraph's span stops at differs between the formats (Markdown before
    /// it, djot after), which is why this reads the remaining bytes rather
    /// than comparing offsets. The paragraph's
    /// start is deliberately not the same case — see
    /// [`insert_thematic_break`](Self::insert_thematic_break) for why that
    /// split is kept.
    fn caret_parts_bare_paragraph(&mut self) -> bool {
        let caret = self.caret.min(self.source.len());
        let Ok(chain) = self.editor.ancestors_at(caret) else {
            return false;
        };
        let mut para_end = None;
        for m in chain {
            match m.kind {
                Kind::Para => para_end = Some(m.span.end.min(self.source.len())),
                Kind::ListItem
                | Kind::TaskListItem
                | Kind::BlockQuote
                | Kind::CodeBlock
                | Kind::Table => return false,
                _ => {}
            }
        }
        match para_end {
            Some(end) if end > caret => !self.source[caret..end].trim().is_empty(),
            _ => false,
        }
    }

    /// The destination of the link under the caret — what a Link prompt shows so
    /// ⌘K on an existing link edits its URL instead of asking for it again.
    /// `None` when the caret stands in no link.
    ///
    /// An autolink carries no separate destination: its text *is* the URL, so
    /// that's what comes back for one.
    pub fn link_destination_at_caret(&mut self) -> Option<String> {
        self.link_destination_at(self.caret)
    }

    /// The destination of the link at `off`.
    /// [`link_destination_at_caret`](Self::link_destination_at_caret) for a place
    /// the caret isn't.
    ///
    /// The offset form exists for the same reason
    /// [`footnote_at`](Self::footnote_at)'s does: a frontend drawing a *piece* of
    /// the document somewhere else — a footnote's text in a popover, say — has
    /// rows and runs but no caret in them, and still needs to know which of those
    /// runs a reader can follow.
    pub fn link_destination_at(&mut self, off: usize) -> Option<String> {
        self.nodes()
            .into_iter()
            .filter(|n| matches!(n.kind.as_str(), "link" | "url" | "email"))
            .filter(|n| n.span.start <= off && off < n.span.end)
            .max_by_key(|n| n.span.start)
            .and_then(|n| n.destination.or(n.text))
    }

    /// The heading the caret is under — the nearest heading at or above it,
    /// whatever its level. See [`heading_at`](Self::heading_at).
    pub fn heading_at_caret(&mut self) -> Option<Heading> {
        self.heading_at(self.caret)
    }

    /// The heading `off` is under: the last heading that starts at or before
    /// it, or the one `off` stands in. `None` above the first heading.
    ///
    /// The question between [`link_destination_at`](Self::link_destination_at)
    /// ("which link is this") and [`locate`](Self::locate) ("where does this
    /// fragment land"): a host writing a link *to* a place needs the heading
    /// that place is under, to name it by `#slug`. Nearest, not enclosing —
    /// a level-3 heading under a level-2 is the answer for the text below the
    /// level-3, because it is the finer place to land.
    pub fn heading_at(&mut self, off: usize) -> Option<Heading> {
        let nodes = self.nodes();
        let h = nodes
            .iter()
            .filter(|n| n.kind == Kind::Heading && n.span.start <= off)
            .max_by_key(|n| n.span.start)?;
        let mut text = String::new();
        inline_text(&nodes, h.first_child, &mut text);
        Some(Heading {
            text: text.trim().to_string(),
            level: h.level.unwrap_or(1),
            span: h.span.clone(),
        })
    }

    /// Where the locator `id` lands in this document — the `#v2` half of a
    /// `chapter.dj#v2`, resolved to the block it names. `None` when nothing here
    /// answers to it.
    ///
    /// The other end of a link, and the reason this exists: without it a
    /// destination has only file granularity, so following a citation into a
    /// chapter drops the reader at the top of it to hunt for the verse. Which is
    /// also why it is a *document* query rather than a caret one — the document
    /// being asked is usually not the one the reader is in.
    ///
    /// Three readings, tried in order, because the same `#some-heading` is
    /// written three ways across the formats leaf opens:
    ///
    /// 1. **A declared id**, exactly as written: djot's `{#v1}` on a block, and
    ///    the auto-ids djot mints for its headings. The only exact answer, so it
    ///    goes first — a document that says `{#v1}` has settled the question.
    /// 2. **A declared id, slugged.** djot spells a heading's auto-id
    ///    `Some-Heading-Here`; nearly every tool that *writes* a link to one
    ///    spells it `#some-heading-here`. Comparing slugs is what lets a link
    ///    authored anywhere land on a djot heading.
    /// 3. **A heading's text, slugged.** Markdown has no ids at all — twig mints
    ///    none and `{#custom}` is literal text in a Markdown heading — so for
    ///    the format most vaults are written in, the heading's own words are the
    ///    only thing a fragment can name. This is the rule every Markdown
    ///    renderer already follows, which is what makes `#a-heading` mean in
    ///    diaryx what it means on the web.
    ///
    /// Ties go to the earliest match, then to the widest: a duplicated id is the
    /// document's mistake and the first one is the answer every anchor
    /// implementation gives, while preferring the wider span picks the section
    /// over the heading that opens it — more for a peek to show, same place to
    /// land.
    pub fn locate(&mut self, id: &str) -> Option<Landing> {
        let id = id.trim();
        if id.is_empty() {
            return None;
        }
        let nodes = self.nodes();

        // Earliest wins, then widest. `Reverse` on the end because `min_by_key`
        // is picking, among nodes that start together, the one that ends last.
        let pick = |matches: &mut dyn Iterator<Item = &FlatNode>| {
            matches
                .min_by_key(|n| (n.span.start, std::cmp::Reverse(n.span.end)))
                .map(|n| Landing {
                    start: n.span.start,
                    end: n.span.end,
                })
        };

        if let Some(landing) = pick(&mut nodes.iter().filter(|n| declared_id(n) == Some(id))) {
            return Some(landing);
        }
        let want = slug(id);
        if want.is_empty() {
            return None;
        }
        if let Some(landing) = pick(
            &mut nodes
                .iter()
                .filter(|n| declared_id(n).map(slug).as_deref() == Some(&*want)),
        ) {
            return Some(landing);
        }

        // A heading by its words. Its span is one line, so the end comes from
        // where the *section* it opens gives out — the next heading that is not
        // under it, or the end of the document. A Markdown heading has no
        // section node to ask (twig only builds those for djot), and a peek that
        // showed the heading alone would answer "what does that say" with the
        // title of the thing it says.
        let heading = nodes
            .iter()
            .filter(|n| n.kind == Kind::Heading)
            .filter(|n| {
                n.content_span
                    .clone()
                    .and_then(|s| self.source.get(s))
                    .is_some_and(|text| slug(text) == want)
            })
            .min_by_key(|n| n.span.start)?;
        let level = heading.level.unwrap_or(u32::MAX);
        let end = nodes
            .iter()
            .filter(|n| n.kind == Kind::Heading)
            .filter(|n| n.span.start > heading.span.start)
            .filter(|n| n.level.unwrap_or(u32::MAX) <= level)
            .map(|n| n.span.start)
            .min()
            .unwrap_or(self.source.len());
        Some(Landing {
            start: heading.span.start,
            end,
        })
    }

    /// Write a footnote at the caret — the toolbar's Footnote button, and the
    /// one gesture in the footnote story that *authors* rather than follows.
    ///
    /// Both halves go in as one twig edit: the `[^1]` where the caret is, and
    /// the `[^1]:` definition at the end of the document. Half a footnote is not
    /// a footnote — a bare reference with nothing defining it renders as literal
    /// brackets — so a single button that wrote only the reference would leave
    /// the author to hand-spell the other half in a document that had just
    /// stopped showing them what the first half meant. One edit also means one
    /// undo takes both back.
    ///
    /// The definition's body is left empty and **the caret lands in it**, which
    /// is the whole point of pressing the button: nobody wants a reference to a
    /// note they have not written yet. Getting back to where they were writing
    /// is [`footnote_definition_at_caret`](Self::footnote_definition_at_caret) —
    /// the same return leg a reader following a reference already uses, so the
    /// author is left standing on the near end of a round trip that works.
    ///
    /// A selection collapses to its *end* rather than being replaced: a
    /// reference annotates the words before it, so "select the claim, add a
    /// footnote" should mark that claim, not consume it.
    pub fn insert_footnote(&mut self) {
        if self.read_only || self.refuse_unsupported("footnote", Gesture::InsertFootnote) {
            return;
        }
        let at = self.selection().map_or(self.caret, |(_, end)| end);
        self.anchor = None;
        self.caret = at;
        self.record_caret();
        let label = self.next_footnote_label();
        match self.editor.insert_footnote(at, &label) {
            Ok(change) => {
                self.last_edit_kind = None;
                self.refresh();
                self.anchor = None;
                // `change.new` runs from the reference to the end of the
                // document, so its start is the `[^1]` just written and
                // `footnote_at` resolves it to the note the same way a reader's
                // tap does — and to the note's *body*, which is already a caret
                // stop even when it is empty (the `[^1]:` marker draws as `[1] `
                // and has none), so this needs no snap on top. The fallback is
                // the reference's own offset: a format that spelled the pair some
                // way leaf can't read back should still leave the caret on the
                // edit rather than at the far end of a document it just grew.
                self.caret = self
                    .footnote_at(change.new.start)
                    .and_then(|note| note.offset)
                    .unwrap_or(change.new.start);
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.clamp_caret();
                self.record_caret();
            }
            Err(e) => self.status = Some(format!("footnote: {e}")),
        }
    }

    /// The label to give a footnote the author has not named: the lowest counting
    /// number no footnote in the document is already wearing.
    ///
    /// twig takes the label rather than minting one, because it holds no opinion
    /// about what a document's footnotes should be called — and it is right not
    /// to. Numbering them is what every author of a numbered note expects, and
    /// re-using a taken number would silently point the new reference at somebody
    /// else's note (twig reuses an existing definition rather than appending a
    /// second one, which is the right rule for citing a note twice on purpose and
    /// exactly the wrong accident to have by default).
    ///
    /// *References* are counted alongside definitions, not just definitions: a
    /// document carrying a dangling `[^2]` has a 2 that means something to
    /// whoever wrote it, and minting a definition for it here would answer a
    /// question nobody asked. Non-numeric labels (`[^why]`) are left out of the
    /// count entirely — they take no number, so they block none.
    fn next_footnote_label(&mut self) -> String {
        let mut taken: Vec<u32> = wysiwyg::footnote_definitions(&mut self.editor)
            .into_iter()
            .filter_map(|note| wysiwyg::footnote_label(&self.source, note.span.start))
            .filter_map(|label| label.parse().ok())
            .collect();
        taken.extend(
            self.nodes()
                .into_iter()
                .filter(|n| n.kind == Kind::FootnoteReference)
                .filter_map(|n| wysiwyg::footnote_reference_label(&self.source, n.span))
                .filter_map(|label| label.parse::<u32>().ok()),
        );
        (1..).find(|n| !taken.contains(n)).unwrap_or(1).to_string()
    }

    /// The footnote reference under the caret, resolved to the note it names.
    /// [`footnote_at`](Self::footnote_at) at the caret's offset.
    pub fn footnote_at_caret(&mut self) -> Option<FootnoteRef> {
        self.footnote_at(self.caret)
    }

    /// The footnote reference at `off`, resolved to the note it names — what a
    /// frontend shows when a reader activates a `[^1]`.
    ///
    /// A reference is not a link node, so
    /// [`link_destination_at_caret`](Self::link_destination_at_caret) does not
    /// (and should not) answer for one: a link names a destination to leave for,
    /// a reference names a note that is already in this document. Following one
    /// is a move within the page, which is why this hands back an `offset`
    /// rather than something to open.
    ///
    /// Offset-based rather than caret-only because the gesture that wants this
    /// most is the one that must not move the caret: a pointer hovering a `[1]`
    /// asks what note it names without disturbing where the reader was typing.
    /// The caret is just the offset a click already placed —
    /// [`footnote_at_caret`](Self::footnote_at_caret) passes it.
    ///
    /// `None` when `off` stands in no reference. A reference whose note the
    /// document never defines is *not* `None` — it answers with the label it
    /// looked for and no text, which is what lets a frontend say so instead of
    /// silently doing nothing.
    pub fn footnote_at(&mut self, off: usize) -> Option<FootnoteRef> {
        // Innermost-wins by latest start, the rule its link sibling uses.
        let span = self
            .nodes()
            .into_iter()
            .filter(|n| n.kind == Kind::FootnoteReference)
            .filter(|n| n.span.start <= off && off < n.span.end)
            .max_by_key(|n| n.span.start)?
            .span;
        let label = wysiwyg::footnote_reference_label(&self.source, span)?.to_string();

        // The note itself. Definitions are roots beside `doc` rather than
        // children of it, so they're asked for directly — see
        // `wysiwyg::footnote_definitions`.
        let note = wysiwyg::footnote_definitions(&mut self.editor)
            .into_iter()
            .find(|m| wysiwyg::footnote_label(&self.source, m.span.start) == Some(&label));
        let Some(note) = note else {
            return Some(FootnoteRef {
                label,
                text: None,
                offset: None,
                end: None,
            });
        };
        let body = wysiwyg::footnote_body_span(&self.source, note.span.clone());
        Some(FootnoteRef {
            label,
            text: body
                .clone()
                .and_then(|b| self.source.get(b))
                .map(str::to_string),
            // The body's start, not the definition's — see `FootnoteRef::offset`.
            offset: body.clone().map(|b| b.start),
            end: body.map(|b| b.end),
        })
    }

    /// The footnote *definition* the caret stands in, and where the reference
    /// that names it is. [`footnote_definition_at`](Self::footnote_definition_at)
    /// at the caret's offset.
    pub fn footnote_definition_at_caret(&mut self) -> Option<FootnoteDef> {
        self.footnote_definition_at(self.caret)
    }

    /// The footnote definition spanning `off`, and where the reference that
    /// names it is — the return leg of [`footnote_at`](Self::footnote_at).
    ///
    /// The mirror image, deliberately: the same gesture that takes a reader from
    /// `[1]` down to the note takes them from the note back up to `[1]`, so
    /// following a footnote is a round trip rather than a fall. It needs no
    /// memory of how the reader arrived — the document says where the reference
    /// is — which is what makes it work for a reader who scrolled to the notes
    /// themselves, and what keeps it right after an edit moves either end.
    ///
    /// `None` when `off` stands in no definition. A definition nothing cites is
    /// *not* `None`, for [`FootnoteRef`]'s reason in reverse: it answers with
    /// its label and no offset, so a frontend can say "nothing refers to this"
    /// rather than offer a jump that goes nowhere.
    pub fn footnote_definition_at(&mut self, off: usize) -> Option<FootnoteDef> {
        // Definitions are roots beside `doc`, so `nodes()` — which walks the
        // document body — never reports one. They're asked for directly, the way
        // `footnote_at` asks for the note it resolves to.
        //
        // Closed at the end, unlike the half-open test its neighbours use. A
        // definition's span stops at its last content byte — the newline ending
        // the line is outside it — so `span.end` is the caret stop at the end of
        // the note's own row, not the first byte of anything after. Excluding it
        // meant the one caret an author is guaranteed to have, the one left
        // sitting at the end of the note they just typed, was in no definition at
        // all: writing a note and then asking to go back to its reference
        // answered nothing. Two definitions in a row still can't both match —
        // there is a blank line between them — and `max_by_key` decides anyway.
        let note = wysiwyg::footnote_definitions(&mut self.editor)
            .into_iter()
            .filter(|m| m.span.start <= off && off <= m.span.end)
            .max_by_key(|m| m.span.start)?;
        let label = wysiwyg::footnote_label(&self.source, note.span.start)?.to_string();

        // The earliest reference carrying this label. `min` rather than a `find`,
        // because `nodes()` reports a flattened walk whose order is twig's
        // business, not document order. Bound first: the walk needs `&mut self`
        // and reading the labels back out needs `&self.source`.
        let nodes = self.nodes();
        let offset = nodes
            .into_iter()
            .filter(|n| n.kind == Kind::FootnoteReference)
            .filter(|n| {
                wysiwyg::footnote_reference_label(&self.source, n.span.clone()) == Some(&*label)
            })
            // Past the `[^`, onto the label — see `FootnoteDef::offset`.
            .map(|n| n.span.start + 2)
            .min();
        Some(FootnoteDef { label, offset })
    }

    /// The destination of the image under the caret — what an image prompt shows
    /// so editing an existing image starts from its current URL instead of blank,
    /// the image analogue of [`link_destination_at_caret`](Self::link_destination_at_caret).
    /// `None` when the caret stands in no image. A caret resting just after a
    /// block image (its trailing stop) is still "in" it — the half-open span test
    /// excludes that offset, which is the intended precision: past the image is
    /// past it.
    pub fn image_destination_at_caret(&mut self) -> Option<String> {
        let off = self.caret;
        self.nodes()
            .into_iter()
            .filter(|n| n.kind == Kind::Image)
            .filter(|n| n.span.start <= off && off < n.span.end)
            .max_by_key(|n| n.span.start)
            .and_then(|n| n.destination)
    }

    /// The language of the fenced code block the caret stands in — what a
    /// language prompt shows so editing it starts from the current value rather
    /// than blank. `None` when the caret is in no code block, or in one whose
    /// fence carries no language (or an indented block, which has no fence).
    pub fn code_language_at_caret(&mut self) -> Option<String> {
        let start = self.code_block_start_at_caret()?;
        wysiwyg::code_language(&self.source, start)
    }

    /// Whether the caret stands in a fenced code block — the one a language
    /// prompt could edit. A frontend gates its "set language" affordance on this
    /// (an indented block, which can't carry a language, reports `false`).
    pub fn caret_in_fenced_code(&mut self) -> bool {
        self.code_block_start_at_caret()
            .is_some_and(|start| wysiwyg::code_info_span(&self.source, start).is_some())
    }

    /// Set (or clear, with `""`) the language of the fenced code block the caret
    /// is in — the prompt's confirm. A no-op when the caret is in no fenced
    /// block, and a reported error for a language the format's fence cannot
    /// carry.
    ///
    /// twig rewrites the info string, so the fence's own width — measured
    /// against a body neither side touches — is kept, and a language holding a
    /// space, a line end or the fence character is refused rather than written
    /// out to reparse as something else. Leaf used to splice over the info span
    /// itself and `trim()` the input, which handled the one bad case it had
    /// thought of.
    pub fn set_code_language(&mut self, lang: &str) {
        // The read-only gate — this door reaches twig without the splice.
        if self.read_only {
            return;
        }
        if self.refuse_unsupported("code language", Gesture::SetCodeLanguage) {
            return;
        }
        if self.code_block_start_at_caret().is_none() {
            return;
        }
        let lang = lang.trim();
        // `None` clears the info string; `Some("")` asks for an empty one. Both
        // write a bare fence, and the prompt's empty value means "clear".
        let want = (!lang.is_empty()).then_some(lang);
        self.record_caret();
        match self.editor.set_code_language(self.caret, want) {
            Ok(_) => {
                self.last_edit_kind = None;
                self.refresh();
                self.anchor = None;
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.clamp_caret();
                self.record_caret();
            }
            Err(e) => self.status = Some(format!("code language: {e}")),
        }
    }

    /// The `span.start` of the code block covering the caret — the anchor
    /// [`wysiwyg::code_info_span`] reads the fence from. `None` when the caret is
    /// in none.
    fn code_block_start_at_caret(&mut self) -> Option<usize> {
        let off = self.caret;
        self.nodes()
            .into_iter()
            .filter(|n| n.kind == Kind::CodeBlock && n.span.start <= off && off <= n.span.end)
            .max_by_key(|n| n.span.start)
            .map(|n| n.span.start)
    }

    /// The source range of the text inside the link covering `off` — what sits
    /// between its `[` and `]`. `None` when twig reports no link there.
    fn link_text_span(&mut self, off: usize) -> Option<std::ops::Range<usize>> {
        self.nodes()
            .into_iter()
            // Two links can touch (`[a](x)[b](y)`), and then one's `span.end` is
            // the other's `span.start`; the link that starts latest at or before
            // `off` is the one `off` is actually in.
            .filter(|n| n.kind == Kind::Link && n.span.start <= off && off < n.span.end)
            .max_by_key(|n| n.span.start)
            .and_then(|n| n.content_span)
    }

    // ── undo / redo ───────────────────────────────────────────────────────────
    // twig owns the history of *bytes* (it owns the buffer) and now carries the
    // caret through it too: `record_caret` stashes each state's caret in twig's
    // opaque per-step blob, and undo/redo hand it back with the source they
    // restore. So leaf keeps no history of its own — no parallel stacks to march
    // in lockstep and silently drift out of it.

    /// Undo the last edit step (⌘Z / ^Z), putting the caret and selection back
    /// where they were when that step began. Whether it did: `false` when
    /// there was nothing to undo, the document is read-only, or twig refused —
    /// the answer a host composing several histories into one walks on by.
    pub fn undo(&mut self) -> bool {
        if self.read_only {
            return false;
        }
        // Stepping through the history ends a group: what comes after is not
        // part of the step just taken back.
        self.close_undo_group();
        let (undone, redoable) = (self.undo_steps, self.redo_steps);
        match self.editor.undo() {
            Ok(Some(change)) => {
                self.after_history(change);
                // `refresh` counted the restore as an edit; it was a step back.
                self.undo_steps = undone.saturating_sub(1);
                self.redo_steps = redoable + 1;
                true
            }
            Ok(None) => {
                self.undo_steps = 0;
                self.status = Some("nothing to undo".into());
                false
            }
            Err(e) => {
                self.status = Some(format!("undo: {e}"));
                false
            }
        }
    }

    /// Redo the last undone edit step (⇧⌘Z / ^Y), putting the caret and
    /// selection back where that step originally left them. Whether it did,
    /// as [`undo`](Self::undo) reports.
    pub fn redo(&mut self) -> bool {
        if self.read_only {
            return false;
        }
        self.close_undo_group();
        let (undone, redoable) = (self.undo_steps, self.redo_steps);
        match self.editor.redo() {
            Ok(Some(change)) => {
                self.after_history(change);
                // `refresh` counted the restore as an edit; it was a step forward.
                self.undo_steps = (undone + 1).min(UNDO_CAP);
                self.redo_steps = redoable.saturating_sub(1);
                true
            }
            Ok(None) => {
                self.redo_steps = 0;
                self.status = Some("nothing to redo".into());
                false
            }
            Err(e) => {
                self.status = Some(format!("redo: {e}"));
                false
            }
        }
    }

    /// Fold the edit just made into the undo step before it — twig's
    /// `coalesce_last_undo`, with the `undo_steps` mirror following
    /// it. twig merges only when there are two steps to merge. Call it after
    /// [`refresh`](Self::refresh) has counted the edit being folded.
    ///
    /// Inside an [undo group](Self::begin_undo_group) it does nothing: the
    /// group's edits are already one step, which [`refresh`](Self::refresh)
    /// folds as they come, so a fold here could only reach across the group's
    /// start into the step before it.
    fn coalesce_last_undo(&mut self) {
        if self.undo_group.is_some() {
            return;
        }
        self.fold_last_undo();
    }

    /// twig's `coalesce_last_undo`, unconditionally, with the mirror following.
    fn fold_last_undo(&mut self) {
        if self.editor.coalesce_last_undo().is_ok() && self.undo_steps >= 2 {
            self.undo_steps -= 1;
        }
    }

    /// Open an undo group: every edit from here to the matching
    /// [`end_undo_group`](Self::end_undo_group) is one step, which a single
    /// [`undo`](Self::undo) takes back whole and a single [`redo`](Self::redo)
    /// puts back — a Replace All over twelve matches, or a Writing Tools
    /// session, rather than twelve presses of ⌘Z.
    ///
    /// Groups nest, and only the outermost end closes one. The step is folded
    /// as each edit lands (twig's `coalesce_last_undo`, one edit at a time),
    /// so a group of a thousand edits holds one step on the history and never
    /// meets twig's cap. Nothing typed before the group folds into it, and
    /// nothing typed after: it is a step of its own, even when it is a single
    /// edit. A group no edit landed in leaves no step at all.
    ///
    /// An [`undo`](Self::undo) or [`redo`](Self::redo) closes any open group,
    /// whatever its depth — the history has moved, and an edit made after it
    /// is not part of the step it moved past. A later `end_undo_group` is
    /// then a no-op.
    pub fn begin_undo_group(&mut self) {
        match &mut self.undo_group {
            Some(g) => g.depth += 1,
            None => {
                self.undo_group = Some(UndoGroup {
                    depth: 1,
                    has_step: false,
                });
                // The first edit inside is not a continuation of the typing
                // before it.
                self.last_edit_kind = None;
            }
        }
    }

    /// Close the undo group [`begin_undo_group`](Self::begin_undo_group)
    /// opened. A no-op when none is open.
    pub fn end_undo_group(&mut self) {
        let Some(g) = &mut self.undo_group else {
            return;
        };
        if g.depth > 1 {
            g.depth -= 1;
        } else {
            self.close_undo_group();
        }
    }

    /// Whether an undo group is open.
    pub fn in_undo_group(&self) -> bool {
        self.undo_group.is_some()
    }

    /// Close any open group, however deeply nested.
    fn close_undo_group(&mut self) {
        if self.undo_group.take().is_some() {
            // Typing after the group is not a continuation of the last edit
            // in it.
            self.last_edit_kind = None;
        }
    }

    /// Refresh the cached source and put the caret back where the step being
    /// undone/redone had it, clearing any active run.
    ///
    /// The caret comes from twig's blob for the restored state (what
    /// `record_caret` stored). `change` is only the fallback for a state with no
    /// blob — a caret at the end of the restored text, which is where this always
    /// landed before the blobs were kept. It is the edit site, not where the user
    /// was standing, so it's a floor and not the behaviour: undoing should hand
    /// back the document *and* the place you were working, which for an edit made
    /// anywhere but under the caret are two different places.
    fn after_history(&mut self, change: Change) {
        self.refresh();
        match self
            .editor
            .caret_blob()
            .ok()
            .and_then(|b| CaretState::from_blob(&b))
        {
            Some(state) => {
                self.caret = state.caret.min(self.source.len());
                self.anchor = state.anchor.map(|a| a.min(self.source.len()));
            }
            None => {
                self.caret = change.new.end.min(self.source.len());
                self.anchor = None;
            }
        }
        self.goal_col = None;
        self.last_edit_kind = None;
        self.dirty = self.source != self.clean_source;
        self.status = None;
        self.clamp_caret();
    }

    // ── the file ──────────────────────────────────────────────────────────────

    #[cfg(feature = "fs")]
    pub fn save(&mut self) {
        if self.is_untitled() {
            // No path to write and no name to invent: ⌘S on an untitled document
            // is a Save As, and only a frontend has a picker to ask with. Say so
            // rather than failing at the filesystem with an empty path.
            self.status = Some("untitled — save as…".into());
            return;
        }
        let path = self.path.clone();
        if self.write(&path) {
            self.mark_saved();
        }
    }

    /// Save As: write the document to `path` and *move* it there — `self.path`
    /// becomes `path`, and every later [`Doc::save`] writes the new file. That's
    /// what Save As means; a copy would leave the user editing a document whose
    /// name is no longer where their keystrokes go.
    ///
    /// The move only happens if the bytes actually landed. A failed write leaves
    /// the path, `dirty`, and the disk watermark exactly as they were, with the
    /// same `save failed: …` status a failed [`Doc::save`] sets — the document
    /// must never come away believing it was saved.
    ///
    /// An existing `path` is overwritten, and the caller is the one that knows
    /// whether to ask first: a Save As picker has already run that prompt, and a
    /// second confirmation from down here would be the same question twice.
    ///
    /// `format` does **not** follow the new extension. The buffer is parsed as
    /// the format it was opened with, and re-reading it as another one is a
    /// conversion — a different, lossy operation that would throw away the undo
    /// history — not a rename. So `notes.md` saved as `notes.dj` holds Markdown
    /// in a `.dj` file, and `format_name()` keeps honestly saying `markdown`
    /// until it's reopened.
    #[cfg(feature = "fs")]
    pub fn save_as(&mut self, path: PathBuf) {
        if !self.write(&path) {
            return;
        }
        self.path = path;
        self.mark_saved();
    }

    /// Put `source` on disk at `path`, reporting whether it got there. The one
    /// place leaf writes a document, so a save and a Save As can't disagree
    /// about what a failure looks like.
    #[cfg(feature = "fs")]
    fn write(&mut self, path: &Path) -> bool {
        match std::fs::write(path, self.source.as_bytes()) {
            Ok(()) => true,
            Err(e) => {
                self.status = Some(format!("save failed: {e}"));
                false
            }
        }
    }

    /// Re-base the document's saved watermark to the current bytes: clears
    /// `dirty`, records `source` as the new clean state (so undoing back to here
    /// clears the flag again), and re-stamps the on-disk hash.
    ///
    /// [`Doc::save`]/[`Doc::save_as`] call this after a write lands. It is also
    /// the hook a **filesystem-free host** calls itself once it has persisted
    /// [`Doc::source`] its own way (a browser download, `localStorage`, a backend
    /// `PUT`) — which is why it is public and touches no filesystem: the bytes
    /// are already where that host wants them, and this just tells the model they
    /// are safe.
    pub fn mark_saved(&mut self) {
        self.clean_source = self.source.clone();
        self.dirty = false;
        // The bytes on disk are now ours, so this is the new watermark: without
        // re-stamping it, every save would report its own work as an external
        // change forever after.
        self.disk_hash = Some(hash_bytes(self.source.as_bytes()));
        self.status = Some(format!("saved {}", self.file_name()));
    }

    /// [`Doc::mark_saved`] for a host whose write is not instantaneous: `saved`
    /// is the source it read before writing, and the edits made since are
    /// still unsaved.
    ///
    /// A host that reads [`Doc::source`], hands it to a write that takes a
    /// while, and then calls `mark_saved` loses whatever was typed meanwhile:
    /// the flag is cleared over text that never reached the disk, so nothing
    /// saves it again. Here `saved` becomes the clean state and the disk
    /// watermark, and `dirty` is whether the buffer has moved on from it.
    pub fn mark_saved_as(&mut self, saved: &str) {
        self.clean_source = saved.to_string();
        self.dirty = self.source != self.clean_source;
        self.disk_hash = Some(hash_bytes(saved.as_bytes()));
        if !self.dirty {
            self.status = Some(format!("saved {}", self.file_name()));
        }
    }

    /// What the file looks like now against the bytes leaf last read or wrote.
    ///
    /// Reads the file and hashes it (see `disk_hash` for why it isn't an mtime),
    /// so this is a filesystem round-trip, not a per-frame question — ask it
    /// when a window regains focus, on a timer, or before a save.
    ///
    /// This *only* reports the file. Whether the document also has unsaved edits
    /// is `dirty`, and the interesting case is the conjunction: `dirty` plus
    /// [`DiskState::Changed`] means a save overwrites someone's work and a
    /// [`Doc::reload`] discards the user's. leaf-core deliberately won't choose —
    /// it has no way to ask — so it hands a frontend both halves and lets it put
    /// the question to the person who can answer it.
    #[cfg(feature = "fs")]
    pub fn disk_state(&self) -> DiskState {
        let Some(want) = self.disk_hash else {
            return DiskState::Untitled;
        };
        match std::fs::read(&self.path) {
            Ok(bytes) if hash_bytes(&bytes) == want => DiskState::Unchanged,
            Ok(_) => DiskState::Changed,
            Err(e) if e.kind() == std::io::ErrorKind::NotFound => DiskState::Missing,
            Err(_) => DiskState::Unreadable,
        }
    }

    /// Re-read the file and replace the document with what's there — the other
    /// answer to a [`DiskState::Changed`].
    ///
    /// **Discards unsaved changes, unconditionally.** It doesn't check `dirty`
    /// first: a frontend that wants to protect unsaved work asks (`dirty` +
    /// [`Doc::disk_state`]) *before* calling this, and one reloading a clean
    /// document shouldn't have to argue with a guard.
    ///
    /// **The undo history survives, and the reload is one step in it.** The
    /// whole buffer is spliced with the file's bytes through the same door every
    /// other edit goes through, as an [`EditKind::Other`] that coalesces with
    /// nothing on either side — so ^Z after a formatter or a `git checkout` has
    /// swapped the document out from under a reader gives them back what they
    /// were looking at, marked dirty, and ^Z again carries on into whatever they
    /// had done before it. This used to build a fresh parse and drop the stack,
    /// on the reasoning that twig's history belongs to the buffer and these are
    /// different bytes; that is true of *rebasing* a step onto them and not of
    /// recording the swap itself as one, which is all this is. A splice twig
    /// won't take falls back to the fresh parse, and only that path still costs
    /// the history.
    ///
    /// The caret keeps its byte offset, clamped to the new length; the selection
    /// is dropped. Anything cleverer would be a lie: leaf doesn't know how the
    /// file changed, so it can't know where the caret "still" is. Clamping keeps
    /// it where the user left it in the common case (a change further down the
    /// file, or none in the text they're sitting in), and never puts it
    /// somewhere invalid. A selection has two such offsets and no such excuse —
    /// silently reinterpreting one over changed bytes would arm the *next*
    /// keystroke to delete something the user never selected.
    ///
    /// Nothing is touched unless the whole reload succeeds; a failure leaves the
    /// document alone with a status.
    #[cfg(feature = "fs")]
    pub fn reload(&mut self) {
        if self.is_untitled() {
            self.status = Some("no file to reload".into());
            return;
        }
        let bytes = match std::fs::read(&self.path) {
            Ok(b) => b,
            Err(e) => {
                self.status = Some(format!("reload failed: {e}"));
                return;
            }
        };
        let Ok(source) = String::from_utf8(bytes) else {
            self.status = Some("reload failed: file is not UTF-8".into());
            return;
        };
        // Already these bytes — someone saved a file back unchanged, or leaf's
        // own write is being read back. Re-baseline against it and stop: a
        // splice of the text onto itself would put an undo step on the stack for
        // something nobody did.
        if source == self.source {
            self.disk_hash = Some(hash_bytes(source.as_bytes()));
            self.clean_source = source;
            self.dirty = false;
            self.status = Some(format!("reloaded {}", self.file_name()));
            return;
        }
        let caret = self.caret;
        // The pre-reload caret, so undoing the swap puts it back where the
        // reader was standing — the same bracketing `splice_exact` does.
        self.record_caret();
        if self
            .editor
            .edit_range(0, self.source.len(), &source)
            .is_ok()
        {
            self.refresh();
        } else {
            // twig wouldn't take the splice. Start over from the bytes, which is
            // what this always did, and is the one path that still costs the
            // history — `format` is the format this document *is*, not what the
            // (unchanged) name now says, see `save_as`.
            match new_editor(source.as_bytes(), self.format) {
                Ok(editor) => {
                    self.editor = editor;
                    // A fresh editor, and so a fresh history — with no
                    // step in it for an open group to fold into.
                    self.undo_steps = 0;
                    self.redo_steps = 0;
                    if let Some(g) = &mut self.undo_group {
                        g.has_step = false;
                    }
                    self.source = source.clone();
                    // Not going through `refresh`, so the revision has to move
                    // here or every frontend keeps painting the old file from
                    // cache.
                    self.revision += 1;
                }
                Err(e) => {
                    self.status = Some(format!("reload failed: {e}"));
                    return;
                }
            }
        }
        self.disk_hash = Some(hash_bytes(source.as_bytes()));
        self.clean_source = self.source.clone();
        self.caret = caret.min(self.source.len());
        self.anchor = None;
        self.goal_col = None;
        self.last_edit_kind = None;
        self.dirty = false;
        self.status = Some(format!("reloaded {}", self.file_name()));
        self.clamp_caret();
        // And the post-reload caret, so a redo restores it.
        self.record_caret();
    }

    /// Re-read the source from twig after it has changed the document. The one
    /// funnel every edit, undo, and redo comes through — so it's where the
    /// revision moves, and anything cached against the text dies here.
    fn refresh(&mut self) {
        if let Ok(s) = self.editor.source_str() {
            self.source = s;
        }
        self.revision += 1;
        // An edit is a step onto the history and the end of anything undone;
        // `undo`/`redo` come through here too and correct this after.
        self.undo_steps = (self.undo_steps + 1).min(UNDO_CAP);
        self.redo_steps = 0;
        // Inside a group, fold each step into the group's first as it lands.
        // Not for `undo`/`redo`, which close the group before they get here.
        if let Some(g) = &mut self.undo_group {
            if g.has_step {
                self.fold_last_undo();
            } else {
                g.has_step = true;
            }
        }
        self.clamp_caret();
    }

    /// Whether [`undo`](Self::undo) has a step to take back — for a native
    /// Edit menu to enable its item by, and exactly when `undo` would move.
    pub fn can_undo(&self) -> bool {
        !self.read_only && self.undo_steps > 0
    }

    /// Whether [`redo`](Self::redo) has an undone step to restore.
    pub fn can_redo(&self) -> bool {
        !self.read_only && self.redo_steps > 0
    }

    // ── caret movement ─────────────────────────────────────────────────────────
    // `extend` grows the selection (Shift+motion): it pins the anchor on the
    // first extended step and moves only the caret; an un-extended motion drops
    // the selection.

    /// Place the caret at byte `offset` (clamped to a char boundary), extending
    /// the selection when `extend` is set. The public form of `move_to`, for a
    /// frontend that hit-tests pixels straight to a source offset.
    pub fn place_caret(&mut self, offset: usize, extend: bool) {
        self.goal_col = None;
        let before = self.caret;
        // A pixel hit-test can land between the visible caret stops — in the
        // blank gap a paragraph break is drawn with, or inside a hidden delimiter.
        // Snap to the nearest real stop so the caret can't come to rest where it
        // would draw in one place and type in another. The `(row, col)` click
        // path (`click`) already snaps this way through `offset_of_pos`; the
        // source view reaches every byte, so it snaps to nothing.
        let target = match self.view {
            View::Wysiwyg => self.vmap.snap_to_stop(offset.min(self.source.len())),
            // The source view reaches every byte, so there is no stop to snap
            // to — but "every byte" still means every *character* boundary. A
            // caret resting inside a multi-byte character draws nowhere real
            // and panics the next time anything slices there.
            View::Source => self.char_boundary_at_or_before(offset),
        };
        self.move_to(target, extend);
        self.clamp_caret();
        self.debug_assert_on_a_stop(before);
    }

    /// Select the whole document (⌘A / Ctrl+A) — everything reachable in the
    /// active view, so in WYSIWYG it starts below hidden frontmatter (copy won't
    /// grab the metadata) while the source view still selects the literal whole.
    pub fn select_all(&mut self) {
        self.anchor = Some(self.caret_floor());
        self.caret = self.source.len();
        self.goal_col = None;
        self.last_edit_kind = None;
        self.status = None;
    }

    /// Select the word (or whitespace / punctuation run) at `offset` — the
    /// double-click gesture. Anchors on the run's start with the caret at its
    /// end so a following Shift-motion extends from the far edge.
    pub fn select_word_at(&mut self, offset: usize) {
        let (s, e) = word_range_at(&self.source, offset.min(self.source.len()));
        self.anchor = Some(s);
        self.caret = e;
        self.goal_col = None;
        self.last_edit_kind = None;
        self.status = None;
        self.clamp_caret();
    }

    /// Select the whole enclosing text block (paragraph, heading, list item's
    /// text…) at `offset` — the triple-click gesture. Reads the range straight
    /// from the AST (twig's `content_span`), so it selects the entire *logical*
    /// paragraph even when that paragraph soft-wraps across several visual rows —
    /// where a visual-row-based select breaks down, because one source offset at
    /// a wrap boundary belongs to two rows at once.
    pub fn select_block_at(&mut self, offset: usize) {
        let off = offset.min(self.source.len());
        let range = self
            .editor
            .ancestors_at(off)
            .ok()
            .and_then(|chain| {
                // Ancestors run root → deepest; the deepest node that is neither
                // an inline span nor a multi-block container is the text block
                // the caret sits in (a paragraph, a heading, a code block…).
                chain
                    .into_iter()
                    .rev()
                    .find(|m| !wysiwyg::is_inline_kind(&m.kind) && !is_block_container(&m.kind))
                    .map(|m| m.content_span.unwrap_or(m.span))
            })
            .unwrap_or_else(|| source_line_range(&self.source, off));
        self.anchor = Some(range.start.min(self.source.len()));
        self.caret = range.end.min(self.source.len());
        self.goal_col = None;
        self.last_edit_kind = None;
        self.status = None;
        self.clamp_caret();
    }

    /// Select the exact source range `[start, end)` — anchor at `start`, caret
    /// at `end` — without snapping either end to a visible caret stop.
    ///
    /// The one caret verb that takes a range it was *handed* rather than one it
    /// worked out, for a host that already knows the bytes it means: a search
    /// hit, an annotation's footprint, a quote re-anchored through
    /// [`Doc::selection_quote`]. [`place_caret`](Self::place_caret) is the
    /// wrong tool for that, and not by a little — it snaps to the nearest
    /// *visible* stop, and where a range butts up against a hidden delimiter
    /// the nearest stop is the one before it, so selecting the "needle" of
    /// `**needle**` comes back with "needl" and an edit against it strands the
    /// "e".
    ///
    /// What `place_caret` does that is bookkeeping rather than snapping still
    /// happens here, because a host handing in a range is not asking to opt out
    /// of the invariants:
    ///
    /// - both ends are clamped into the document and up to
    ///   [`caret_floor`](Self::caret_floor) — in WYSIWYG the leading
    ///   frontmatter is hidden, and a caret parked in it draws nowhere and
    ///   types into the metadata;
    /// - both land on character boundaries, so nothing slices a `é` in half;
    /// - the sticky vertical goal column is dropped, and any armed inline mark
    ///   disarmed, since a range from outside inherits neither.
    ///
    /// An empty range is a caret rather than a selection —
    /// [`selection`](Self::selection) reports `None` for it, as it does for any
    /// anchor that has met the caret.
    pub fn select_range(&mut self, start: usize, end: usize) {
        let floor = self.caret_floor();
        let anchor = self.char_boundary_at_or_before(start.clamp(floor, self.source.len()));
        let caret = self.char_boundary_at_or_before(end.clamp(floor, self.source.len()));
        self.anchor = Some(anchor);
        self.caret = caret;
        self.goal_col = None;
        self.status = None;
        self.last_edit_kind = None;
        self.clear_pending();
    }

    /// `offset` itself if it is a character boundary, else the boundary before
    /// it. An offset that isn't one draws nowhere real and panics the next time
    /// anything slices there.
    fn char_boundary_at_or_before(&self, offset: usize) -> usize {
        let mut o = offset.min(self.source.len());
        while o > 0 && !self.source.is_char_boundary(o) {
            o -= 1;
        }
        o
    }

    /// The lowest source offset the caret may occupy in the active view. In
    /// WYSIWYG, leading frontmatter is hidden and unreachable, so the floor is
    /// the first rendered offset; the source view reaches everything, so it's 0.
    fn caret_floor(&self) -> usize {
        match self.view {
            View::Wysiwyg => self.vmap.content_start.min(self.source.len()),
            View::Source => 0,
        }
    }

    /// Land in a table cell with its whole content selected — the anchor at the
    /// cell's start, the caret at its end — so a Tab/Return hop into a cell reads
    /// like tabbing into a form field: the text comes up selected, so typing
    /// replaces it and an arrow collapses to an edge. An empty cell (`start ==
    /// end`) collapses to a plain caret home (an empty selection is no selection).
    fn select_cell(&mut self, start: usize, end: usize) {
        self.select_range(start, end);
    }

    fn move_to(&mut self, offset: usize, extend: bool) {
        if extend {
            if self.anchor.is_none() {
                self.anchor = Some(self.caret);
            }
        } else {
            self.anchor = None;
        }
        self.caret = offset.min(self.source.len()).max(self.caret_floor());
        self.status = None;
        // A caret move ends the current typing/deletion run, so the next edit
        // starts a fresh undo group rather than coalescing across the gap.
        self.last_edit_kind = None;
        // Moving away disarms any sticky mark — "start bold" applies only where
        // it was asked for, not wherever the caret next lands.
        self.clear_pending();
    }

    // In the source view, motion walks source bytes / source lines. In the
    // WYSIWYG view it walks the rendered glyph grid (the visual map), which is
    // what steps the caret cleanly over hidden delimiters.

    pub fn move_left(&mut self, extend: bool) {
        self.goal_col = None;
        if !extend && let Some((s, _e)) = self.selection() {
            self.move_to(s, false);
            return;
        }
        let target = match self.view {
            View::Source => {
                if self.caret > 0 {
                    prev_boundary(&self.source, self.caret)
                } else {
                    0
                }
            }
            // Walks caret *stops*, not columns: decoration (a table border, a
            // cell's padding) is stepped over in one press, and a hidden
            // delimiter never holds the caret up — though the end of a mark's
            // content is a stop of its own (`VisualMap::mark_ends`), so
            // leaving `**bold**` from past its `**` is a press onto the end of
            // the bold and another onto the `d`.
            View::Wysiwyg => self
                .vmap
                .caret_stop_before(self.caret)
                .unwrap_or(self.caret),
        };
        let before = self.caret;
        self.move_to(target, extend);
        self.debug_assert_on_a_stop(before);
    }

    pub fn move_right(&mut self, extend: bool) {
        self.goal_col = None;
        if !extend && let Some((_s, e)) = self.selection() {
            self.move_to(e, false);
            return;
        }
        let target = match self.view {
            View::Source => {
                if self.caret < self.source.len() {
                    next_boundary(&self.source, self.caret)
                } else {
                    self.caret
                }
            }
            View::Wysiwyg => self.vmap.caret_stop_after(self.caret).unwrap_or(self.caret),
        };
        let before = self.caret;
        self.move_to(target, extend);
        self.debug_assert_on_a_stop(before);
    }

    /// Move to the start of the previous word (⌥← / Ctrl+←).
    pub fn move_word_left(&mut self, extend: bool) {
        self.goal_col = None;
        let before = self.caret;
        let target = self.word_left_from(self.caret);
        self.move_to(target, extend);
        self.debug_assert_on_a_stop(before);
    }

    /// Move to the end of the next word (⌥→ / Ctrl+→).
    pub fn move_word_right(&mut self, extend: bool) {
        self.goal_col = None;
        let before = self.caret;
        let target = self.word_right_from(self.caret);
        self.move_to(target, extend);
        self.debug_assert_on_a_stop(before);
    }

    // Word boundaries are found in the space the *view* is in. The source view
    // walks the source, because there the source is what's rendered. WYSIWYG
    // walks the rendered text instead: `**` is invisible to the user, so it has
    // to be invisible to word motion too — a caret parked inside one draws in
    // the column after `bold` and types two bytes earlier, and a word-delete
    // that stops there shreds the markup into `a ** c`.

    /// The word boundary to the left of `off` in the active view's space.
    fn word_left_from(&self, off: usize) -> usize {
        match self.view {
            View::Source => prev_word(&self.source, off),
            View::Wysiwyg => self.glyph_word_left(off),
        }
    }

    /// The word boundary to the right of `off` in the active view's space.
    fn word_right_from(&self, off: usize) -> usize {
        match self.view {
            View::Source => next_word(&self.source, off),
            View::Wysiwyg => self.glyph_word_right(off),
        }
    }

    /// The character class of the glyph drawn at stop `off`.
    ///
    /// Read from the source, because a stop points at the source byte its glyph
    /// came from — the source *is* where the rendered character is written. What
    /// makes the walk glyph space rather than source space is that it only ever
    /// visits stops, and the hidden bytes between them have none.
    fn class_at(&self, off: usize) -> Class {
        self.source
            .get(off..)
            .and_then(|s| s.chars().next())
            .map_or(Class::Space, classify)
    }

    /// [`next_word`] in glyph space: skip any leading separators, then consume
    /// the following word run, with the stop table standing in for the source's
    /// characters.
    fn glyph_word_right(&self, from: usize) -> usize {
        let Some(mut off) = self.vmap.stop_at_or_after(from) else {
            return from;
        };
        let mut in_word = false;
        loop {
            match self.class_at(off) {
                Class::Word => in_word = true,
                _ if in_word => return off,
                _ => {}
            }
            match self.vmap.stop_after(off) {
                Some(next) => off = next,
                None => return off,
            }
        }
    }

    /// [`prev_word`] in glyph space: skip separators walking left, then consume
    /// the preceding word run.
    fn glyph_word_left(&self, from: usize) -> usize {
        let Some(mut off) = self.vmap.stop_at_or_before(from) else {
            return from;
        };
        let mut in_word = false;
        while let Some(prev) = self.vmap.stop_before(off) {
            match self.class_at(prev) {
                Class::Word => in_word = true,
                _ if in_word => return off,
                _ => {}
            }
            off = prev;
        }
        off
    }

    /// After a motion that walks the visual map, the caret must be *on* the map.
    /// A stop is the only offset where the caret draws and edits in the same
    /// place, and it's the invariant both a caret parked inside an emoji and one
    /// parked inside a `**` were quietly breaking.
    ///
    /// Only when the caret actually moved: a walk with nowhere to go leaves it
    /// where it was, which is wherever the floor or a frontend put it rather
    /// than somewhere this motion chose.
    fn debug_assert_on_a_stop(&self, before: usize) {
        debug_assert!(
            self.view != View::Wysiwyg
                || self.vmap.num_rows() == 0
                || self.caret == before
                || self.vmap.is_stop(self.caret),
            "motion left the caret at {}, which is not a caret stop: it would draw in \
             one place and type in another",
            self.caret
        );
    }

    // Up and Down run off the ends of the document rather than stopping dead at
    // them: Up from the first row lands at the document's start, Down from the
    // last at its end. That's Cocoa's rule (`moveUp:`/`moveDown:` past the edge
    // are `moveToBeginningOfDocument:`/`moveToEndOfDocument:`), and holding ↓
    // reaching the end of the text is what a reader means by it.
    //
    // The views used to disagree here by accident rather than by decision: the
    // source view fell into the edge behaviour through `row_col_to_offset`
    // clamping an out-of-range row to the end of the string, while WYSIWYG had
    // no row below to walk to and did nothing at all. They share the rule now,
    // each in its own space — the source view reaches every byte, WYSIWYG only
    // the offsets it draws.

    pub fn move_up(&mut self, extend: bool) {
        let (row, col) = self.caret_pos();
        let goal = self.goal_col.unwrap_or(col);
        let target = match self.view {
            View::Source => match row.checked_sub(1) {
                Some(r) => row_col_to_offset(&self.source, r, goal),
                None => self.reachable_start(),
            },
            // A table's border rules are drawn but hold no caret, so Up steps
            // over them to the row that does.
            View::Wysiwyg => match self.vmap.navigable_above(row) {
                Some(r) => self.row_target(r, goal),
                None => self.reachable_start(),
            },
        };
        self.step_vertical(target, goal, extend);
    }

    pub fn move_down(&mut self, extend: bool) {
        let (row, col) = self.caret_pos();
        let goal = self.goal_col.unwrap_or(col);
        let target = match self.view {
            View::Source => match self.source_row_below(row) {
                Some(r) => row_col_to_offset(&self.source, r, goal),
                None => self.reachable_end(),
            },
            View::Wysiwyg => match self.vmap.navigable_below(row) {
                Some(r) => self.row_target(r, goal),
                None => self.reachable_end(),
            },
        };
        self.step_vertical(target, goal, extend);
    }

    /// Land a vertical motion at `target`, latching the `goal` column it aimed
    /// with so the rest of the run keeps aiming there.
    ///
    /// A motion with nowhere to go changes *nothing*, the goal column included:
    /// the latch used to run before the early return at the top of the document,
    /// so an Up that did nothing still armed a column, and the next Down aimed
    /// at one the caret had never been in.
    fn step_vertical(&mut self, target: usize, goal: usize, extend: bool) {
        let before = self.caret;
        if target == before {
            return;
        }
        self.goal_col = Some(goal);
        self.move_to(target, extend);
        self.debug_assert_on_a_stop(before);
    }

    /// The source line below `row`, or `None` when `row` is the last one. Lines
    /// are counted by newline, so a trailing one leaves a real, empty last line
    /// for the caret to sit on — the document ends below it, not on it.
    fn source_row_below(&self, row: usize) -> Option<usize> {
        let last = self.source.bytes().filter(|&b| b == b'\n').count();
        (row < last).then_some(row + 1)
    }

    /// Where a vertical motion aiming at the `goal` column lands on visual row
    /// `r`: the column clamped to the row, mapped to its offset, then held
    /// inside the row's own [bounds](Self::row_bounds) — a wrapped row's last
    /// column belongs to the row below, and a gutter's column 0 points at the
    /// block rather than at this row.
    fn row_target(&self, r: usize, goal: usize) -> usize {
        let (start, end) = self.row_bounds(r);
        self.vmap
            .offset_of_pos(r, goal.min(self.vmap.row_width(r)))
            .clamp(start, end)
    }

    /// The first and last offsets the caret can reach in the active view.
    ///
    /// Not the same span in both: the source view shows every byte, so it can
    /// reach every byte. WYSIWYG reaches only what it draws — hidden frontmatter
    /// sits below the first stop, and a document's trailing newline is drawn
    /// nowhere and so sits past the last.
    fn reachable_start(&self) -> usize {
        match self.view {
            View::Source => 0,
            View::Wysiwyg => self.vmap.stop_at_or_after(0).unwrap_or(self.caret),
        }
    }

    fn reachable_end(&self) -> usize {
        match self.view {
            View::Source => self.source.len(),
            View::Wysiwyg => self
                .vmap
                .stop_at_or_before(self.source.len())
                .unwrap_or(self.caret),
        }
    }

    /// The `[start, end]` offsets visual row `r` *draws* — everything on it,
    /// including the space a soft wrap ate off its end, which is drawn on this
    /// row however much the offset past it belongs to the next one.
    fn row_span(&self, r: usize) -> (usize, usize) {
        let start = self
            .vmap
            .row_start(r)
            .unwrap_or_else(|| self.vmap.offset_of_pos(r, 0));
        let end = self.vmap.offset_of_pos(r, self.vmap.row_width(r));
        (start.min(end), end)
    }

    /// [`row_span`](Self::row_span) narrowed to where the caret can stand: a
    /// soft wrap's shared offset opens the row below (see `pos_of_offset`), so
    /// this row's last position is the one before it — the offset before the
    /// space the wrap ate, where the caret draws just past the row's last word
    /// and types there too.
    ///
    /// Aiming at the shared offset instead is what stalled End: it is the row's
    /// last *column*, so End pressed on the row reached it and then read back as
    /// the row below's start, where a second press ran on to that row's end and
    /// the next to the one after — End walking down the paragraph a row a press.
    fn row_bounds(&self, r: usize) -> (usize, usize) {
        let (start, end) = self.row_span(r);
        let wraps = self
            .vmap
            .navigable_below(r)
            .and_then(|b| self.vmap.row_start(b))
            .is_some_and(|off| off == end);
        match wraps {
            true => (start, self.vmap.stop_before(end).unwrap_or(end).max(start)),
            false => (start, end),
        }
    }

    /// The `[start, end]` of the line Home and End aim at: the visual row in
    /// WYSIWYG, the logical line in the source view. Both ends are caret stops.
    ///
    /// A soft-wrapped row is a line here, because it is one to the eye and the
    /// eye is what these keys are aimed by — a reader pressing End means the end
    /// of the line they can see. (`select_block_at` wants the opposite and reads
    /// the AST for it: a triple-click grabs the whole paragraph, however many
    /// rows it folds into.)
    fn line_bounds(&self) -> (usize, usize) {
        let (row, _) = self.caret_pos();
        match self.view {
            View::Source => {
                let start = line_start(&self.source, row);
                (start, line_end_from(&self.source, start))
            }
            View::Wysiwyg => self.row_bounds(row),
        }
    }

    /// The same line as [`line_bounds`](Self::line_bounds), as far as it is
    /// *drawn* — what a kill takes.
    ///
    /// The two part only at a soft wrap, over the space the wrap ate: the caret
    /// can't stand after it (that offset opens the row below, and End stopping
    /// there would walk), but it is on this row, and a kill that spared it would
    /// leave a double space behind where the row's text had been. Deleting it
    /// joins nothing — a wrap is drawn, not written.
    fn line_span(&self) -> (usize, usize) {
        let (row, _) = self.caret_pos();
        match self.view {
            View::Source => self.line_bounds(),
            View::Wysiwyg => self.row_span(row),
        }
    }

    /// The first offset in `[start, end]` holding something other than
    /// whitespace, or `end` when the line holds nothing else — where Home aims.
    ///
    /// Walks the space the view is in, as word motion does: WYSIWYG steps stops,
    /// so a hidden delimiter is never taken for the line's first character (nor
    /// landed on), and the source view steps the source it is showing.
    fn first_non_space(&self, start: usize, end: usize) -> usize {
        let mut off = start;
        while off < end {
            if self.class_at(off) != Class::Space {
                return off;
            }
            off = match self.view {
                View::Source => next_boundary(&self.source, off),
                View::Wysiwyg => match self.vmap.stop_after(off) {
                    Some(next) => next,
                    None => return end,
                },
            };
        }
        end
    }

    /// Home: to the first character on the line, or to column 0 when the caret
    /// is already on it — the two-press toggle every editor spells this way.
    /// The indentation is somewhere the caret has to be able to reach and almost
    /// never where a reader is headed, so it costs the second press.
    pub fn move_home(&mut self, extend: bool) {
        self.goal_col = None;
        let (start, end) = self.line_bounds();
        let text = self.first_non_space(start, end);
        let target = if self.caret == text { start } else { text };
        let before = self.caret;
        self.move_to(target, extend);
        self.debug_assert_on_a_stop(before);
    }

    /// End: to the end of the line.
    pub fn move_end(&mut self, extend: bool) {
        self.goal_col = None;
        let (_, end) = self.line_bounds();
        let before = self.caret;
        self.move_to(end, extend);
        self.debug_assert_on_a_stop(before);
    }

    /// Hop to the next (Tab) or previous (Shift+Tab) table cell, landing with the
    /// cell's whole content selected (see [`Self::select_cell`]). Returns `false`
    /// when the caret isn't in a table, or is already in the last/first cell — the
    /// frontend then does whatever Tab normally does (indent), so Tab keeps its
    /// meaning everywhere else.
    pub fn cell_hop(&mut self, forward: bool) -> bool {
        let Some((grid, r, c)) = self.table_grid_at(self.caret) else {
            return false;
        };
        // Flatten to document (row-major) order and step one cell either way.
        let i: usize = grid[..r].iter().map(Vec::len).sum::<usize>() + c;
        let flat: Vec<(usize, usize)> = grid.into_iter().flatten().collect();
        let next = if forward {
            i.checked_add(1)
        } else {
            i.checked_sub(1)
        };
        let Some(&(start, end)) = next.and_then(|j| flat.get(j)) else {
            return false; // at the table's edge; leave Tab to the frontend
        };
        self.select_cell(start, end);
        true
    }

    /// Move the caret to the cell directly above (`down == false`) or below in
    /// the same column, landing with the cell's whole content selected (see
    /// [`Self::select_cell`]). Returns `false` at the grid's top/bottom edge (or
    /// when the caret isn't in a table), so the frontend can fall through — the
    /// vertical counterpart of [`Self::cell_hop`].
    ///
    /// A ragged row that is short a column clamps to its last cell, so Down never
    /// falls out of the table over a gap the row above happened to have.
    pub fn cell_move_vertical(&mut self, down: bool) -> bool {
        let Some((grid, r, c)) = self.table_grid_at(self.caret) else {
            return false;
        };
        let target = match down {
            true => r + 1,
            false if r == 0 => return false,
            false => r - 1,
        };
        let Some(row) = grid.get(target) else {
            return false;
        };
        let Some(&(start, end)) = row.get(c).or_else(|| row.last()) else {
            return false;
        };
        self.select_cell(start, end);
        true
    }

    /// The table containing `off` as a row-major grid of `(start, end)` cell
    /// caret homes, plus the `(row, col)` the caret sits in — `None` when `off`
    /// isn't in a table. Read straight off the visual map's laid-out grid, so
    /// every cell (an empty one included, whose derived home twig gives no
    /// `content_span` for) is present and in the order Tab walks them.
    // Grid, row, column — three returns that only ever travel together, and a
    // named type for the pair of them would be read at one call site.
    #[allow(clippy::type_complexity)]
    fn table_grid_at(&self, off: usize) -> Option<(Vec<Vec<(usize, usize)>>, usize, usize)> {
        for t in &self.vmap.tables {
            let mut pos = None;
            let grid: Vec<Vec<(usize, usize)>> = t
                .grid
                .iter()
                .enumerate()
                .map(|(r, row)| {
                    row.cells
                        .iter()
                        .enumerate()
                        .map(|(c, cell)| {
                            if pos.is_none() && off >= cell.start && off <= cell.end {
                                pos = Some((r, c));
                            }
                            (cell.start, cell.end)
                        })
                        .collect()
                })
                .collect();
            if let Some((r, c)) = pos {
                return Some((grid, r, c));
            }
        }
        None
    }

    // ── table key policy ──────────────────────────────────────────────────────
    // The three keys a table gives its own meaning — Tab, Return, Shift+Return —
    // as one policy every frontend shares, rather than each re-deriving it. Each
    // reports whether it acted *as a table key*; a `false` hands the key back to
    // the frontend's ordinary handling (indent, newline) so it keeps its meaning
    // everywhere else.

    /// Tab / Shift+Tab inside a table. Tab steps to the next cell, appending a
    /// fresh row and entering it when it runs off the last one; Shift+Tab steps
    /// back and simply stays put at the very first cell. `false` when the caret
    /// isn't in a table.
    pub fn cell_tab(&mut self, forward: bool) -> bool {
        if !self.caret_in_table() {
            return false;
        }
        if self.cell_hop(forward) {
            return true;
        }
        // Off the last cell: grow the table by a row and step into its first
        // cell. (Shift+Tab at the first cell has nowhere to go and just holds.)
        if forward {
            self.append_row_and_enter(0);
        }
        true
    }

    /// Return inside a table: drop to the cell below in the same column,
    /// appending a new row when the caret is already in the last one. `false`
    /// when the caret isn't in a table, so the frontend inserts a newline.
    pub fn cell_return(&mut self) -> bool {
        if !self.caret_in_table() {
            return false;
        }
        if self.cell_move_vertical(true) {
            return true;
        }
        // Already on the last row: grow one below and drop into the same column.
        let col = self.table_grid_at(self.caret).map_or(0, |(_, _, c)| c);
        self.append_row_and_enter(col);
        true
    }

    /// Append a row below the caret's (last) row and land in `col` of it. The
    /// caret is in the last row, so twig's "insert below" makes the fresh row the
    /// table's new last — but twig re-spells the whole table, moving every byte,
    /// so the destination is read back from the rebuilt grid by the table's
    /// position (stable across a row insert), not from the pre-edit caret.
    fn append_row_and_enter(&mut self, col: usize) {
        let table = self.caret_table_index();
        self.table_insert_row(true);
        self.rebuild_map();
        let Some((start, end)) = table
            .and_then(|ti| self.vmap.tables.get(ti))
            .and_then(|t| t.grid.last())
            .and_then(|row| row.cells.get(col.min(row.cells.len().saturating_sub(1))))
            .map(|cell| (cell.start, cell.end))
        else {
            return;
        };
        self.select_cell(start, end);
    }

    /// The index, among the document's tables, of the one the caret sits in —
    /// `None` when it's in none. Used to re-find a table after an edit re-spells
    /// it (a row insert leaves the table order unchanged).
    fn caret_table_index(&self) -> Option<usize> {
        let off = self.caret;
        self.vmap.tables.iter().position(|t| {
            t.grid
                .iter()
                .any(|row| row.cells.iter().any(|c| off >= c.start && off <= c.end))
        })
    }

    /// Shift+Return inside a table: insert a hard line break *within* the current
    /// cell, via twig's `insert_line_break`. `false` when the caret isn't in a
    /// table, so the frontend inserts an ordinary line break.
    ///
    /// A table row is a single source line, so the newline-spelled hard break
    /// can't live in a cell. twig spells the in-cell break the format's way
    /// (`<br>` for Markdown) and reparses it as a *semantic* `hard_break`, so the
    /// break round-trips as structure the renderer reads back as a line — not the
    /// opaque raw HTML the old raw-splice left behind.
    ///
    /// Djot has no idiomatic in-cell break, so twig refuses it
    /// (`UnsupportedFormat`) rather than emit a `<br>` that any other djot reader
    /// would render as the literal text `<br>`. The gesture is still *consumed*
    /// there — returning `false` would let the frontend insert a real newline,
    /// which splits the one-line row — it just leaves the cell unchanged and says
    /// so on the status line. A rollback (`EditConflict`) is swallowed the same.
    ///
    /// Which formats refuse is [`Capabilities::cell_line_break`], and the two
    /// have to be read together: djot is not the only `false`, and naming it in
    /// the message was already a guess that HTML — which spells the break as its
    /// own `<br>` — would have made wrong.
    pub fn cell_line_break(&mut self) -> bool {
        if self.read_only || !self.caret_in_table() {
            return false;
        }
        self.record_caret();
        match self.editor.insert_line_break(self.caret) {
            Ok(change) => {
                self.last_edit_kind = None;
                self.refresh();
                self.caret = change.new.end;
                self.anchor = None;
                self.goal_col = None;
                self.clamp_caret();
                self.dirty = self.source != self.clean_source;
                self.status = None;
                self.record_caret();
            }
            Err(twig::Error::UnsupportedFormat) => {
                self.status = Some(format!(
                    "in-cell line breaks aren't supported in {}",
                    self.format_name()
                ));
            }
            Err(_) => {}
        }
        true
    }

    /// Rebuild the visual map at the width the last build used. A structural edit
    /// bumps the revision and swaps the source in, but leaves the *map* stale;
    /// when a single gesture edits and then moves over the result (Tab appending
    /// a row, then stepping into it), the move needs the map to already show the
    /// edit rather than waiting for the frontend's next frame.
    fn rebuild_map(&mut self) {
        let wrap = self.vmap_key.as_ref().and_then(|(_, w, _)| *w);
        self.build_map(wrap);
    }

    /// Move the caret to the very start of the document (⌘↑ on macOS,
    /// Ctrl+Home on Windows/Linux).
    pub fn move_doc_start(&mut self, extend: bool) {
        self.goal_col = None;
        self.move_to(0, extend);
    }

    /// Move the caret to the very end of the document (⌘↓ on macOS,
    /// Ctrl+End on Windows/Linux).
    pub fn move_doc_end(&mut self, extend: bool) {
        self.goal_col = None;
        let end = self.source.len();
        self.move_to(end, extend);
    }

    /// Point the caret at the body cell `(row, col)` the mouse landed on —
    /// `col` being a cell of the terminal grid, which is what a display column
    /// is. A click on the far cell of a wide character lands at that
    /// character's start; the mapping's own doc-comments carry the rule.
    pub fn click(&mut self, row: usize, col: usize, extend: bool) {
        self.goal_col = None;
        let target = match self.view {
            View::Source => row_col_to_offset(&self.source, row, col),
            View::Wysiwyg => self.vmap.offset_of_pos(row, col),
        };
        let before = self.caret;
        self.move_to(target, extend);
        self.debug_assert_on_a_stop(before);
    }

    /// A click in the blank space under the document's last block.
    ///
    /// Not a click *on* anything, so it lands on nothing in particular: the
    /// caret goes onto an empty paragraph under the last block, wherever the
    /// pointer was horizontally — and if the document does not end with one,
    /// one is opened, which is the only way to get out from under a block Enter
    /// cannot leave. Enter inside a fenced code block is a literal newline (see
    /// [`newline`](Self::newline)), so a document that *ends* in a fence had no
    /// way out at all; and a click under any last block used to land at the
    /// pointer's x on the block's last line, which is what a click on that line
    /// means and not what a click under it does.
    ///
    /// "Ends with an empty paragraph" is two trailing newlines: the first closes
    /// the last line and the second opens the blank line the visual map lays
    /// out as a navigable empty row (see `emit_trailing_blank_lines`). A
    /// document ending inside an *unclosed* fence gets the fence closed first,
    /// since a newline written there would only be another line of code. An
    /// empty document has nothing to be under, and the caret simply goes to its
    /// start.
    ///
    /// In the source view the gesture is the ordinary one: the caret goes to the
    /// end of the source, and nothing is written. A read-only document likewise.
    pub fn click_past_end(&mut self) {
        self.goal_col = None;
        let len = self.source.len();
        if self.view == View::Source || self.read_only || self.source.trim().is_empty() {
            self.move_to(len, false);
            return;
        }
        let mut tail = String::new();
        if let Some(fence) = self.unclosed_fence_at_end() {
            if !self.source.ends_with('\n') {
                tail.push('\n');
            }
            tail.push_str(&fence);
            tail.push('\n');
        }
        let joined = format!("{}{tail}", self.source);
        let trailing = joined.len() - joined.trim_end_matches('\n').len();
        for _ in trailing..2 {
            tail.push('\n');
        }
        if !tail.is_empty() && !self.splice(len, len, &tail, EditKind::Other) {
            return;
        }
        let end = self.source.len();
        self.move_to(end, false);
    }

    /// The closing fence a document ending inside an unclosed fenced code block
    /// needs — the opening fence's own run, behind the quote prefix the block
    /// wears — or `None` when the last block is closed, indented, or not a code
    /// block at all.
    ///
    /// Unclosed is when twig's content span reaches the block's end: a closing
    /// fence line would lie between the two. `code_info_span`'s read of the
    /// fence is not used because it starts at the block's span, which inside a
    /// quote is the quote marker rather than the fence.
    fn unclosed_fence_at_end(&mut self) -> Option<String> {
        let content_end = self.source.trim_end_matches('\n').len();
        let block = self
            .nodes()
            .into_iter()
            .filter(|n| n.kind == Kind::CodeBlock && n.span.end >= content_end)
            .max_by_key(|n| n.span.start)?;
        if block.content_span.as_ref()?.end < block.span.end {
            return None;
        }
        let line = self.source[block.span.start..].lines().next()?;
        let opening = line.trim_start_matches(['>', ' ', '\t']);
        let fence = opening.chars().next().filter(|c| matches!(c, '`' | '~'))?;
        let run: String = opening.chars().take_while(|&c| c == fence).collect();
        let prefix = self.quote_prefix_at(block.span.start);
        Some(format!("{prefix}{run}"))
    }

    /// Settle `scroll` for a frame about to be drawn: follow the caret onto the
    /// screen if it has moved since the last frame, and never scroll past the
    /// last of `rows`.
    ///
    /// Only if it has *moved* — that's the whole point. Revealing the caret on
    /// every frame ties the viewport to it, and a scroll wheel that fights the
    /// caret for the viewport loses: the view snaps back the instant it tries to
    /// pass the caret's row, so the document can't be scrolled beyond what's
    /// already on screen. A caret move is the frontend's cue to follow; a scroll
    /// with the caret sitting still is the reader's cue to leave it alone.
    pub fn follow_caret(&mut self, caret_row: usize, height: usize, rows: usize) {
        if self.drawn_caret != Some(self.caret) {
            if caret_row < self.scroll {
                self.scroll = caret_row;
            } else if height > 0 && caret_row >= self.scroll + height {
                self.scroll = caret_row + 1 - height;
            }
            self.drawn_caret = Some(self.caret);
        }
        self.scroll = self.scroll.min(rows.saturating_sub(1));
    }

    /// The caret's screen position `(row, col)` in the active view's grid, with
    /// `col` a display column: the cell to draw the caret in, which on a line of
    /// `你好` or emoji is not the count of characters before it.
    pub fn caret_pos(&self) -> (usize, usize) {
        match self.view {
            View::Source => offset_to_row_col(&self.source, self.caret),
            View::Wysiwyg => self.vmap.pos_of_offset(self.caret),
        }
    }

    fn clamp_caret(&mut self) {
        if self.caret > self.source.len() {
            self.caret = self.source.len();
        }
        // In WYSIWYG the caret can't sit inside hidden frontmatter; lift it (and
        // any selection anchor) to the first rendered offset.
        let floor = self.caret_floor();
        if self.caret < floor {
            self.caret = floor;
        }
        if let Some(a) = self.anchor
            && a < floor
        {
            self.anchor = Some(floor);
        }
        while self.caret > 0 && !self.source.is_char_boundary(self.caret) {
            self.caret -= 1;
        }
    }
}

// ── byte-offset ⇄ (row, col) helpers ─────────────────────────────────────────

// Left/right motion and backspace/delete step by *grapheme cluster*, not
// codepoint, so an emoji (a ZWJ sequence) or a base letter plus its combining
// marks moves and deletes as the single character a user sees. Grapheme
// boundaries are a superset of char boundaries, so the caret stays valid for twig.

/// How an insert of `text` groups for undo: a single typed character folds into
/// the run of typing around it, while a newline or a multi-character insert is a
/// step of its own.
fn typed_edit_kind(text: &str) -> EditKind {
    if text.chars().take(2).count() == 1 && text != "\n" {
        EditKind::Insert
    } else {
        EditKind::Other
    }
}

fn prev_boundary(s: &str, i: usize) -> usize {
    let mut cursor = GraphemeCursor::new(i, s.len(), true);
    cursor.prev_boundary(s, 0).ok().flatten().unwrap_or(0)
}

fn next_boundary(s: &str, i: usize) -> usize {
    let mut cursor = GraphemeCursor::new(i, s.len(), true);
    cursor.next_boundary(s, 0).ok().flatten().unwrap_or(s.len())
}

// ── word boundaries ──────────────────────────────────────────────────────────
// The shared primitive behind word-wise motion, word deletion, and
// double-click-to-select-a-word. A "word" is a maximal run of one character
// class; whitespace and punctuation are their own classes, so motion skips
// cleanly between them the way native text fields do.

#[derive(PartialEq, Eq, Clone, Copy)]
enum Class {
    Word,
    Space,
    Other,
}

/// The source range of an inline node's own visible text — the part of it a
/// WYSIWYG caret can reach, as against the delimiters that only spell it.
/// `None` for a node with no interior to empty (a `str`, a break).
///
/// twig reports no `content_span` for `verbatim`/`inline_math`, whose text sits
/// one delimiter in from the span — the same place the renderer maps it to. A
/// longer fence (`` ``a`` ``) breaks that assumption, so the guess is checked
/// against the source rather than trusted: a range guessed wrong here is text
/// deleted wrong.
fn inline_content_span(n: &FlatNode, source: &str) -> Option<std::ops::Range<usize>> {
    if let Some(span) = n.content_span.clone() {
        return Some(span);
    }
    match n.kind.as_str() {
        "verbatim" | "inline_math" => {
            let text = n.text.as_ref()?;
            let start = n.span.start + 1;
            let range = start..start + text.len();
            (source.get(range.clone()) == Some(text.as_str())).then_some(range)
        }
        _ => None,
    }
}

/// The `id` a node declares, or `None` for one that declares none — the
/// attribute djot writes for a `{#v1}` and mints for a heading.
///
/// A bare attribute (`{#v1 hidden}`'s `hidden`) has no value, and a bare `id`
/// names nothing, so it reads as absent rather than as the empty string.
fn declared_id(n: &FlatNode) -> Option<&str> {
    n.attrs.iter().find(|(k, _)| k == "id")?.1.as_deref()
}

/// A heading's words reduced to the form a link fragment spells them in:
/// lowercase, runs of anything else collapsed to a single `-`, with none left
/// dangling at either end. `## Some Heading Here` → `some-heading-here`.
///
/// The rule every Markdown renderer follows, and applied to djot's own auto-ids
/// too so that `#some-heading-here` and `#Some-Heading-Here` are one question.
/// Unicode-aware (`is_alphanumeric`, not an ASCII test), because a heading in
/// any other language is still a heading someone will link to. Underscores
/// survive for the same reason they do on the web: they are word characters
/// wherever identifiers are written.
fn slug(text: &str) -> String {
    let mut out = String::new();
    let mut pending = false;
    for c in text.chars() {
        if c.is_alphanumeric() || c == '_' {
            if pending && !out.is_empty() {
                out.push('-');
            }
            pending = false;
            out.extend(c.to_lowercase());
        } else {
            pending = true;
        }
    }
    out
}

/// The text of the inline run starting at `first` and its siblings, markup
/// stripped: each leaf's payload in order, a line break as a space. `nodes` is
/// the arena `Doc::nodes` returns, indexed by id.
fn inline_text(nodes: &[FlatNode], first: Option<NodeId>, out: &mut String) {
    let mut next = first;
    while let Some(id) = next {
        let Some(n) = nodes.get(id.0 as usize) else {
            return;
        };
        match (&n.text, &n.kind) {
            (Some(text), _) => out.push_str(text),
            (None, Kind::SoftBreak | Kind::HardBreak) => out.push(' '),
            (None, _) => inline_text(nodes, n.first_child, out),
        }
        next = n.next_sibling;
    }
}

fn is_block_container(kind: &Kind) -> bool {
    matches!(
        kind,
        Kind::Doc
            | Kind::Section
            | Kind::BlockQuote
            | Kind::BulletList
            | Kind::OrderedList
            | Kind::TaskList
            | Kind::ListItem
            | Kind::TaskListItem
            // Every `container` — a directive in any of its three forms, or a
            // promoted HTML element. A *text* directive is really inline, so
            // claiming it here is a small overreach, and the deliberate one this
            // function's kind-only peer `is_inline_kind` documents: the pair is
            // consulted together, and answering "block container" for something
            // inline is what keeps an ancestor walk from stopping short of the
            // paragraph that actually holds it.
            | Kind::Container
    )
}

/// The `[start, end)` byte range of the source line containing `off` (newline
/// excluded) — the fallback when `off` sits outside any AST block (e.g. a blank
/// line between paragraphs).
fn source_line_range(s: &str, off: usize) -> std::ops::Range<usize> {
    let off = off.min(s.len());
    let start = s[..off].rfind('\n').map(|p| p + 1).unwrap_or(0);
    let end = s[off..].find('\n').map(|p| off + p).unwrap_or(s.len());
    start..end
}

/// How many leading bytes an outdent takes off `line`: a whole indent level
/// where the line has one, and whatever it has where it has less.
///
/// A leading tab counts as a level on its own. It's indentation some other
/// editor wrote, and one tab is one level everywhere it came from — measuring it
/// in spaces it doesn't contain would leave it untouchable.
fn outdent_width(line: &str, unit: usize) -> usize {
    if line.starts_with('\t') {
        return 1;
    }
    line.bytes().take(unit).take_while(|b| *b == b' ').count()
}

/// A list marker found at the head of a line, together with everything before it
/// that a sibling line has to repeat.
///
/// The three offsets differ only inside a block quote, where `>   - b` opens with
/// a `> ` quote marker the line's own text doesn't own. Outside one they collapse:
/// `line_start == marker_start`, and `text` is the plain `"  - "`.
#[derive(Clone, Debug)]
struct ListMarker {
    /// The line's first byte.
    line_start: usize,
    /// Where the marker proper begins, past any quote prefix. The offset to hand
    /// the AST: a quoted item's span opens at its bullet, not at the `>`.
    marker_start: usize,
    /// `line_start` through the marker's trailing space — quote prefix, indent
    /// and bullet together, which is what the next item's line opens with.
    text: String,
}

impl ListMarker {
    /// Where the item's content starts — one past the marker's trailing space.
    fn content_start(&self) -> usize {
        self.line_start + self.text.len()
    }
}

fn classify(c: char) -> Class {
    if c == '_' || c.is_alphanumeric() {
        Class::Word
    } else if c.is_whitespace() {
        Class::Space
    } else {
        Class::Other
    }
}

/// The offset at the end of the next word to the right of `i` (⌥→ / Ctrl+→):
/// skip any leading separators, then consume the following word run.
fn next_word(s: &str, i: usize) -> usize {
    let mut off = i;
    let mut in_word = false;
    for c in s[i..].chars() {
        if classify(c) == Class::Word {
            in_word = true;
        } else if in_word {
            break;
        }
        off += c.len_utf8();
    }
    off
}

/// The offset at the start of the word to the left of `i` (⌥← / Ctrl+←):
/// skip separators walking left, then consume the preceding word run.
fn prev_word(s: &str, i: usize) -> usize {
    let mut off = i;
    let mut in_word = false;
    for c in s[..i].chars().rev() {
        if classify(c) == Class::Word {
            in_word = true;
        } else if in_word {
            break;
        }
        off -= c.len_utf8();
    }
    off
}

/// The `[start, end)` run of same-class characters surrounding `off` — the
/// word (or whitespace/punctuation run) a double-click selects. At end-of-text
/// the run ending there is used.
fn word_range_at(s: &str, off: usize) -> (usize, usize) {
    if s.is_empty() {
        return (0, 0);
    }
    let off = off.min(s.len());
    let reference = if off < s.len() {
        s[off..].chars().next()
    } else {
        s[..off].chars().next_back()
    };
    let Some(rc) = reference else {
        return (off, off);
    };
    let class = classify(rc);

    let mut start = off;
    for c in s[..start].chars().rev() {
        if classify(c) == class {
            start -= c.len_utf8();
        } else {
            break;
        }
    }
    let mut end = off;
    for c in s[end..].chars() {
        if classify(c) == class {
            end += c.len_utf8();
        } else {
            break;
        }
    }
    (start, end)
}

/// `(row, col)` of byte offset `off`, `col` counted in *display columns* from
/// the line's start — terminal cells, not characters, so the column names the
/// cell the caret is drawn in even on a line of `你好` or emoji.
fn offset_to_row_col(s: &str, off: usize) -> (usize, usize) {
    let off = off.min(s.len());
    let mut row = 0;
    let mut line_start = 0;
    for (i, &b) in s.as_bytes().iter().enumerate() {
        if i >= off {
            break;
        }
        if b == b'\n' {
            row += 1;
            line_start = i + 1;
        }
    }
    (row, wysiwyg::text_width(&s[line_start..off]))
}

/// The byte offset at display column `col` of `row` (clamped to that line's
/// end) — the inverse of [`offset_to_row_col`], which it has to agree with.
///
/// A column landing *inside* a character — the second cell of `你`, or any cell
/// but the first of an emoji — resolves to that character's start, which is the
/// column the caret would have been drawn at to begin with. So both cells of a
/// wide character mean the character, and every offset survives the round trip
/// out to a column and back. The walk steps by grapheme cluster for the same
/// reason the caret does: a cluster is the character, and the cells belong to it
/// rather than to the codepoints spelling it.
fn row_col_to_offset(s: &str, row: usize, col: usize) -> usize {
    let start = line_start(s, row);
    let end = line_end_from(s, start);
    let mut off = start;
    let mut at = 0; // the display column `off` sits at
    while off < end {
        let next = next_boundary(s, off).min(end);
        let cells = wysiwyg::text_width(&s[off..next]);
        if at + cells > col {
            break; // `col` is one of this cluster's own cells
        }
        at += cells;
        off = next;
    }
    off
}

fn line_start(s: &str, row: usize) -> usize {
    if row == 0 {
        return 0;
    }
    let mut r = 0;
    for (i, &b) in s.as_bytes().iter().enumerate() {
        if b == b'\n' {
            r += 1;
            if r == row {
                return i + 1;
            }
        }
    }
    s.len()
}

fn line_end_from(s: &str, start: usize) -> usize {
    s[start..].find('\n').map(|p| start + p).unwrap_or(s.len())
}

/// twig's node-kind name for an inline mark, back to the [`InlineKind`] a
/// frontend names when it calls [`Doc::toggle`] — the inverse of the mapping
/// twig applies writing the mark out, so the toolbar can light the same button
/// that made the node.
///
/// `None` for every other kind, including the inline nodes that aren't marks at
/// all (`str`, `link`, `image`, the math and break kinds): they're things a
/// caret stands in, not formatting a button toggles.
/// Whether a match from an ancestor chain is an inline run whose delimiters
/// the rich view draws nothing for — a mark (`**`, `_`, `==`), or an
/// attributed span: `<span data-size="large">…</span>`, djot's `[…]{…}`. The
/// span is a [`Kind::Container`], which the kind alone cannot tell from a
/// block `<div>`, so the chain's caller passes [`Doc::run_span_ids`] and the
/// answer is the node's own. Every delete and caret step that walks over a
/// `**` walks over a span's tags by this test; without it Backspace after
/// `</span>` took the `>` and left the paragraph unparseable.
fn hides_delims(m: &QueryMatch, run_spans: &[NodeId]) -> bool {
    inline_kind(&m.kind).is_some() || run_spans.contains(&NodeId(m.node_id))
}

fn inline_kind(kind: &Kind) -> Option<InlineKind> {
    Some(match kind {
        Kind::Strong => InlineKind::Strong,
        Kind::Emph => InlineKind::Emph,
        Kind::Verbatim => InlineKind::Verbatim,
        Kind::Mark => InlineKind::Mark,
        Kind::Superscript => InlineKind::Superscript,
        Kind::Subscript => InlineKind::Subscript,
        Kind::Insert => InlineKind::Insert,
        Kind::Delete => InlineKind::Delete,
        _ => return None,
    })
}

/// leaf's [`MarkColor`] as twig's — the palette twig writes as the emoji after
/// a highlight's opening `==`.
///
/// Two enums for one closed vocabulary, and the duplication is the boundary
/// working: core's is what a *frontend* names (`style::MarkColor`, beside the
/// [`Role`](crate::Role) that carries it into the glyph map) and twig's is what
/// the editor writes. Spelled as a match rather than routed through the two
/// crates' name strings so that a colour added on either side is a compile
/// error here, where the pairing is decided, rather than a runtime `None` that
/// would read as "clear the colour".
fn twig_mark_color(color: MarkColor) -> twig::MarkColor {
    match color {
        MarkColor::Red => twig::MarkColor::Red,
        MarkColor::Orange => twig::MarkColor::Orange,
        MarkColor::Yellow => twig::MarkColor::Yellow,
        MarkColor::Green => twig::MarkColor::Green,
        MarkColor::Blue => twig::MarkColor::Blue,
        MarkColor::Purple => twig::MarkColor::Purple,
        MarkColor::Brown => twig::MarkColor::Brown,
    }
}

/// Where an offset lands after a splice it didn't make — twig's own rule, from
/// [`Change`]: shift anything at or past the replaced range's end by the length
/// the replacement gained or lost, and leave anything before it alone.
///
/// An offset *inside* the replaced range has no text of its own to ride any
/// more, and lands at the end of what replaced it: for
/// [`Doc::set_mark_color`] that is a caret standing on the colour prefix when
/// the prefix is cleared, which then sits where the highlighted text begins.
/// One node's attribute list, twig's own `(key, value)` pairs owned — what
/// every presentation gesture reads, edits one key of, and passes back whole.
type Attrs = Vec<(String, Option<String>)>;

/// The name of the leaf directive a page break is — [`Doc::insert_page_break`]
/// writes it and the walker draws it, and a frontend that paginates matches a
/// [`DirectiveMark`](crate::wysiwyg::DirectiveMark) against it. One spelling,
/// stated once.
pub const PAGE_BREAK: &str = "page-break";

/// `attrs` with `key` set to `value`, or removed when `value` is `None`, and
/// every other attribute kept in its place — the read-edit-write half of twig's
/// replace-not-merge contract for a `data-` key.
///
/// **A key that is already there is rewritten where it stands**, and only a key
/// the node did not have goes on the end. That is what makes the proposal's
/// worked example true: `class="lead center" id="intro"
/// data-line-height="1.5"`, right-aligned, is `class="lead right" id="intro"
/// data-line-height="1.5"` — the same document with one token changed, and a
/// one-line diff. Removing the key and pushing it back would reorder the
/// author's attributes on every press, so a document that passed through the
/// editor came out shuffled even where nothing about it had changed.
///
/// A duplicate key — which no format leaf opens can spell, but twig reports
/// verbatim — collapses onto the first of its copies, since twig is handed one
/// value for one key either way.
fn with_attr(attrs: &[(String, Option<String>)], key: &str, value: Option<&str>) -> Attrs {
    let mut out: Attrs = Vec::with_capacity(attrs.len() + 1);
    let mut written = false;
    for (k, v) in attrs {
        if k != key {
            out.push((k.clone(), v.clone()));
            continue;
        }
        if let Some(new) = value.filter(|_| !written) {
            out.push((k.clone(), Some(new.to_string())));
            written = true;
        }
    }
    if let Some(new) = value.filter(|_| !written) {
        out.push((key.to_string(), Some(new.to_string())));
    }
    out
}

/// [`with_attr`] for a `class` token: every token `mine` claims is removed, and
/// `token` added, with the rest of the list kept in order.
///
/// `class` is a space-separated token list, and leaf owns three of the tokens in
/// it. A paragraph that arrives as `class="lead center"` and is right-aligned
/// goes out as `class="lead right"`; one whose last owned token goes and which
/// carried nothing else loses the key, so a block that has lost its whole
/// vocabulary is spelled bare again. `class` itself keeps its place among the
/// attributes, because [`with_attr`] does the writing.
fn with_class_token(
    attrs: &[(String, Option<String>)],
    mine: impl Fn(&str) -> bool,
    token: Option<&str>,
) -> Attrs {
    let kept: Vec<&str> = attrs
        .iter()
        .find(|(k, _)| k == "class")
        .and_then(|(_, v)| v.as_deref())
        .unwrap_or_default()
        .split_whitespace()
        .filter(|t| !mine(t))
        .collect();
    let class = kept.into_iter().chain(token).collect::<Vec<_>>().join(" ");
    with_attr(
        attrs,
        "class",
        (!class.is_empty()).then_some(class.as_str()),
    )
}

/// An owned attribute list as the borrowed pairs twig's two attribute ops take.
///
/// A **bare** attribute — one twig reports with no value, such as HTML's `<p
/// hidden>` — is passed back as an empty one. Twig refuses a `None` outright
/// (djot has no bare attribute, so no format reads one back everywhere), and
/// `hidden=""` is the same document where `hidden` is; dropping it instead
/// would lose what the author wrote, which is the one thing these gestures
/// promise not to do.
fn attr_pairs(attrs: &[(String, Option<String>)]) -> Vec<(&str, Option<&str>)> {
    attrs
        .iter()
        .map(|(k, v)| (k.as_str(), Some(v.as_deref().unwrap_or_default())))
        .collect()
}

fn reanchor(off: usize, change: &Change) -> usize {
    if off < change.old.start {
        return off;
    }
    if off < change.old.end {
        return change.new.end;
    }
    (off + change.new.end).saturating_sub(change.old.end)
}

/// [`reanchor`] for an edit that respells the markup *around* a block and
/// leaves the block's own bytes alone — which is every attribute gesture.
///
/// `block` is that block's content span before and after the splice, so an
/// offset standing in the text keeps its distance from the text's start and how
/// many bytes twig wrote above it never enters the arithmetic. That is the whole
/// rule, and it is why nothing here knows how long a `<div …>` is: a second key
/// on the same div lengthens the attribute line, clearing the last one takes the
/// div away entirely, and both are the same sum. `None` where the splice named
/// no block at either end, which is every djot case — the `{…}` line is written
/// above the block, and the block itself only shifts past it.
///
/// Anywhere else it is `reanchor`'s own answer: untouched before the splice,
/// shifted by its delta after it, and at the splice's end for an offset that
/// stood in markup being rewritten — a caret inside djot's `{…}` line has no
/// text to keep.
fn reanchor_in_block(
    off: usize,
    change: &Change,
    block: Option<(&Range<usize>, &Range<usize>)>,
) -> usize {
    if let Some((was, now)) = block
        && was.start <= off
        && off <= was.end
    {
        return now.start + (off - was.start).min(now.end - now.start);
    }
    reanchor(off, change)
}

/// A watermark for a file's contents (see `Doc::disk_hash`).
///
/// `DefaultHasher` is not stable across Rust releases, which doesn't matter: a
/// watermark is compared only against one taken by the same process moments
/// earlier, and never outlives it. 64 bits leaves a collision — an external edit
/// that hashes to exactly what leaf wrote — at odds no filesystem race gets near.
fn hash_bytes(bytes: &[u8]) -> u64 {
    use std::hash::{Hash, Hasher};
    let mut h = std::collections::hash_map::DefaultHasher::new();
    bytes.hash(&mut h);
    h.finish()
}

#[cfg(feature = "fs")]
fn detect_format(path: &Path) -> Result<Format> {
    let ext = path
        .extension()
        .and_then(|e| e.to_str())
        .unwrap_or("")
        .to_ascii_lowercase();
    Ok(match ext.as_str() {
        "dj" | "djot" => Format::Djot,
        "md" | "markdown" => Format::Markdown,
        "xml" => Format::Xml,
        "html" | "htm" => Format::Html,
        other => return Err(anyhow!("unknown document extension: .{other}")),
    })
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::style::{FontFamily, LineSpacing, SizeStep};

    /// A document open in `view`. WYSIWYG motion reads the visual map, which the
    /// renderer stamps each frame, so the map is built here too — a WYSIWYG doc
    /// without one is a view no user is ever in.
    fn doc_in(view: View, name: &str, body: &str) -> Doc {
        // The fixture name doubles as the temp file's, so two tests picking the
        // same one raced under the parallel runner and read each other's body —
        // a green suite proving the wrong thing. The counter makes that
        // unreachable rather than asking every future caller to notice.
        static SEQ: std::sync::atomic::AtomicUsize = std::sync::atomic::AtomicUsize::new(0);
        let seq = SEQ.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
        let mut p = std::env::temp_dir();
        p.push(format!("leaf_test_{name}_{seq}.md"));
        std::fs::write(&p, body).unwrap();
        let mut d = Doc::open(p).unwrap();
        d.view = view;
        if view == View::Wysiwyg {
            d.build_visual(80);
        }
        d
    }

    // Source-view document for the source-behaviour tests. `Doc::open` now
    // defaults to WYSIWYG (leaf's default view), so pin the source view here;
    // `wysiwyg_doc` builds the rich-text variant on top of this.
    fn doc_with(name: &str, body: &str) -> Doc {
        doc_in(View::Source, name, body)
    }

    /// Every visual row's drawn text — what the reader actually sees, which is
    /// the only thing the reveal preference is supposed to change.
    fn drawn_rows(d: &Doc) -> Vec<String> {
        d.vmap
            .rows
            .iter()
            .map(|r| r.glyphs.iter().map(|g| g.ch).collect())
            .collect()
    }

    /// Put the caret at the first byte of `needle` and rebuild, so the row under
    /// it becomes the revealed line.
    fn caret_at(d: &mut Doc, needle: &str) {
        d.caret = d.source.find(needle).expect("needle in source");
        d.build_visual(80);
    }

    #[test]
    fn blockquote_after_a_list_is_not_bulleted() {
        // twig nests a following top-level block quote under the `bullet_list`
        // (a direct child, not a `list_item`). The map must render it de-nested —
        // `│ quote`, never `• │ quote` — with a blank separator, like any block
        // that follows a list. Regression for the "combined list + blockquote" bug.
        let mut d = doc_in(View::Wysiwyg, "bq_after_list", "- item\n\n> quote\n");
        d.build_visual(80);
        let rows: Vec<String> = d
            .vmap
            .rows
            .iter()
            .map(|r| r.glyphs.iter().map(|g| g.ch).collect())
            .collect();
        assert!(
            rows.iter().any(|r| r == "│ quote"),
            "block quote should render on its own gutter, got rows: {rows:?}"
        );
        assert!(
            !rows.iter().any(|r| r.contains('•') && r.contains('│')),
            "no row should carry both a bullet and a quote gutter, got rows: {rows:?}"
        );
    }

    // ── the map is built at most once per (revision, wrap) ───────────────────
    //
    // A frontend repaints for reasons that have nothing to do with the text — a
    // blinking caret, a scroll — and rebuilding the map is O(document). These
    // pin *that the cache fires*, which a passing suite can't tell you: a cache
    // that never hits is invisible to every other test in this file.
    //
    // The probe is to wreck the built map and ask for it again. A rebuild
    // repairs it; a cache hit hands the wreckage straight back. Nothing else
    // can distinguish the two from outside.

    #[test]
    fn a_rebuild_with_nothing_changed_reuses_the_map() {
        let mut d = doc_in(View::Wysiwyg, "cache_hit", "# Title\n\nbody\n");
        d.build_visual(80);
        assert!(!d.vmap.rows.is_empty());
        d.vmap.rows.clear(); // wreck it
        d.build_visual(80);
        assert!(
            d.vmap.rows.is_empty(),
            "the map was rebuilt though nothing changed — the cache never fired"
        );
    }

    #[test]
    fn an_edit_rebuilds_the_map() {
        let mut d = doc_in(View::Wysiwyg, "cache_edit", "# Title\n\nbody\n");
        d.build_visual(80);
        let before = d.revision();
        d.vmap.rows.clear();
        d.insert("x");
        d.build_visual(80);
        assert!(d.revision() > before, "an edit must move the revision");
        assert!(
            !d.vmap.rows.is_empty(),
            "an edited document must not paint from a stale map"
        );
    }

    #[test]
    fn a_width_change_rebuilds_the_map() {
        // The map is a function of the wrap width too, so a resize is a miss
        // even though the text is untouched.
        let mut d = doc_in(
            View::Wysiwyg,
            "cache_width",
            "one two three four five six\n",
        );
        d.build_visual(80);
        d.vmap.rows.clear();
        d.build_visual(12);
        assert!(!d.vmap.rows.is_empty(), "a resize must rebuild the map");
        // And the unwrapped map is its own key, not the same as any width.
        d.vmap.rows.clear();
        d.build_visual_unwrapped();
        assert!(!d.vmap.rows.is_empty(), "unwrapped is a different map");
    }

    #[test]
    fn a_motion_does_not_rebuild_the_map() {
        // The whole point: moving the caret changes nothing the map is built
        // from. If a motion bumped the revision, every arrow key would cost a
        // full rebuild and the cache would be worthless.
        let mut d = doc_in(View::Wysiwyg, "cache_motion", "# Title\n\nbody text\n");
        d.build_visual(80);
        let rev = d.revision();
        d.move_right(false);
        d.move_right(true);
        d.move_down(false);
        assert_eq!(d.revision(), rev, "a motion must not move the revision");
        d.vmap.rows.clear();
        d.build_visual(80);
        assert!(
            d.vmap.rows.is_empty(),
            "a motion should not rebuild the map"
        );
    }

    #[test]
    fn saving_does_not_rebuild_the_map() {
        // Saving changes `dirty`, not the text.
        let mut d = doc_in(View::Wysiwyg, "cache_save", "# Title\n\nbody\n");
        d.insert("x");
        d.build_visual(80);
        let rev = d.revision();
        d.save();
        assert_eq!(d.revision(), rev, "a save must not move the revision");
        assert!(!d.dirty, "the save should have cleaned the document");
    }

    #[test]
    fn a_reload_rebuilds_the_map() {
        // Reload replaces the text without going through `refresh`, so it has to
        // move the revision itself — else the editor paints the old file.
        let mut d = doc_in(View::Wysiwyg, "cache_reload", "# Title\n\nbody\n");
        d.build_visual(80);
        let rev = d.revision();
        std::fs::write(&d.path, "# Other\n\nwholly new\n").unwrap();
        d.reload();
        assert!(d.revision() > rev, "a reload must move the revision");
        d.build_visual(80);
        let text: String = d
            .vmap
            .rows
            .iter()
            .flat_map(|r| r.glyphs.iter().map(|g| g.ch))
            .collect();
        assert!(
            text.contains("wholly new"),
            "the reloaded text should be on screen, got {text:?}"
        );
    }

    // ── golden-case harness ──────────────────────────────────────────────────
    // The pattern the whole parity suite can reuse: write a fixture with the
    // caret marked by `|`, run one action, and compare the rendered result —
    // also caret-marked — against the expected string. One readable line per
    // behavior, and it exercises the exact `Doc` ops both frontends call.

    /// Split a `|`-marked fixture into `(source, caret_offset)`.
    fn parse_caret(marked: &str) -> (String, usize) {
        let caret = marked.find('|').expect("fixture needs a `|` caret marker");
        (marked.replacen('|', "", 1), caret)
    }

    /// Render a doc's source with `|` at the caret (and `[`…`]` around any
    /// selection) so a result reads like the fixtures.
    fn render_caret(d: &Doc) -> String {
        // (offset, rank, char); rank keeps coincident markers ordered `[ | ]`
        // so the caret always renders inside its own selection.
        let mut marks: Vec<(usize, u8, char)> = vec![(d.caret, 1, '|')];
        if let Some((s, e)) = d.selection() {
            marks.push((s, 0, '['));
            marks.push((e, 2, ']'));
        }
        // Insert right-to-left: descending offset, then descending rank.
        marks.sort_by(|a, b| b.0.cmp(&a.0).then(b.1.cmp(&a.1)));
        let mut out = d.source.clone();
        for (at, _, ch) in marks {
            out.insert(at, ch);
        }
        out
    }

    /// Load a `|`-marked fixture, run `action`, return the caret-marked result.
    fn golden(name: &str, marked: &str, action: impl FnOnce(&mut Doc)) -> String {
        golden_in(View::Source, name, marked, action)
    }

    /// [`golden`] in a chosen view — the editing ops are the view's to share, so
    /// the same fixture has to read the same way in both.
    fn golden_in(view: View, name: &str, marked: &str, action: impl FnOnce(&mut Doc)) -> String {
        let (src, caret) = parse_caret(marked);
        let mut d = doc_in(view, name, &src);
        d.caret = caret;
        action(&mut d);
        render_caret(&d)
    }

    #[test]
    fn word_motion_walks_word_by_word() {
        let g = |m, f: fn(&mut Doc)| golden("word_motion", m, f);
        assert_eq!(
            g("hello wor|ld", |d| d.move_word_left(false)),
            "hello |world"
        );
        assert_eq!(
            g("hello| world", |d| d.move_word_left(false)),
            "|hello world"
        );
        assert_eq!(
            g("hel|lo world", |d| d.move_word_right(false)),
            "hello| world"
        );
        assert_eq!(
            g("hello| world", |d| d.move_word_right(false)),
            "hello world|"
        );
        // Punctuation is its own class, so motion stops at the boundary.
        assert_eq!(g("|foo.bar", |d| d.move_word_right(false)), "foo|.bar");
    }

    #[test]
    fn word_motion_extends_the_selection_when_asked() {
        assert_eq!(
            golden("word_sel", "hello |world", |d| d.move_word_right(true)),
            "hello [world|]"
        );
    }

    #[test]
    fn delete_word_removes_a_whole_word() {
        let g = |m, f: fn(&mut Doc)| golden("del_word", m, f);
        assert_eq!(g("hello world|", |d| d.delete_word_back()), "hello |");
        assert_eq!(g("hello |world", |d| d.delete_word_forward()), "hello |");
        assert_eq!(g("foo |bar baz", |d| d.delete_word_back()), "|bar baz");
    }

    // ── Home / End ───────────────────────────────────────────────────────────

    #[test]
    fn home_toggles_between_the_line_s_text_and_its_margin() {
        // Source: the indentation is what the toggle is for. WYSIWYG resolves an
        // indent to the markup it spells everywhere it means one, so the fixture
        // with whitespace left to walk is a code block, which is verbatim.
        let g = |m, f: fn(&mut Doc)| golden("smart_home", m, f);
        assert_eq!(g("    inden|ted", |d| d.move_home(false)), "    |indented");
        assert_eq!(g("    |indented", |d| d.move_home(false)), "|    indented");
        assert_eq!(g("|    indented", |d| d.move_home(false)), "    |indented");
        // A line with no indentation has one place to go, so the toggle is a
        // no-op rather than a trip to nowhere.
        assert_eq!(g("hel|lo", |d| d.move_home(false)), "|hello");
        assert_eq!(g("|hello", |d| d.move_home(false)), "|hello");

        let mut d = wysiwyg_doc("smart_home_wys", "```\n    indented\n```\n");
        let indent = d.source.find("    indented").unwrap();
        d.caret = indent + 6; // inside "indented"
        d.move_home(false);
        assert_eq!(
            d.caret,
            indent + 4,
            "wysiwyg: Home aims at the code line's text"
        );
        d.move_home(false);
        assert_eq!(
            d.caret, indent,
            "wysiwyg: the second press takes the indent"
        );
        d.move_home(false);
        assert_eq!(d.caret, indent + 4, "wysiwyg: the toggle swaps back");
    }

    #[test]
    fn end_takes_the_line_the_view_is_showing() {
        // The line differs by view for the same document, and that is the point:
        // a bare newline inside a paragraph is a soft break, which WYSIWYG draws
        // as a space on one row and the source view as two lines.
        let mut d = doc_with("end_src", "one two\nthree\n");
        d.caret = 1;
        d.move_end(false);
        assert_eq!(d.caret, 7, "source: the end of the source line");

        let mut d = wysiwyg_doc("end_wys", "one two\nthree\n");
        d.caret = 1;
        d.move_end(false);
        assert_eq!(
            d.caret, 13,
            "wysiwyg: the end of the row, soft break and all"
        );
    }

    #[test]
    fn home_and_end_extend_the_selection_when_asked() {
        for (view, tag) in VIEWS {
            let mut d = doc_in(view, &format!("home_end_ext_{tag}"), "hello world");
            d.caret = 6;
            d.move_end(true);
            assert_eq!(d.selection(), Some((6, 11)), "{tag}: End extends");
            let mut d = doc_in(view, &format!("home_ext_{tag}"), "hello world");
            d.caret = 6;
            d.move_home(true);
            assert_eq!(d.selection(), Some((0, 6)), "{tag}: Home extends");
        }
    }

    // ── kill to the line's start / end ───────────────────────────────────────

    #[test]
    fn kill_to_the_line_start_and_end_in_both_views() {
        for (view, tag) in VIEWS {
            // The gap that reads as a paragraph break in each view: the source
            // view's lines are the renderer's rows only where the source says so.
            let gap = if view == View::Source { "\n" } else { "\n\n" };
            let mut d = doc_in(
                view,
                &format!("kill_end_{tag}"),
                &format!("one two{gap}three\n"),
            );
            d.caret = 3;
            d.delete_to_line_end();
            assert_eq!(
                d.source,
                format!("one{gap}three\n"),
                "{tag}: ^K to the line's end"
            );
            assert_eq!(d.caret, 3, "{tag}: the caret stays where it kills from");

            let mut d = doc_in(
                view,
                &format!("kill_start_{tag}"),
                &format!("one two{gap}three\n"),
            );
            d.caret = 7; // the end of the first line
            d.delete_to_line_start();
            assert_eq!(
                d.source,
                format!("{gap}three\n"),
                "{tag}: ⌘⌫ to the line's start"
            );
            assert_eq!(d.caret, 0, "{tag}");
        }
    }

    #[test]
    fn a_kill_at_the_line_s_edge_leaves_the_lines_joined() {
        // The decision: at the boundary both kills do nothing, rather than
        // eating the line break. "Line" is the view's own — in WYSIWYG it ends
        // at a soft wrap as often as at a newline, where there is nothing
        // written to delete — and a source newline is only half of the blank
        // line between two paragraphs, so taking it leaves a soft break rather
        // than the join it looks like. Backspace and Delete are the keys for it.
        for (view, tag) in VIEWS {
            let gap = if view == View::Source { "\n" } else { "\n\n" };
            let src = format!("one{gap}three\n");
            let mut d = doc_in(view, &format!("kill_edge_end_{tag}"), &src);
            d.caret = 3; // the end of "one"
            d.delete_to_line_end();
            assert_eq!(
                d.source, src,
                "{tag}: ^K at the line's end joined it to the next"
            );

            let mut d = doc_in(view, &format!("kill_edge_start_{tag}"), &src);
            d.caret = 3 + gap.len(); // the start of "three"
            d.delete_to_line_start();
            assert_eq!(
                d.source, src,
                "{tag}: ⌘⌫ at the line's start joined it to the last"
            );
        }
    }

    #[test]
    fn a_kill_takes_the_selection_when_there_is_one() {
        // What every other delete here does with one, so these two as well.
        for (view, tag) in VIEWS {
            for (name, kill) in [
                (
                    "end",
                    (|d: &mut Doc| d.delete_to_line_end()) as fn(&mut Doc),
                ),
                ("start", |d: &mut Doc| d.delete_to_line_start()),
            ] {
                let mut d = doc_in(view, &format!("kill_sel_{name}_{tag}"), "one two three\n");
                d.anchor = Some(4);
                d.caret = 7; // "two"
                kill(&mut d);
                assert_eq!(
                    d.source, "one  three\n",
                    "{tag}: {name} ignored the selection"
                );
                assert_eq!(d.selection(), None, "{tag}: {name}");
            }
        }
    }

    #[test]
    fn a_kill_takes_the_markup_it_empties_with_it() {
        // The same hazard a word-delete has: a WYSIWYG range covers what the
        // user can see, which for `**bold**` is the word and never the
        // delimiters, so a kill that stopped at the text would leave `a ****` —
        // markup wrapped around nothing.
        let mut d = wysiwyg_doc("kill_widen", "a **bold**\n");
        d.caret = d.source.find("bold").unwrap();
        d.delete_to_line_end();
        assert_eq!(d.source, "a \n");
    }

    #[test]
    fn a_kill_is_undone_in_one_step() {
        for (view, tag) in VIEWS {
            let mut d = doc_in(view, &format!("kill_undo_{tag}"), "one two three\n");
            d.caret = 3;
            d.delete_to_line_end();
            assert_eq!(d.source, "one\n", "{tag}");
            d.undo();
            assert_eq!(d.source, "one two three\n", "{tag}: a kill takes one undo");
        }
    }

    #[test]
    fn select_block_grabs_the_whole_paragraph_from_any_wrapped_row() {
        // Regression: triple-click used move_home/move_end over visual rows, so
        // it only worked on a paragraph's first row (a wrap-boundary offset maps
        // to the earlier row). select_block_at reads the AST, so every offset in
        // the paragraph selects the whole thing.
        let body = "one two three four five six seven eight\n";
        let mut d = doc_with("sel_block", body);
        d.view = View::Wysiwyg;
        d.build_visual(12); // force the paragraph to wrap into several rows
        assert!(d.vmap.num_rows() > 1, "test needs a wrapped paragraph");
        let para = (0, "one two three four five six seven eight".len());
        for off in [0usize, 8, 19, 28, 38] {
            d.caret = 0;
            d.anchor = None;
            d.select_block_at(off);
            assert_eq!(
                d.selection(),
                Some(para),
                "offset {off} should select the paragraph"
            );
        }
    }

    #[test]
    fn select_block_uses_content_span_for_a_heading() {
        let mut d = doc_with("sel_head", "# Title\n\nbody\n");
        d.select_block_at(4); // inside "Title"
        // content_span excludes the "# " marker.
        assert_eq!(d.selected_text(), Some("Title"));
        d.select_block_at(10); // inside "body"
        assert_eq!(d.selected_text(), Some("body"));
    }

    #[test]
    fn select_all_spans_the_document() {
        let mut d = doc_with("sel_all", "abc\n\ndef\n");
        d.select_all();
        assert_eq!(d.selection(), Some((0, d.source.len())));
    }

    #[test]
    fn select_word_at_picks_the_surrounding_word() {
        let mut d = doc_with("sel_word", "hello world\n");
        d.select_word_at(8); // inside "world"
        assert_eq!(d.selection(), Some((6, 11)));
        // Double-clicking at end-of-word still grabs the word to its left.
        d.select_word_at(5); // the space between the words
        assert_eq!(d.selection(), Some((5, 6)));
    }

    #[test]
    fn word_helpers_respect_utf8_boundaries() {
        // "café" is 5 bytes ('é' is two); motion must land on char boundaries.
        assert_eq!(
            golden("utf8", "|café ok", |d| d.move_word_right(false)),
            "café| ok"
        );
        assert_eq!(golden("utf8b", "café |ok", |d| d.delete_word_back()), "|ok");
    }

    #[test]
    fn typing_inserts_at_the_caret_and_advances_it() {
        let mut d = doc_with("type", "hello\n");
        d.insert("Hi ");
        assert_eq!(d.source, "Hi hello\n");
        assert_eq!(d.caret, 3);
        assert!(d.dirty);
    }

    #[test]
    fn backspace_deletes_the_char_before_the_caret() {
        let mut d = doc_with("bs", "hello\n");
        d.caret = 3; // after "hel"
        d.backspace();
        assert_eq!(d.source, "helo\n");
        assert_eq!(d.caret, 2);
    }

    #[test]
    fn typing_replaces_the_selection() {
        let mut d = doc_with("replace", "a word b\n");
        d.anchor = Some(2);
        d.caret = 6; // "word" selected
        d.insert("X");
        assert_eq!(d.source, "a X b\n");
        assert_eq!(d.caret, 3);
        assert_eq!(d.anchor, None);
    }

    #[test]
    fn toggle_bold_wraps_then_unwraps_the_selection() {
        let mut d = doc_with("bold", "a word b\n");
        d.anchor = Some(2);
        d.caret = 6;
        d.toggle(InlineKind::Strong);
        assert_eq!(d.source, "a **word** b\n");
        // The toggled region stays selected, so a second toggle reverses it.
        d.toggle(InlineKind::Strong);
        assert_eq!(d.source, "a word b\n");
        d.toggle(InlineKind::Strong);
        assert_eq!(d.source, "a **word** b\n");
    }

    #[test]
    fn toggle_code_wraps_then_unwraps_the_selection() {
        let mut d = doc_with("code_rt", "a word b\n");
        d.anchor = Some(2);
        d.caret = 6;
        d.toggle(InlineKind::Verbatim);
        assert_eq!(d.source, "a `word` b\n");
        d.toggle(InlineKind::Verbatim);
        assert_eq!(d.source, "a word b\n");
    }

    #[test]
    fn sticky_bold_with_no_selection_wraps_the_next_typed_text() {
        // ⌘b at a bare caret, then type: the text comes out bold with no
        // selection ever made — the word-processor "start bold here" gesture.
        let mut d = doc_with("sticky_wrap", "xy\n");
        d.caret = 1; // between x and y
        d.toggle(InlineKind::Strong);
        assert_eq!(d.source, "xy\n", "arming a mark must not edit the document");
        d.insert("A");
        assert_eq!(d.source, "x**A**y\n");
    }

    #[test]
    fn sticky_bold_lights_the_toolbar_before_any_typing() {
        // The button must light the instant ⌘b is pressed, or the mode is
        // invisible until the first character lands.
        let mut d = doc_with("sticky_light", "xy\n");
        d.caret = 1;
        assert!(!d.active_inline_marks().contains(InlineKind::Strong));
        d.toggle(InlineKind::Strong);
        assert!(d.active_inline_marks().contains(InlineKind::Strong));
    }

    #[test]
    fn sticky_bold_toggled_off_types_normally_again() {
        // ⌘b, type, ⌘b, type: the first run is bold, the second is not — all
        // in the flow of typing, the exact sequence the user described.
        let mut d = doc_with("sticky_off", "\n");
        d.caret = 0;
        d.toggle(InlineKind::Strong);
        d.insert("a");
        d.insert("b"); // continues inside the run, no re-arming
        assert_eq!(d.source, "**ab**\n");
        d.toggle(InlineKind::Strong); // ⌘b again — shed bold
        d.insert("c");
        assert_eq!(d.source, "**ab**c\n");
    }

    #[test]
    fn continued_typing_after_a_sticky_run_stays_in_the_run() {
        // Once a mark is realised the caret sits inside the run, so plain typing
        // extends it rather than starting a second, adjacent bold span.
        let mut d = doc_with("sticky_cont", "\n");
        d.caret = 0;
        d.toggle(InlineKind::Emph);
        d.insert("h");
        d.insert("i");
        assert_eq!(d.source, "*hi*\n");
    }

    #[test]
    fn moving_the_caret_disarms_a_sticky_mark() {
        // Arming a mark and then moving away must not style text elsewhere.
        let mut d = doc_with("sticky_disarm", "xy\n");
        d.caret = 0;
        d.toggle(InlineKind::Strong);
        d.move_right(false); // caret 0 → 1, disarms
        assert!(!d.active_inline_marks().contains(InlineKind::Strong));
        d.insert("A");
        assert_eq!(d.source, "xAy\n", "the mark must not follow the caret");
    }

    #[test]
    fn stacked_sticky_marks_apply_together() {
        // ⌘b then ⌘i before typing: the text comes out both bold and italic.
        let mut d = doc_with("sticky_stack", "\n");
        d.caret = 0;
        d.toggle(InlineKind::Strong);
        d.toggle(InlineKind::Emph);
        d.insert("x");
        // Land the caret on the styled character and confirm both marks are live.
        d.anchor = Some(d.source.find('x').unwrap());
        d.caret = d.anchor.unwrap() + 1;
        let marks = d.active_inline_marks();
        assert!(marks.contains(InlineKind::Strong), "bold: {}", d.source);
        assert!(marks.contains(InlineKind::Emph), "italic: {}", d.source);
    }

    // ── the mark-edge rule (see `Doc::splice`) ───────────────────────────────

    #[test]
    fn a_space_typed_in_a_bold_run_never_leaves_the_delimiters_showing() {
        // The reported bug, keystroke for keystroke: ⌘b, "bold", space, "hey".
        // The space inside the run made `**bold **`, which is *not* bold — four
        // literal asterisks — so the rich view drew them, correctly and
        // uselessly, until the next character happened to close the run again.
        let mut d = wysiwyg_doc("edge_typing", "a \n");
        d.caret = 2;
        d.toggle(InlineKind::Strong);
        for c in "bold".chars() {
            d.insert(&c.to_string());
        }
        assert_eq!(d.source, "a **bold**\n");
        d.insert(" ");
        assert_eq!(
            d.source, "a **bold** \n",
            "the space belongs outside the run"
        );
        assert!(
            d.active_inline_marks().contains(InlineKind::Strong),
            "bold is still what's being typed, so the button stays lit"
        );
        // What the writer is looking at while all this happens: their words.
        d.build_visual(80);
        let drawn: String = d.vmap.rows[0].glyphs.iter().map(|g| g.ch).collect();
        assert_eq!(drawn, "a bold ", "no delimiter ever surfaces: {}", d.source);
        for c in "hey".chars() {
            d.insert(&c.to_string());
        }
        assert_eq!(
            d.source, "a **bold hey**\n",
            "one bold phrase, not two runs"
        );
    }

    #[test]
    fn typing_past_a_space_can_still_leave_the_bold_behind() {
        // The other half: the marks stay armed across the space, so ⌘b turns
        // them off again there and the next word is plain — the run isn't
        // rejoined by a caret that was told not to.
        let mut d = wysiwyg_doc("edge_shed", "\n");
        d.caret = 0;
        d.toggle(InlineKind::Strong);
        for c in "bold ".chars() {
            d.insert(&c.to_string());
        }
        assert_eq!(d.source, "**bold** \n");
        d.toggle(InlineKind::Strong);
        assert!(!d.active_inline_marks().contains(InlineKind::Strong));
        d.insert("x");
        assert_eq!(d.source, "**bold** x\n");
    }

    #[test]
    fn a_space_typed_first_of_all_still_leaves_the_mark_armed() {
        // ⌘b and then a space before any word: the space is not marked (nothing
        // is), and the word after it is.
        let mut d = wysiwyg_doc("edge_space_first", "a\n");
        d.caret = 1;
        d.toggle(InlineKind::Strong);
        d.insert(" ");
        assert_eq!(d.source, "a \n");
        assert!(d.active_inline_marks().contains(InlineKind::Strong));
        d.insert("b");
        assert_eq!(d.source, "a **b**\n");
    }

    #[test]
    fn a_space_typed_at_either_edge_of_an_existing_mark_steps_outside_it() {
        let mut d = wysiwyg_doc("edge_tail", "x **bold**\n");
        d.caret = 8; // the caret's home at the end of the run's text
        d.insert(" ");
        assert_eq!(
            d.source, "x **bold** \n",
            "the space lands past the delimiters"
        );
        assert_eq!(d.caret, 11, "and the caret stands past it, outside the run");

        let mut d = wysiwyg_doc("edge_head", "x **bold** y\n");
        d.caret = 4; // in front of the "b"
        d.insert(" ");
        assert_eq!(d.source, "x  **bold** y\n");
        assert_eq!(d.caret, 3, "in front of the run, where the space was typed");
    }

    #[test]
    fn a_delete_that_backs_a_space_onto_a_delimiter_moves_the_delimiter() {
        // Backspace over the last letter of a bold phrase.
        let mut d = wysiwyg_doc("edge_bksp", "a **bold h**\n");
        d.caret = 10; // past the "h"
        d.backspace();
        assert_eq!(d.source, "a **bold** \n");
        assert_eq!(d.caret, 11, "the caret keeps the place on screen it had");
        assert!(d.active_inline_marks().contains(InlineKind::Strong));
        d.insert("x");
        assert_eq!(d.source, "a **bold x**\n", "and typing rejoins the run");
    }

    #[test]
    fn deleting_the_last_of_a_run_takes_its_delimiters_with_it() {
        // `**b**` with the `b` gone is `****`: two delimiters with nothing to
        // mark, which is only text. The marks live on in the caret instead.
        let mut d = wysiwyg_doc("edge_empty", "a **b** c\n");
        d.caret = 5;
        d.backspace();
        assert_eq!(d.source, "a  c\n");
        assert!(d.active_inline_marks().contains(InlineKind::Strong));
        d.insert("x");
        assert_eq!(d.source, "a **x** c\n");
    }

    #[test]
    fn typing_over_a_whole_bold_word_keeps_it_bold() {
        let mut d = wysiwyg_doc("edge_replace", "a **bold** c\n");
        d.anchor = Some(4);
        d.caret = 8; // the word, not its delimiters
        d.insert("x");
        assert_eq!(d.source, "a **x** c\n");
    }

    #[test]
    fn a_code_span_keeps_the_space_it_is_given() {
        // Backticks are not whitespace-sensitive the way `**` is: `` `code ` ``
        // is still verbatim, so nothing is re-spelt. The repair asks the parser
        // rather than a table of kinds, and this is the answer it gets.
        let mut d = wysiwyg_doc("edge_code", "a `code` c\n");
        d.caret = 7;
        d.insert(" ");
        assert_eq!(d.source, "a `code ` c\n");
    }

    #[test]
    fn a_delete_from_a_runs_outer_edge_reaches_into_the_run() {
        // A run's closing delimiter has a caret home on each side of it, one
        // column apart on screen — and a plain ← off the space after a bold word
        // lands on the outer one. The character drawn behind the caret there is
        // still the last letter of the phrase, so that is what Backspace takes;
        // the byte behind it is a `*` nobody can see.
        let mut d = wysiwyg_doc("edge_outer_close", "**bold** x\n");
        d.caret = 9;
        d.move_left(false);
        assert_eq!(d.caret, 8, "← rests past the delimiters, not inside them");
        d.backspace();
        assert_eq!(
            d.source, "**bol** x\n",
            "a letter of the phrase, not its `*`"
        );
        assert_eq!(d.caret, 5);

        // And the mirror in front of the opening delimiter, where Delete's
        // character is the first letter of the run.
        let mut d = wysiwyg_doc("edge_outer_open", "x**bold**\n");
        d.caret = 1;
        d.delete_forward();
        assert_eq!(d.source, "x**old**\n");
        assert_eq!(d.caret, 3, "inside the run, in front of what is left of it");
    }

    #[test]
    fn a_delete_at_a_run_edge_never_eats_a_delimiter() {
        // The byte beside the caret at either edge of a bold word is a `*` the
        // rich view draws nothing for. Taking it is not the character delete the
        // key was pressed for — it unspells the run and puts a literal asterisk
        // on screen (`a *bold** c`). The visible character is the one that goes.
        let mut d = wysiwyg_doc("edge_open_bksp", "a **bold** c\n");
        d.caret = 4; // in front of the "b"
        d.backspace();
        assert_eq!(d.source, "a**bold** c\n", "the space goes, the run stands");

        let mut d = wysiwyg_doc("edge_close_del", "a **bold** c\n");
        d.caret = 8; // past the "d"
        d.delete_forward();
        assert_eq!(d.source, "a **bold**c\n");
        assert_eq!(d.caret, 8, "and the caret stays inside the run");
        d.insert("x");
        assert_eq!(d.source, "a **boldx**c\n");

        // A code span's backticks are hidden the same way, so they are covered
        // by the same rule and not by a list of kinds.
        let mut d = wysiwyg_doc("edge_open_code", "a `code` c\n");
        d.caret = 3;
        d.backspace();
        assert_eq!(d.source, "a`code` c\n");
    }

    #[test]
    fn the_source_view_deletes_the_delimiter_byte_it_is_shown() {
        // The asterisks are on the screen there and the caret can stand between
        // them, so a delete takes exactly the byte it is aimed at.
        let mut d = doc_with("edge_open_src", "a **bold** c\n");
        d.caret = 4;
        d.backspace();
        assert_eq!(d.source, "a *bold** c\n");

        let mut d = doc_with("edge_close_src", "a **bold** c\n");
        d.caret = 8;
        d.delete_forward();
        assert_eq!(d.source, "a **bold* c\n");
    }

    #[test]
    fn backspacing_the_space_out_of_a_bold_phrase_leaves_the_caret_in_it() {
        // The reported bug, keystroke for keystroke: ⌘b, "bold", space, Backspace.
        // The space had stepped outside the run (the mark-edge rule), taking the
        // caret with it, so the delete put it back down on the far side of the
        // closing `**` — one place on screen, and the wrong side of it. Typing
        // came out plain and the toolbar went dark, with nothing to see.
        let mut d = wysiwyg_doc("edge_bksp_space", "\n");
        d.caret = 0;
        d.toggle(InlineKind::Strong);
        for c in "bold".chars() {
            d.insert(&c.to_string());
        }
        d.insert(" ");
        assert_eq!(d.source, "**bold** \n");
        d.backspace();
        assert_eq!(
            d.source, "**bold**\n",
            "the space goes, the delimiters stay"
        );
        assert_eq!(d.caret, 6, "and the caret comes back inside the run");
        assert!(
            d.active_inline_marks().contains(InlineKind::Strong),
            "so the button is still lit"
        );
        d.insert("x");
        assert_eq!(
            d.source, "**boldx**\n",
            "and the next character is still bold"
        );
    }

    #[test]
    fn a_second_backspace_there_deletes_a_letter_of_the_phrase() {
        // What the stranded caret did next: the byte behind it was the closing
        // `*`, so a second press took that instead of a letter — `**bold*`, the
        // styling gone and an asterisk on the screen where the word had been.
        let mut d = wysiwyg_doc("edge_bksp_twice", "\n");
        d.caret = 0;
        d.toggle(InlineKind::Strong);
        for c in "bold ".chars() {
            d.insert(&c.to_string());
        }
        assert_eq!(d.source, "**bold** \n");
        d.backspace();
        d.backspace();
        assert_eq!(d.source, "**bol**\n", "the delete lands inside the run");
        assert_eq!(d.caret, 5);
    }

    #[test]
    fn a_delete_that_ends_at_a_nested_run_settles_inside_every_delimiter() {
        // `***both***` closes two runs with one stack of asterisks: the caret has
        // to walk in through all of them, or it lands between the emph and the
        // strong and types half-marked.
        let mut d = wysiwyg_doc("edge_bksp_nested", "***both*** \n");
        d.caret = 11;
        d.backspace();
        assert_eq!(d.source, "***both***\n");
        assert_eq!(d.caret, 7, "past the last letter, inside both runs");
        d.insert("x");
        assert_eq!(d.source, "***bothx***\n");
    }

    #[test]
    fn a_delete_that_ends_mid_run_leaves_the_caret_where_it_fell() {
        // The settle only moves a caret a run actually closed over. Ordinary
        // deletes — inside a run, or in plain prose — are untouched.
        let mut d = wysiwyg_doc("edge_bksp_mid", "a **bold** c\n");
        d.caret = 8;
        d.backspace();
        assert_eq!(d.source, "a **bol** c\n");
        assert_eq!(d.caret, 7);

        let mut d = wysiwyg_doc("edge_bksp_plain", "plain\n");
        d.caret = 5;
        d.backspace();
        assert_eq!(d.source, "plai\n");
        assert_eq!(d.caret, 4);
    }

    #[test]
    fn the_source_view_leaves_a_delete_where_it_landed() {
        // The delimiters are on the screen there, so the offset past them is a
        // place the caret can be seen to be — nothing to settle.
        let mut d = doc_with("edge_bksp_src", "**bold** \n");
        d.caret = 9;
        d.backspace();
        assert_eq!(d.source, "**bold**\n");
        assert_eq!(d.caret, 8);
    }

    #[test]
    fn the_mark_edge_rule_clears_every_delimiter_of_a_nested_run() {
        // `***both***` closes two runs with one stack of asterisks; a space that
        // clears only the inner one lands against the outer's and breaks that
        // instead.
        let mut d = wysiwyg_doc("edge_nested", "a ***both***\n");
        d.caret = 9;
        d.insert(" ");
        assert_eq!(d.source, "a ***both*** \n");
        assert_eq!(d.caret, 13);
        d.insert("x");
        assert_eq!(d.source, "a ***both x***\n");
    }

    #[test]
    fn the_mark_edge_repair_undoes_with_the_keystroke_that_caused_it() {
        // The delimiter shuffle is not an edit the writer made, so it is not a
        // step they have to undo past.
        let mut d = wysiwyg_doc("edge_undo", "a **bold**\n");
        d.caret = 8;
        d.insert(" ");
        assert_eq!(d.source, "a **bold** \n");
        d.undo();
        assert_eq!(d.source, "a **bold**\n");
    }

    #[test]
    fn the_source_view_types_the_space_where_it_was_asked_to() {
        // The rule is a rich-view courtesy. In the source view the delimiters are
        // on the screen and the user is editing the bytes they can see.
        let mut d = doc_with("edge_src", "a **bold** c\n");
        d.caret = 8;
        d.insert(" ");
        assert_eq!(d.source, "a **bold ** c\n");
    }

    #[test]
    fn toggling_a_mark_over_a_selection_leaves_its_edge_whitespace_out() {
        // Double-clicking a word takes the space after it; bolding that must not
        // spell `**word **`, which is not bold at all.
        let mut d = wysiwyg_doc("edge_sel", "a word b\n");
        d.anchor = Some(2);
        d.caret = 7; // "word "
        d.toggle(InlineKind::Strong);
        assert_eq!(d.source, "a **word** b\n");
        d.toggle(InlineKind::Strong);
        assert_eq!(d.source, "a word b\n");
        d.toggle(InlineKind::Strong);
        assert_eq!(
            d.source, "a **word** b\n",
            "reapplying the mark must not wrap stale delimiter offsets"
        );
        // And a selection of nothing but whitespace has no word to mark.
        let mut d = wysiwyg_doc("edge_sel_ws", "a word b\n");
        d.anchor = Some(6);
        d.caret = 7;
        d.toggle(InlineKind::Strong);
        assert_eq!(d.source, "a word b\n");
        assert!(d.status.is_some());
    }

    #[test]
    fn set_block_turns_a_paragraph_into_a_heading_at_the_caret() {
        let mut d = doc_with("head_set", "hello\n");
        d.caret = 2; // caret inside the paragraph, no selection
        d.set_block(BlockKind::Heading(1));
        assert_eq!(d.source, "# hello\n");
    }

    #[test]
    fn set_block_heading_works_in_wysiwyg_view() {
        // The app defaults to WYSIWYG; the caret is a source offset either way.
        let mut d = wysiwyg_doc("head_wys", "hello\n");
        d.caret = 2;
        d.set_block(BlockKind::Heading(1));
        assert_eq!(d.source, "# hello\n");
    }

    #[test]
    fn toggle_heading_applies_switches_and_reverts() {
        let mut d = doc_with("head_toggle", "hello\n");
        d.caret = 2;
        d.toggle_heading(1);
        assert_eq!(d.source, "# hello\n"); // paragraph → H1
        d.toggle_heading(2);
        assert_eq!(d.source, "## hello\n"); // H1 → H2 (different level switches)
        d.toggle_heading(2);
        assert_eq!(d.source, "hello\n"); // same level reverts to paragraph
    }

    #[test]
    fn preserve_enter_at_a_line_end_lands_the_caret_on_the_new_blank_line() {
        // Regression: Enter at the end of a soft-break line (mid-paragraph) opened
        // the blank line but the caret rendered on the *next* line, because the
        // separator was a non-navigable decoration row. In Preserve flow that
        // blank line is a real caret home — the caret must resolve onto it, and
        // typing there makes the soft break that continues the paragraph.
        let src = "line one:\nsecond line\n";
        let mut d = wysiwyg_doc("pre_enter_lineend", src);
        d.set_line_flow(LineFlow::Preserve);
        d.build_visual_unwrapped(); // the GUI path (pixel-wrapped)
        d.caret = 9; // the visual end of row 0, at the soft-break '\n'
        d.newline();
        d.build_visual_unwrapped();
        assert_eq!(d.source, "line one:\n\nsecond line\n");
        assert_eq!(
            d.caret, 10,
            "caret sits on the new blank line, not the next line"
        );
        // The blank line is row 1, and the caret resolves onto it — not row 2.
        assert_eq!(
            d.vmap.pos_of_offset(10),
            (1, 0),
            "caret renders on the blank row"
        );
        assert!(
            !d.vmap.rows[1].decoration,
            "the blank line is navigable in Preserve"
        );
        // Typing there makes a soft break: one paragraph, three lines.
        d.insert("new clause,");
        assert_eq!(d.source, "line one:\nnew clause,\nsecond line\n");
    }

    #[test]
    fn preserve_enter_makes_a_soft_break_not_a_paragraph() {
        // Mid-paragraph: Enter splits the line with a single `\n`, a soft break
        // that keeps it one paragraph — where Fold would open a second paragraph.
        let mut d = wysiwyg_doc("pre_enter_mid", "abcdef\n");
        d.set_line_flow(LineFlow::Preserve);
        d.caret = 3;
        d.newline();
        assert_eq!(d.source, "abc\ndef\n", "mid-line Enter is a soft break");

        // End-of-paragraph: Enter then typing continues the same paragraph on a
        // new line (a soft break), not a fresh paragraph.
        let mut d = wysiwyg_doc("pre_enter_end", "abc\n");
        d.set_line_flow(LineFlow::Preserve);
        d.caret = 3;
        d.newline();
        d.insert("def");
        assert_eq!(
            d.source, "abc\ndef\n",
            "end-of-line Enter + typing is a soft break"
        );
    }

    #[test]
    fn preserve_double_enter_still_makes_a_paragraph() {
        // Two Enters in a row promote to a real paragraph break: the second lands
        // on the blank line the first opened and takes the empty-line branch.
        let mut d = wysiwyg_doc("pre_enter_dbl", "abc\n");
        d.set_line_flow(LineFlow::Preserve);
        d.caret = 3;
        d.newline();
        d.newline();
        d.insert("def");
        assert_eq!(
            d.source, "abc\n\ndef\n",
            "double Enter is a paragraph break"
        );
    }

    #[test]
    fn preserve_backspace_joins_across_a_soft_break() {
        // Backspace is the symmetric undo of a Preserve Enter: over the `\n` of a
        // soft break it deletes the single newline and joins the two lines.
        let mut d = wysiwyg_doc("pre_bs", "abc\ndef\n");
        d.set_line_flow(LineFlow::Preserve);
        d.build_visual(80);
        d.caret = 4; // start of "def", just past the soft break
        d.backspace();
        assert_eq!(
            d.source, "abcdef\n",
            "Backspace joins across the soft break"
        );
        assert_eq!(d.caret, 3, "caret lands where the lines meet");
    }

    #[test]
    fn fold_enter_still_starts_a_new_paragraph() {
        // The default flow is unchanged: a lone `\n` would render as an invisible
        // space, so Enter keeps opening the paragraph break that actually shows.
        let mut d = wysiwyg_doc("fold_enter", "abcdef\n");
        d.caret = 3;
        d.newline();
        assert_eq!(
            d.source, "abc\n\ndef\n",
            "Fold mid-line Enter is a paragraph break"
        );
    }

    #[test]
    fn wysiwyg_one_enter_starts_a_new_paragraph() {
        // Regression: one Enter left the caret between the two newlines, so typing
        // made a soft break (one paragraph) and you needed a second Enter.
        let mut d = wysiwyg_doc("wys_enter", "abc\n");
        d.caret = 3;
        d.newline();
        d.insert("def");
        assert_eq!(d.source, "abc\n\ndef\n"); // two paragraphs, not "abc\ndef\n"
    }

    #[test]
    fn enter_at_the_end_of_a_bold_run_keeps_its_closing_delimiter_attached() {
        // Regression: Enter at the caret's natural End-of-line resting place
        // after a bold run with nothing following it (on screen: right after
        // "bold", before the hidden closing "**") spliced the paragraph break
        // at that very byte offset — which sits *before* the closing "**" in
        // the source, since the delimiter is hidden and emits no glyph of its
        // own for `push_row`'s "end of row" fallback to count. That severed the
        // mark: "**bold**\n" became "**bold\n\n**\n", stranding the closing
        // "**" alone on the new line instead of leaving "**bold**" intact with
        // a fresh empty paragraph after it.
        let mut d = wysiwyg_doc("bold_eol_enter", "**bold**\n");
        d.move_end(false); // the WYSIWYG End key, from caret 0
        assert_eq!(
            d.caret, 6,
            "caret rests right after \"bold\", before the hidden \"**\""
        );
        d.newline();
        assert!(
            d.source.starts_with("**bold**"),
            "the closing ** must stay attached to \"bold\": got {:?}",
            d.source
        );
        assert_eq!(
            d.source, "**bold**\n\n\n",
            "a fresh empty paragraph follows the still-intact bold run"
        );
    }

    #[test]
    fn source_view_enter_is_a_single_newline() {
        let mut d = doc_with("src_enter", "abc\n");
        d.caret = 3;
        d.newline();
        assert_eq!(d.source, "abc\n\n");
    }

    #[test]
    fn heading_applies_at_the_end_of_a_paragraph() {
        // The caret at a line end sits at the doc level; set_block must still find
        // the block on that line.
        let mut d = doc_with("head_end", "abc\n");
        d.caret = 3; // end of "abc"
        d.toggle_heading(1);
        assert_eq!(d.source, "# abc\n");
    }

    #[test]
    fn heading_on_an_empty_new_paragraph_creates_one() {
        let mut d = wysiwyg_doc("head_empty", "abc\n");
        d.caret = 3;
        d.newline(); // caret now on a fresh, empty paragraph
        d.toggle_heading(1);
        d.insert("Title");
        assert!(d.source.contains("# Title"), "got {:?}", d.source);
    }

    #[test]
    fn a_heading_typed_on_a_blank_line_keeps_the_caret_on_its_own_row() {
        // The reported bug, end to end: click a blank line with another one under
        // it, press H1, type. The text landed in the heading and the caret's
        // offset was right (the source view drew it there), but the rich view
        // drew it two rows lower, on the trailing blank line — the empty `# `
        // heading had left every row below it short by the marker's two bytes,
        // and the blank line ended up claiming the heading's own end offset.
        let mut d = wysiwyg_doc("head_blank", "one\n\ntwo\n\n\n\n");
        d.build_visual_unwrapped();
        d.caret = d.vmap.offset_of_pos(4, 0); // the first of the two blank lines
        d.toggle_heading(1);
        for c in "title".chars() {
            d.insert(&c.to_string());
            d.build_visual_unwrapped(); // as a frontend does, one frame per key
        }
        assert_eq!(d.source, "one\n\ntwo\n\n# title\n\n");
        assert_eq!(
            d.caret_pos(),
            (4, 5),
            "the caret draws at the end of the heading"
        );
    }

    #[test]
    fn clicking_an_empty_heading_types_after_its_marker() {
        // The same anchor from the other side: the empty heading's row is its own
        // caret home, so a click on it must land past the hidden `# `. Landing in
        // front of the hashes made the first keystroke un-heading the line.
        let mut d = wysiwyg_doc("head_click", "# \n");
        d.build_visual_unwrapped();
        d.caret = d.vmap.offset_of_pos(0, 0);
        d.insert("x");
        assert_eq!(d.source, "# x\n");
    }

    #[test]
    fn wysiwyg_enter_after_a_heading_makes_a_paragraph() {
        let mut d = wysiwyg_doc("head_enter", "# Title\n");
        d.caret = 7; // end of the heading
        d.newline();
        d.insert("body");
        assert_eq!(d.source, "# Title\n\nbody\n");
    }

    #[test]
    fn wysiwyg_enter_continues_a_bullet_list() {
        let mut d = wysiwyg_doc("wys_bullet", "- item\n");
        d.caret = 6; // end of "item"
        d.newline();
        d.insert("two");
        assert_eq!(d.source, "- item\n- two\n");
    }

    #[test]
    fn wysiwyg_enter_increments_an_ordered_list() {
        let mut d = wysiwyg_doc("wys_ol", "1. one\n");
        d.caret = 6; // end of "one"
        d.newline();
        d.insert("two");
        assert_eq!(d.source, "1. one\n2. two\n");
    }

    #[test]
    fn wysiwyg_backspace_after_leaving_a_list_collapses_the_gap_cleanly() {
        // Regression for the "extra newline" left between a list and the paragraph
        // below it. Enter, Enter leaves the list on a fresh empty paragraph
        // (`- item\n\n\n\nnext`, a navigable blank between the two blocks); one
        // Backspace should then take the caret cleanly back to the end of the list
        // item, `- item\n\nnext`, not delete a single newline and strand it on the
        // odd `- item\n\n\nnext` — a blank line the eye reads as one separator but
        // no caret can land on. The map is rebuilt between keystrokes exactly as a
        // frontend does, since Backspace reads the stop table to place the delete.
        let mut d = wysiwyg_doc("wys_exit_bksp", "- item\n\nnext\n");
        d.caret = 6; // end of "item"
        d.newline();
        d.build_visual(80);
        d.newline(); // leave the list onto a fresh empty paragraph
        d.build_visual(80);
        assert_eq!(
            d.source, "- item\n\n\n\nnext\n",
            "double-Enter opens the empty paragraph"
        );
        d.backspace();
        assert_eq!(
            d.source, "- item\n\nnext\n",
            "one Backspace collapses the whole gap"
        );
        assert_eq!(
            d.caret, 6,
            "and lands the caret back at the end of the list item"
        );
    }

    #[test]
    fn wysiwyg_backspace_on_stacked_blank_lines_still_removes_just_one() {
        // The stop-wise delete must not over-reach when there is no block boundary
        // to cross: two blank lines in a row are one caret stop apart, so pressing
        // Enter on an empty line and then Backspace removes exactly the one newline
        // it added — the lone-Enter / lone-Backspace symmetry, preserved.
        let mut d = wysiwyg_doc("wys_stack", "abc\n\n\n");
        d.caret = 5; // the empty paragraph the first Enter already opened
        d.build_visual(80);
        d.newline();
        d.build_visual(80);
        assert_eq!(
            d.source, "abc\n\n\n\n",
            "Enter on the blank line adds one newline"
        );
        d.backspace();
        assert_eq!(
            d.source, "abc\n\n\n",
            "Backspace takes back exactly that one newline"
        );
    }

    #[test]
    fn wysiwyg_enter_on_an_empty_list_item_exits_the_list() {
        let mut d = wysiwyg_doc("wys_exit", "- a\n- \n");
        d.caret = 6; // end of the empty "- " item
        d.newline();
        d.insert("p");
        assert_eq!(d.source, "- a\n\np\n");
    }

    #[test]
    fn wysiwyg_enter_does_not_mistake_a_setext_underline_for_a_list() {
        // `text\n- \n` is a setext heading — the `- ` is its underline, not a
        // list item, though it reads as a `- ` marker byte-for-byte. Enter must
        // not take the list-exit path (which would splice the `- ` away as if
        // leaving an empty item); the AST guard sends it to a normal break and
        // leaves the underline intact.
        let mut d = wysiwyg_doc("wys_setext", "text\n- \n");
        assert!(
            d.nodes().iter().any(|n| n.kind == Kind::Heading),
            "precondition: twig parses this as a heading, not a list",
        );
        d.caret = 7; // on the `- ` underline line
        d.newline();
        assert!(
            d.source.contains("- "),
            "the setext underline survives, not spliced away as a list item: {:?}",
            d.source,
        );
    }

    #[test]
    fn wysiwyg_enter_in_a_code_block_is_a_literal_newline() {
        let mut d = wysiwyg_doc("wys_code", "```\nabc\n```\n");
        d.caret = 7; // end of "abc" inside the fence
        d.newline();
        d.insert("def");
        assert_eq!(d.source, "```\nabc\ndef\n```\n");
    }

    #[test]
    fn wysiwyg_enter_continues_a_block_quote() {
        // Enter opens a new *paragraph* inside the quote, not a second line of
        // the same one. `> quote\n> more` is a soft break, which under
        // `LineFlow::Fold` renders as a space — the keystroke would look like it
        // did nothing. The quoted blank line is what makes the break visible, and
        // it's the same thing Enter does in running prose.
        let mut d = wysiwyg_doc("wys_quote", "> quote\n");
        d.caret = 7; // end of "quote"
        d.newline();
        d.insert("more");
        assert_eq!(d.source, "> quote\n>\n> more\n");
        // Still one quote, now holding two paragraphs — not a quote and a stray
        // line that fell out of it.
        let quotes = d
            .nodes()
            .iter()
            .filter(|n| n.kind == Kind::BlockQuote)
            .count();
        assert_eq!(quotes, 1);
    }

    #[test]
    fn set_block_makes_a_heading_at_the_caret() {
        let mut d = doc_with("head", "Title\n\nbody\n");
        d.caret = 0;
        d.set_block(BlockKind::Heading(2));
        assert_eq!(d.source, "## Title\n\nbody\n");
        d.set_block(BlockKind::Paragraph);
        assert_eq!(d.source, "Title\n\nbody\n");
    }

    // ── block containers (quote / list) ──────────────────────────────────────

    #[test]
    fn toggle_blockquote_wraps_the_block_at_the_caret_and_reverses() {
        let g = |m, f: fn(&mut Doc)| golden("quote", m, f);
        assert_eq!(g("hel|lo\n", |d| d.toggle_blockquote()), "> hel|lo\n");
        assert_eq!(g("> hel|lo\n", |d| d.toggle_blockquote()), "hel|lo\n");
        // A caret at a line end sits at the doc level; the block is still found.
        assert_eq!(g("hello|\n", |d| d.toggle_blockquote()), "> hello|\n");
    }

    #[test]
    fn toggle_blockquote_keeps_the_caret_in_a_hard_wrapped_paragraph() {
        // Every source line of the paragraph gets its own `> `, so a caret left
        // on its old byte offset falls one prefix per line above it too far
        // back — inside the markup it just asked for rather than in its word.
        assert_eq!(
            golden("quote_wrap", "aaa\nb|bb\nccc\n", |d| d.toggle_blockquote()),
            "> aaa\n> b|bb\n> ccc\n"
        );
    }

    #[test]
    fn toggle_blockquote_works_in_wysiwyg_view() {
        let g = |n, m, f: fn(&mut Doc)| golden_in(View::Wysiwyg, n, m, f);
        assert_eq!(
            g("q_wys", "hel|lo\n", |d| d.toggle_blockquote()),
            "> hel|lo\n"
        );
        assert_eq!(
            g("q_wys2", "> hel|lo\n", |d| d.toggle_blockquote()),
            "hel|lo\n"
        );
    }

    #[test]
    fn toggle_list_makes_a_list_and_converts_between_the_kinds() {
        let g = |m, f: fn(&mut Doc)| golden("list", m, f);
        assert_eq!(g("hel|lo\n", |d| d.toggle_list(false)), "- hel|lo\n");
        assert_eq!(g("hel|lo\n", |d| d.toggle_list(true)), "1. hel|lo\n");
        // The *other* kind converts in place instead of nesting, which is what
        // makes the two buttons one three-state control.
        assert_eq!(g("- hel|lo\n", |d| d.toggle_list(true)), "1. hel|lo\n");
        assert_eq!(g("1. hel|lo\n", |d| d.toggle_list(false)), "- hel|lo\n");
        // Its own kind, over the only item the list holds, takes it off.
        assert_eq!(g("- hel|lo\n", |d| d.toggle_list(false)), "hel|lo\n");
    }

    #[test]
    fn toggle_list_works_in_wysiwyg_view() {
        let g = |n, m, f: fn(&mut Doc)| golden_in(View::Wysiwyg, n, m, f);
        assert_eq!(
            g("l_wys", "hel|lo\n", |d| d.toggle_list(true)),
            "1. hel|lo\n"
        );
        assert_eq!(
            g("l_wys2", "1. hel|lo\n", |d| d.toggle_list(false)),
            "- hel|lo\n"
        );
        assert_eq!(
            g("l_wys3", "- hel|lo\n", |d| d.toggle_list(false)),
            "hel|lo\n"
        );
    }

    #[test]
    fn a_list_over_a_selection_numbers_each_block_and_stays_selected() {
        // The selection has to grow with the markup: twig takes a container off
        // only a range covering every block it holds, so the second press can
        // reverse the first only if the result is what's selected.
        let mut d = doc_with("list_sel", "abc\n\ndef\n");
        d.select_all();
        d.toggle_list(true);
        assert_eq!(d.source, "1. abc\n\n2. def\n");
        assert_eq!(d.selection(), Some((0, d.source.len())));
        d.toggle_list(true);
        assert_eq!(d.source, "abc\n\ndef\n");
    }

    #[test]
    fn toggle_blockquote_nests_a_partly_covered_quote() {
        // twig's rule: covering only some of a container's blocks nests, because
        // taking the quote off would drag its uncovered siblings out with it.
        let mut d = doc_with("quote_nest", "> a\n>\n> b\n");
        d.caret = 2; // in the first quoted paragraph only
        d.toggle_blockquote();
        assert_eq!(d.source, "> > a\n>\n> b\n");
    }

    #[test]
    fn a_container_toggle_opens_an_empty_one_on_a_blank_line() {
        // A blank line used to be no block for twig to wrap —
        // `toggle_block_container` answered `NotFound` — so Quote and the list
        // buttons did nothing on the very line the H1 button works on, and leaf
        // lent twig a scratch paragraph to wrap and took it back out again.
        // twig 3.2.0 opens an empty container there itself, so what is left here
        // is where the caret lands: inside the marker that was just written.
        let mut d = doc_with("quote_blank", "\nabc\n");
        d.caret = 0;
        d.toggle_blockquote();
        assert_eq!(d.source, "> \nabc\n");
        assert_eq!(
            d.caret, 2,
            "the caret belongs inside the quote it just opened"
        );
        assert!(d.status.is_none(), "{:?}", d.status);
        assert!(d.dirty);

        // And the paragraph below is still its own block: an empty container one
        // soft break from `abc` would take that paragraph into the quote with it.
        let mut d = wysiwyg_doc("quote_blank_rows", "\nabc\n");
        d.caret = 0;
        d.toggle_blockquote();
        d.build_visual(80);
        assert_eq!(drawn_rows(&d), ["│ ", "", "abc"]);

        // The same from the other side: a blank line directly under a paragraph
        // earns the blank line an empty block needs, rather than being read as a
        // soft break inside that paragraph.
        let mut d = doc_with("list_blank_below", "abc\n");
        d.caret = 4;
        d.toggle_list(false);
        assert_eq!(d.source, "abc\n\n- ");
        assert_eq!(d.caret, 7);
    }

    #[test]
    fn enter_at_the_end_of_a_quote_stays_in_the_quote() {
        // The gesture the rendering fix is for. `newline` inside a quote already
        // wrote the right source — `> a\n` becomes `> a\n>\n> \n`, twig's own
        // spelling — but the two marker lines it adds belonged to no node until
        // twig 3.2.0, so the gutter stopped at `a` and the line the writer had
        // just made drew as plain prose under the quote.
        let mut d = wysiwyg_doc("quote_enter", "> a\n");
        d.caret = 3; // past `a`, at the end of the quoted line
        d.newline();
        assert_eq!(d.source, "> a\n>\n> \n");
        d.build_visual(80);
        assert_eq!(drawn_rows(&d), ["│ a", "│ ", "│ "]);
        // And the caret is on the new line, not stranded on the old one.
        assert_eq!(d.caret, 8);
    }

    #[test]
    fn opening_a_container_on_a_blank_line_is_one_undo_step() {
        // It was three edits — scratch, wrap, unscratch — coalesced into one, and
        // now it is twig's single edit. Either way one ⌘z has to put the blank
        // line back rather than undoing into a half-built document.
        for open in [
            &(|d: &mut Doc| d.toggle_blockquote()) as &dyn Fn(&mut Doc),
            &|d: &mut Doc| d.toggle_list(false),
            &|d: &mut Doc| d.toggle_list(true),
        ] {
            let mut d = doc_with("container_blank_undo", "a\n\n\n\nb\n");
            d.caret = 3;
            open(&mut d);
            assert_ne!(d.source, "a\n\n\n\nb\n");
            d.undo();
            assert_eq!(d.source, "a\n\n\n\nb\n");
        }
    }

    #[test]
    fn a_container_toggle_is_one_undo_step() {
        let mut d = doc_with("quote_undo", "hello\n");
        d.caret = 3;
        d.insert("X"); // a typing run the structural edit must not fold into
        d.toggle_blockquote();
        assert_eq!(d.source, "> helXlo\n");
        d.undo();
        assert_eq!(d.source, "helXlo\n");
    }

    // ── links ────────────────────────────────────────────────────────────────

    #[test]
    fn insert_link_wraps_the_selection_and_leaves_its_text_selected() {
        let mut d = doc_with("link_sel", "word here\n");
        d.anchor = Some(0);
        d.caret = 4;
        d.insert_link("http://x.dev");
        assert_eq!(d.source, "[word](http://x.dev) here\n");
        // The text, not the destination — so a second press re-points the link
        // the first one made rather than nesting one inside it.
        assert_eq!(d.selected_text(), Some("word"));
        d.insert_link("http://y.dev");
        assert_eq!(d.source, "[word](http://y.dev) here\n");
        assert_eq!(d.selected_text(), Some("word"));
    }

    #[test]
    fn insert_image_at_the_caret_spells_the_markup_and_lands_past_it() {
        let mut d = doc_with("img_caret", "before after\n");
        d.caret = 7; // between "before " and "after"
        d.insert_image("cat.png", "a cat");
        assert_eq!(d.source, "before ![a cat](cat.png)after\n");
        // The caret sits just past the inserted image, nothing selected.
        assert_eq!(d.selection(), None);
        assert_eq!(d.caret, 7 + "![a cat](cat.png)".len());
    }

    /// The bug a real vault hit: a filename with spaces in it. Markdown ends a
    /// destination at the first space, so the `format!` this used to be wrote
    /// something that was not an image at all — and the reader saw the markup as
    /// text. twig owns the spelling now, and moves it into the angle form.
    #[test]
    fn insert_image_spells_a_destination_with_spaces_so_it_stays_an_image() {
        let mut d = doc_with("img_space", "x\n");
        d.caret = 0;
        d.insert_image("Jesus Commands the Apostles to Rest.jpg", "");
        assert_eq!(
            d.source,
            "![](<Jesus Commands the Apostles to Rest.jpg>)x\n"
        );
        // And it reads back as an image pointing at the unescaped path — the angle
        // brackets are spelling, not part of the destination.
        d.caret = 2;
        assert_eq!(
            d.image_destination_at_caret(),
            Some("Jesus Commands the Apostles to Rest.jpg".to_string())
        );
    }

    /// A `)` in a caption or a filename must not close the image early.
    #[test]
    fn insert_image_escapes_a_paren_in_either_half() {
        let mut d = doc_with("img_paren", "x\n");
        d.caret = 0;
        d.insert_image("a)b.png", "");
        assert_eq!(d.source, "![](a\\)b.png)x\n");
        d.caret = 2;
        assert_eq!(d.image_destination_at_caret(), Some("a)b.png".to_string()));
    }

    #[test]
    fn insert_image_uses_the_selection_as_alt_text() {
        let mut d = doc_with("img_sel", "caption here\n");
        d.anchor = Some(0);
        d.caret = 7; // "caption"
        d.insert_image("p.png", "ignored fallback");
        assert_eq!(d.source, "![caption](p.png) here\n");
    }

    #[test]
    fn insert_image_with_no_alt_leaves_empty_brackets() {
        let mut d = doc_with("img_noalt", "\n");
        d.caret = 0;
        d.insert_image("logo.svg", "");
        assert_eq!(d.source, "![](logo.svg)\n");
    }

    // ── move_block ─────────────────────────────────────────────────────────────

    /// A move, then its undo: one step takes the whole thing back.
    fn moved(name: &str, body: &str, from: usize, to: usize) -> (String, Doc) {
        let mut d = doc_with(name, body);
        d.caret = from;
        let steps = d.undo_steps;
        d.move_block(from, to);
        let after = d.source.clone();
        assert_eq!(d.undo_steps, steps + 1, "one undo step");
        assert!(d.dirty);
        d.undo();
        assert_eq!(d.source, body, "one undo restores the original");
        (after, d)
    }

    #[test]
    fn move_block_carries_a_paragraph_between_two_paragraphs_and_to_the_end() {
        let body = "a\n\nb\n\nc\n";
        assert_eq!(moved("mv_p1", body, 6, 3).0, "a\n\nc\n\nb\n");
        assert_eq!(moved("mv_p2", body, 0, body.len()).0, "b\n\nc\n\na\n");
        assert_eq!(moved("mv_p3", body, 3, 0).0, "b\n\na\n\nc\n");
    }

    #[test]
    fn move_block_carries_an_image_block() {
        let body = "a\n\n![p](x.png)\n\nc\n";
        assert_eq!(moved("mv_img1", body, 4, 0).0, "![p](x.png)\n\na\n\nc\n");
        assert_eq!(
            moved("mv_img2", body, 4, body.len()).0,
            "a\n\nc\n\n![p](x.png)\n"
        );
    }

    #[test]
    fn move_block_carries_a_table() {
        let body = "a\n\n| h |\n|---|\n| c |\n\nc\n";
        assert_eq!(
            moved("mv_tbl1", body, 5, 0).0,
            "| h |\n|---|\n| c |\n\na\n\nc\n"
        );
        assert_eq!(
            moved("mv_tbl2", body, 5, body.len()).0,
            "a\n\nc\n\n| h |\n|---|\n| c |\n"
        );
    }

    #[test]
    fn move_block_carries_a_code_block() {
        let body = "a\n\n```rs\nx\n```\n\nc\n";
        assert_eq!(moved("mv_code1", body, 8, 0).0, "```rs\nx\n```\n\na\n\nc\n");
        assert_eq!(
            moved("mv_code2", body, 8, body.len()).0,
            "a\n\nc\n\n```rs\nx\n```\n"
        );
    }

    #[test]
    fn move_block_onto_its_own_boundary_is_a_quiet_no_op() {
        let mut d = doc_with("mv_noop", "a\n\nb\n");
        let steps = d.undo_steps;
        d.move_block(0, 0);
        d.move_block(0, 3);
        assert_eq!(d.source, "a\n\nb\n");
        assert_eq!(d.undo_steps, steps);
        assert_eq!(d.status, None);
        assert!(!d.dirty);
    }

    #[test]
    fn move_block_into_a_fence_is_refused_with_a_status() {
        let mut d = doc_with("mv_fence", "a\n\n```\nx\ny\n```\n");
        d.move_block(0, 7);
        assert_eq!(d.source, "a\n\n```\nx\ny\n```\n");
        assert!(
            d.status
                .as_deref()
                .is_some_and(|s| s.starts_with("move block"))
        );
    }

    #[test]
    fn move_block_rides_the_caret_with_the_block() {
        // Down: "second" is line 0 of its block, caret 3 bytes from its end.
        let mut d = doc_with("mv_caret1", "first\n\nsecond\n\nthird\n");
        d.caret = 10; // "sec|ond"
        d.move_block(10, d.source.len());
        assert_eq!(d.source, "first\n\nthird\n\nsecond\n");
        assert_eq!(&d.source[d.caret..], "ond\n");
        // Up, across a wrapped paragraph's second line.
        let mut d = doc_with("mv_caret2", "first\n\nsecond\nline two\n");
        d.caret = 16; // "li|ne two"
        d.move_block(16, 0);
        assert_eq!(d.source, "second\nline two\n\nfirst\n");
        assert_eq!(&d.source[d.caret..], "ne two\n\nfirst\n");
        assert_eq!(d.selection(), None);
    }

    #[test]
    fn move_block_keeps_the_caret_on_its_line_through_a_quotes_prefix() {
        let mut d = doc_with("mv_quote_caret", "para\n\n> a\n");
        d.caret = 2; // "pa|ra"
        d.move_block(2, 9); // after a, inside the quote
        assert_eq!(d.source, "> a\n>\n> para\n");
        assert_eq!(&d.source[d.caret..], "ra\n");
    }

    #[test]
    fn move_block_up_and_down_step_over_siblings() {
        let mut d = doc_with("mv_updown", "a\n\nb\n\nc\n");
        d.caret = 3;
        d.move_block_down();
        assert_eq!(d.source, "a\n\nc\n\nb\n");
        assert_eq!(&d.source[d.caret..], "b\n");
        d.move_block_down();
        assert_eq!(d.source, "a\n\nc\n\nb\n", "nothing below the last block");
        assert_eq!(d.status.as_deref(), Some("move block: nothing below"));
        d.move_block_up();
        d.move_block_up();
        assert_eq!(d.source, "b\n\na\n\nc\n");
        assert_eq!(&d.source[d.caret..], "b\n\na\n\nc\n");
        d.move_block_up();
        assert_eq!(d.status.as_deref(), Some("move block: nothing above"));
    }

    #[test]
    fn move_block_up_and_down_reorder_list_items_with_their_children() {
        let mut d = doc_with("mv_items", "- a\n  - x\n- b\n- c\n");
        d.caret = 12; // in "b"
        d.move_block_up();
        assert_eq!(d.source, "- b\n- a\n  - x\n- c\n");
        assert_eq!(&d.source[d.caret..], "b\n- a\n  - x\n- c\n");
        d.caret = 6; // in "a"
        d.move_block_down();
        assert_eq!(d.source, "- b\n- c\n- a\n  - x\n");
        assert_eq!(&d.source[d.caret..], "a\n  - x\n");
        // A nested item leaves its list upward, as an item of the outer one.
        d.caret = 16; // in "x"
        d.move_block_up();
        assert_eq!(d.source, "- b\n- c\n- x\n- a\n");
    }

    #[test]
    fn move_block_on_a_lone_item_that_stays_a_bullet_is_no_step() {
        let mut d = doc_with("mv_lone_item", "x\n\n- b\n\ny\n");
        d.caret = 5;
        let steps = d.undo_steps;
        d.move_block_up();
        assert_eq!(d.source, "x\n\n- b\n\ny\n");
        assert_eq!(
            d.undo_steps, steps,
            "a move that rewrote nothing is no undo step"
        );
        assert_eq!(d.status.as_deref(), Some("move block: nothing above"));
    }

    #[test]
    fn move_block_up_leaves_a_container_at_its_first_block_and_down_at_its_last() {
        let mut d = doc_with("mv_leave", "x\n\n> a\n>\n> b\n\ny\n");
        d.caret = 5; // "a"
        d.move_block_up();
        assert_eq!(d.source, "x\n\na\n\n> b\n\ny\n");
        d.caret = 8; // "b"
        d.move_block_down();
        assert_eq!(d.source, "x\n\na\n\nb\n\ny\n");
        // A tail block leaves its item into the next item's tail, then the list.
        let mut d = doc_with("mv_tail", "- a\n\n  t\n- b\n");
        d.caret = 7;
        d.move_block_down();
        assert_eq!(d.source, "- a\n- b\n\n  t\n");
        d.move_block_down();
        assert_eq!(d.source, "- a\n- b\n\nt\n");
        // And a paragraph after a quote steps over the whole quote, not into it.
        let mut d = doc_with("mv_over", "x\n\n> a\n>\n> b\n\ny\n");
        d.caret = 14;
        d.move_block_up();
        assert_eq!(d.source, "x\n\ny\n\n> a\n>\n> b\n");
        assert_eq!(d.caret, 3);
        d.move_block_down();
        assert_eq!(d.status, None);
        assert_eq!(d.source, "x\n\n> a\n>\n> b\n\ny\n");
        // Above a quote that opens the document is offset 0 — before the
        // quote, not inside it.
        let mut d = doc_with("mv_over_top", "> a\n\ny\n");
        d.caret = 5;
        d.move_block_up();
        assert_eq!(d.source, "y\n\n> a\n");
        assert_eq!(d.caret, 0);
    }

    #[test]
    fn move_block_up_from_the_first_block_of_a_quote_that_opens_the_document_leaves_it() {
        let mut d = doc_with("mv_top_quote", "> a\n>\n> b\n");
        d.caret = 2;
        d.move_block_up();
        assert_eq!(d.source, "a\n\n> b\n");
        assert_eq!(d.caret, 0);
        assert_eq!(d.status, None);
    }

    #[test]
    fn move_block_on_a_blank_line_or_read_only_does_nothing() {
        let mut d = doc_with("mv_blank", "a\n\nb\n");
        d.caret = 2;
        d.move_block_down();
        assert_eq!(d.source, "a\n\nb\n");
        assert_eq!(d.status.as_deref(), Some("move block: no block here"));
        d.read_only = true;
        d.caret = 0;
        d.move_block_down();
        d.move_block(0, 5);
        assert_eq!(d.source, "a\n\nb\n");
    }

    #[test]
    fn move_block_never_lands_above_hidden_frontmatter() {
        let mut d = wysiwyg_doc("mv_fm", "---\nt: x\n---\n\na\n\nb\n");
        let b = d.source.find('b').unwrap();
        d.move_block(b, 0);
        assert_eq!(d.source, "---\nt: x\n---\n\nb\n\na\n");
    }

    #[test]
    fn block_range_at_is_the_block_a_move_picks_up() {
        let mut d = doc_with("mv_range", "a\n\n- b\n  - c\n\nd\n");
        assert_eq!(d.block_range_at(0), Some(0..1));
        assert_eq!(d.block_range_at(5), Some(3..12), "the item with its child");
        assert_eq!(d.block_range_at(10), Some(7..12), "the nested item alone");
        assert_eq!(d.block_range_at(2), None, "a blank line");
    }

    #[test]
    fn drop_target_at_splits_a_block_at_its_middle_row_and_ends_below_everything() {
        let long = "two ".repeat(40).trim_end().to_string(); // wraps to three rows at 80
        let mut d = wysiwyg_doc("drop", &format!("one\n\n{long} four\n\nfive\n"));
        let rows = d.vmap.rows.len();
        assert_eq!(rows, 7, "one, gap, three wrapped rows, gap, five");
        assert_eq!(d.drop_target_at(0), Some(DropTarget { offset: 0, row: 0 }));
        let two = d.source.find("two").unwrap();
        assert_eq!(
            d.drop_target_at(1),
            Some(DropTarget {
                offset: two,
                row: 2
            }),
            "the gap resolves to the block under it"
        );
        assert_eq!(
            d.drop_target_at(2),
            Some(DropTarget {
                offset: two,
                row: 2
            })
        );
        assert_eq!(
            d.drop_target_at(3),
            Some(DropTarget {
                offset: two,
                row: 2
            })
        );
        let four_end = d.source.find("four").unwrap() + 4;
        assert_eq!(
            d.drop_target_at(4),
            Some(DropTarget {
                offset: four_end,
                row: 5
            })
        );
        assert_eq!(
            d.drop_target_at(rows + 3),
            Some(DropTarget {
                offset: d.source.len(),
                row: rows
            })
        );
        // And the offsets are ones move_block takes.
        let five = d.source.find("five").unwrap();
        let t = d.drop_target_at(2).unwrap();
        d.move_block(five, t.offset);
        assert_eq!(d.source, format!("one\n\nfive\n\n{long} four\n"));
    }

    #[test]
    fn append_media_lands_at_the_end_as_its_own_block() {
        let mut d = doc_with("append_mid", "first word and more\n");
        d.caret = 0; // an editor nobody has tapped: the caret is at the start
        d.append_media(MediaKind::Image, "cat.png", "");
        assert_eq!(d.source, "first word and more\n\n![](cat.png)");
        assert_eq!(d.selection(), None);
        assert_eq!(d.caret, "first word and more\n\n![](cat.png)".len());
    }

    #[test]
    fn append_media_needs_no_separator_after_a_blank_line_or_in_an_empty_document() {
        let mut d = doc_with("append_blank", "para\n\n");
        d.append_media(MediaKind::Image, "a.png", "");
        assert_eq!(d.source, "para\n\n![](a.png)");

        let mut e = doc_with("append_empty", "");
        e.append_media(MediaKind::Image, "b.png", "");
        assert_eq!(e.source, "![](b.png)");

        let mut f = doc_with("append_noeol", "no newline at end");
        f.anchor = Some(0);
        f.caret = 2; // a selection, which the verb ignores
        f.append_media(MediaKind::Video, "clip.mp4", "");
        assert_eq!(
            f.source,
            "no newline at end\n\n<video src=\"clip.mp4\" controls></video>"
        );
    }

    #[test]
    fn insert_media_spells_a_video_as_html_and_reads_it_back_as_a_block() {
        // The round trip is the point: it's no use writing markup the reader
        // can't pick up again. This is the pair that only holds from twig 2.5.1
        // on — before it, the one-line form went in fine and came back as a
        // paragraph of raw tags, publishing no media at all.
        let mut d = doc_with("vid_rt", "\n");
        d.caret = 0;
        d.insert_media(MediaKind::Video, "clip.mp4", "a clip");
        assert_eq!(
            d.source,
            "<video src=\"clip.mp4\" controls>a clip</video>\n"
        );

        d.build_visual(80);
        assert_eq!(d.vmap.media.len(), 1, "reads back as one block media");
        assert_eq!(d.vmap.media[0].kind, MediaKind::Video);
        assert_eq!(d.vmap.media[0].destination, "clip.mp4");
        assert_eq!(d.vmap.media[0].alt, "a clip");
    }

    #[test]
    fn insert_media_spells_audio_with_its_own_tag() {
        let mut d = doc_with("aud_rt", "\n");
        d.caret = 0;
        d.insert_media(MediaKind::Audio, "take.mp3", "");
        assert_eq!(d.source, "<audio src=\"take.mp3\" controls></audio>\n");
        d.build_visual(80);
        assert_eq!(d.vmap.media[0].kind, MediaKind::Audio);
    }

    #[test]
    fn insert_media_uses_the_selection_as_fallback_text() {
        // The same courtesy `insert_image` does with alt: select a caption,
        // insert, and the caption labels the thing rather than being replaced.
        let mut d = doc_with("vid_sel", "the talk here\n");
        d.anchor = Some(0);
        d.caret = 8; // "the talk"
        d.insert_media(MediaKind::Video, "talk.mp4", "ignored fallback");
        assert_eq!(
            d.source,
            "<video src=\"talk.mp4\" controls>the talk</video> here\n"
        );
    }

    #[test]
    fn insert_media_with_an_image_kind_is_just_insert_image() {
        let mut d = doc_with("img_via_media", "\n");
        d.caret = 0;
        d.insert_media(MediaKind::Image, "logo.svg", "x");
        assert_eq!(d.source, "![x](logo.svg)\n");
    }

    // ── thematic breaks ─────────────────────────────────────────────────────

    /// The node the source parses as at `caret` — what confirms an inserted
    /// `---` actually reads back as a rule, not stray text or a setext heading.
    ///
    /// The *narrowest* node covering the offset. Every ancestor covers it too,
    /// and since twig 2.8 that includes the `doc` root, which now carries a real
    /// span (it reported none before, so taking the first match used to land on
    /// the block by luck and now always answers `"doc"`).
    fn kind_at(d: &mut Doc, caret: usize) -> Option<Kind> {
        d.nodes()
            .into_iter()
            .filter(|n| n.span.start <= caret && caret < n.span.end)
            .min_by_key(|n| n.span.end - n.span.start)
            .map(|n| n.kind)
    }

    #[test]
    fn a_task_box_toggles_at_the_caret_and_reads_back() {
        let mut d = doc_with("task_toggle", "- [ ] todo\n- [x] done\n");
        d.caret = 8; // inside "todo"
        assert_eq!(d.task_checked_at_caret(), Some(false));
        d.toggle_task_checked();
        assert_eq!(d.source, "- [x] todo\n- [x] done\n");
        assert_eq!(d.task_checked_at_caret(), Some(true));
        d.toggle_task_checked();
        assert_eq!(d.source, "- [ ] todo\n- [x] done\n");
    }

    #[test]
    fn a_click_toggles_a_box_without_taking_the_caret_with_it() {
        // The whole reason `toggle_task_at` exists apart from the caret form:
        // ticking a box elsewhere must not move the cursor out of what's being
        // typed.
        let mut d = doc_with("task_click", "- [ ] first\n- [ ] second\n");
        d.caret = 8; // inside "first"
        let second = d.source.find("second").unwrap();
        d.toggle_task_at(second);
        assert_eq!(d.source, "- [ ] first\n- [x] second\n");
        assert_eq!(d.caret, 8, "the caret stayed in the first item");
    }

    #[test]
    fn a_paragraph_becomes_a_task_item_in_one_undo_step() {
        let mut d = doc_with("task_para", "first\n\nplain para\n");
        d.caret = d.source.find("para").unwrap();
        d.toggle_task_item();
        assert_eq!(d.source, "first\n\n- [ ] plain para\n");
        assert_eq!(d.task_checked_at_caret(), Some(false));
        assert_eq!(d.status, None);
        d.undo();
        assert_eq!(
            d.source, "first\n\nplain para\n",
            "one step takes both back"
        );
    }

    #[test]
    fn a_blank_line_becomes_an_empty_task_item() {
        let mut d = doc_with("task_blank", "first\n\n");
        d.caret = d.source.len();
        d.toggle_task_item();
        assert_eq!(d.task_checked_at_caret(), Some(false), "{:?}", d.source);
        assert_eq!(d.status, None);
    }

    #[test]
    fn a_plain_item_gains_and_loses_a_box() {
        let mut d = doc_with("task_mint", "- plain\n");
        d.caret = 4;
        assert_eq!(d.task_checked_at_caret(), None);
        d.toggle_task_item();
        assert_eq!(d.source, "- [ ] plain\n");
        assert_eq!(
            d.task_checked_at_caret(),
            Some(false),
            "a new box arrives unticked"
        );
        d.toggle_task_item();
        assert_eq!(d.source, "- plain\n");
    }

    #[test]
    fn ticking_a_box_that_isnt_there_reports_rather_than_minting_one() {
        // `set checked` must not silently convert a bullet into a task — that is
        // `toggle_task_item`'s job, and twig refuses it here.
        let mut d = doc_with("task_none", "- plain\n");
        d.caret = 4;
        d.toggle_task_checked();
        assert_eq!(d.source, "- plain\n", "nothing written");
        assert!(
            d.status.is_some(),
            "the refusal should reach the status line"
        );
    }

    #[test]
    fn a_task_item_in_a_quote_is_found_past_the_quote_marker() {
        let mut d = doc_with("task_quote", "> - [ ] nested\n");
        d.caret = d.source.find("nested").unwrap();
        assert_eq!(d.task_checked_at_caret(), Some(false));
        d.toggle_task_checked();
        assert_eq!(d.source, "> - [x] nested\n");
    }

    #[test]
    fn insert_thematic_break_parts_the_paragraph_around_the_caret() {
        // A rule is a block, so twig's `insert_thematic_break` alone lands it
        // after the whole paragraph. `split_block` parts the paragraph first and
        // the rule is aimed at the *first* half, which is what a rule button is
        // understood to do — and what leaf spelled by hand until twig grew both
        // halves of the gesture.
        let mut d = doc_with("hr_mid", "before after\n");
        d.caret = 7; // between "before " and "after"
        d.insert_thematic_break();
        assert_eq!(d.source, "before \n\n---\n\nafter\n");
        assert_eq!(d.selection(), None);
        assert_eq!(
            kind_at(&mut d, "before \n\n".len()),
            Some(Kind::ThematicBreak)
        );
    }

    #[test]
    fn insert_thematic_break_at_a_paragraph_s_end_splits_nothing() {
        // At the end there is nothing to part, and a split there writes the
        // separator anyway — a blank line and the empty slot the next paragraph
        // would fill — which the rule then landed above: `para\n\n* * *\n\n\n`,
        // two blank lines nothing fills. Now the rule lands after the paragraph,
        // where the split-and-aim was sending it regardless. Both formats, and
        // both shapes of a last line — terminated, and still being typed —
        // because the two reach the split through different doors: Markdown's
        // paragraph span stops before its newline, so `para\n` at 4 never split
        // there, but `para` at 4 did.
        //
        // What stands under the rule is the line the caret goes on to, not a
        // slot: nothing above the rule but the one blank line, and the caret on
        // the line beneath it.
        for (fmt, rule) in [(Format::Markdown, "---"), (Format::Djot, "* * *")] {
            for src in ["para\n", "para"] {
                let mut d = Doc::from_source(src.into(), fmt).unwrap();
                d.caret = 4;
                d.insert_thematic_break();
                assert_eq!(d.source, format!("para\n\n{rule}\n\n"), "{fmt:?} {src:?}");
                assert_eq!(d.caret, d.source.len());
            }
            // Mid-document the slot sat between the rule and the next block;
            // the caret's line does now, a blank line either side of it.
            let mut d = Doc::from_source("para\n\nnext\n".into(), fmt).unwrap();
            d.caret = 4;
            d.insert_thematic_break();
            assert_eq!(d.source, format!("para\n\n{rule}\n\n\n\nnext\n"), "{fmt:?}");
            // Trailing whitespace is nothing to part either.
            let mut d = Doc::from_source("para  \n".into(), fmt).unwrap();
            d.caret = 4;
            d.insert_thematic_break();
            assert_eq!(d.source, format!("para  \n\n{rule}\n\n"), "{fmt:?}");
        }
    }

    #[test]
    fn insert_thematic_break_at_a_paragraph_s_start_lands_before_it() {
        // The split at the start parts nothing, but it is kept on purpose:
        // `|para` becomes `\npara` with the caret on a blank line, and twig
        // (3.5.2) writes a rule aimed at a blank line ON that line — the only
        // way "before the paragraph" is reachable through a gesture that only
        // places after. Before 3.5.2 this came out as `\n\n---\n\npara`.
        for (fmt, rule) in [(Format::Markdown, "---"), (Format::Djot, "* * *")] {
            let mut d = Doc::from_source("para\n".into(), fmt).unwrap();
            d.caret = 0;
            d.insert_thematic_break();
            assert_eq!(d.source, format!("{rule}\n\npara\n"), "{fmt:?}");
            let mut d = Doc::from_source("prev\n\npara\n".into(), fmt).unwrap();
            d.caret = 6;
            d.insert_thematic_break();
            assert_eq!(d.source, format!("prev\n\n{rule}\n\npara\n"), "{fmt:?}");
        }
    }

    #[test]
    fn insert_thematic_break_on_a_blank_line_takes_that_line() {
        // The gap between two blocks is where a click lands the caret; the
        // rule goes on the blank, one blank each side, and the caret on the
        // line opened under it.
        let mut d = doc_with("hr_gap", "a\n\nb\n");
        d.caret = 2;
        d.insert_thematic_break();
        assert_eq!(d.source, "a\n\n---\n\n\n\nb\n");
        assert_eq!(d.caret, "a\n\n---\n\n".len());
    }

    #[test]
    fn insert_thematic_break_leaves_the_caret_on_a_line_under_the_rule() {
        // The caret used to be left where twig's splice ended, the home past
        // the rule, which is drawn on the rule's own row: the writer saw the
        // caret beside the rule, and mid-document what they typed next ran
        // into the block below. Now it goes on to an empty row under the
        // rule's gap from every place a rule is asked for — the end of a
        // paragraph, the empty paragraph Enter opened, a list and the line
        // Enter leaves a list for, closing the document and before another
        // block — and what is typed there is a paragraph of its own.
        for (body, caret, typed) in [
            ("para\n", 4, "para\n\n---\n\nX"),
            ("para\n\n\n", 6, "para\n\n---\n\nX"),
            ("- one\n- two\n", 11, "- one\n- two\n\n---\n\nX"),
            ("- one\n- two\n\n\n", 13, "- one\n- two\n\n---\n\nX"),
            ("a\n\nb\n", 1, "a\n\n---\n\nX\n\nb\n"),
            ("a\n\n\n\nb\n", 3, "a\n\n---\n\nX\n\nb\n"),
        ] {
            let mut d = wysiwyg_doc("hr_caret", body);
            d.build_visual(80);
            d.caret = caret;
            d.insert_thematic_break();
            d.build_visual(80);
            let rule = d.vmap.pos_of_offset(d.source.find("---").unwrap()).0;
            let (row, col) = d.vmap.pos_of_offset(d.caret);
            assert_eq!((row, col), (rule + 2, 0), "{body:?} → {:?}", d.source);
            assert!(d.vmap.rows[rule + 1].decoration, "the gap under the rule");
            assert!(d.vmap.rows[row].glyphs.is_empty(), "an empty line");
            d.insert("X");
            assert_eq!(d.source, typed, "{body:?}");
            let rule_at = d.source.find("---").unwrap();
            assert_eq!(kind_at(&mut d, rule_at), Some(Kind::ThematicBreak));
        }
        // djot's rule takes in its newline; the line under it is the same.
        let mut d = Doc::from_source("a\n\nb\n".into(), Format::Djot).unwrap();
        d.view = View::Wysiwyg;
        d.caret = 1;
        d.insert_thematic_break();
        d.build_visual(80);
        assert_eq!(d.source, "a\n\n* * *\n\n\n\nb\n");
        assert_eq!(d.vmap.pos_of_offset(d.caret), (4, 0));
    }

    #[test]
    fn a_rule_parting_a_paragraph_leaves_the_caret_before_its_second_half() {
        // The line under the rule is the paragraph's second half, so that is
        // where the caret goes: the text it was standing in front of is still
        // in front of it.
        let mut d = wysiwyg_doc("hr_parted", "before after\n");
        d.caret = 7;
        d.insert_thematic_break();
        assert_eq!(d.source, "before \n\n---\n\nafter\n");
        assert_eq!(d.caret, d.source.find("after").unwrap());
        // At the start of a paragraph too, the rule landing above it.
        let mut d = wysiwyg_doc("hr_parted_start", "prev\n\npara\n");
        d.caret = 6;
        d.insert_thematic_break();
        assert_eq!(d.caret, d.source.find("para").unwrap());
    }

    #[test]
    fn the_line_under_a_new_rule_is_one_undo_step_with_it() {
        let mut d = wysiwyg_doc("hr_undo", "a\n\nb\n");
        d.caret = 1;
        d.insert_thematic_break();
        assert_eq!(d.source, "a\n\n---\n\n\n\nb\n");
        d.undo();
        assert_eq!(d.source, "a\n\nb\n");
        assert!(!d.can_undo());
        d.redo();
        assert_eq!(d.source, "a\n\n---\n\n\n\nb\n");
    }

    #[test]
    fn a_rule_in_preserve_flow_leaves_the_caret_on_the_first_line_under_it() {
        // Preserve draws every blank line as somewhere to type, the first under
        // the rule included, so that is the caret's line and the one written.
        let mut d = wysiwyg_doc("hr_preserve", "para\n");
        d.set_line_flow(LineFlow::Preserve);
        d.caret = 4;
        d.insert_thematic_break();
        assert_eq!(d.source, "para\n\n---\n\n");
        assert_eq!(d.caret, "para\n\n---\n".len());
        d.build_visual(80);
        let rule = d.vmap.pos_of_offset(d.source.find("---").unwrap()).0;
        assert_eq!(d.vmap.pos_of_offset(d.caret), (rule + 1, 0));
        // Before another block, one blank line more keeps it off that block.
        let mut d = wysiwyg_doc("hr_preserve_mid", "a\n\nb\n");
        d.set_line_flow(LineFlow::Preserve);
        d.caret = 1;
        d.insert_thematic_break();
        assert_eq!(d.source, "a\n\n---\n\n\nb\n");
        assert_eq!(d.caret, "a\n\n---\n".len());
    }

    #[test]
    fn insert_page_break_leaves_the_caret_on_a_line_under_it() {
        // The rule button's placement, for the other block the toolbar writes:
        // the caret was left at the end of twig's splice, beside the break at
        // the end of the document and on the next block's first letter before
        // one. djot spells the break as a fence of two lines; the line under
        // it is under the closing `:::`.
        for (fmt, body, want, typed) in [
            (
                Format::Markdown,
                "a\n\nb\n",
                "a\n\n::page-break\n\n\n\nb\n",
                "a\n\n::page-break\n\nX\n\nb\n",
            ),
            (
                Format::Markdown,
                "a\n",
                "a\n\n::page-break\n\n",
                "a\n\n::page-break\n\nX",
            ),
            (
                Format::Djot,
                "a\n\nb\n",
                "a\n\n::: page-break\n:::\n\n\n\nb\n",
                "a\n\n::: page-break\n:::\n\nX\n\nb\n",
            ),
        ] {
            let mut d = Doc::from_source(body.into(), fmt).unwrap();
            d.view = View::Wysiwyg;
            d.caret = 1;
            d.insert_page_break();
            assert_eq!(d.source, want, "{fmt:?}");
            d.build_visual(80);
            let (row, col) = d.vmap.pos_of_offset(d.caret);
            assert_eq!(col, 0);
            assert!(d.vmap.rows[row].glyphs.is_empty(), "{fmt:?}: an empty line");
            assert!(
                d.vmap.rows[row - 2].leaf_directive.is_some(),
                "{fmt:?}: under the break"
            );
            d.insert("X");
            assert_eq!(d.source, typed, "{fmt:?}");
            d.undo();
            d.undo();
            assert_eq!(
                d.source, body,
                "{fmt:?}: the break and its line are one step"
            );
        }
    }

    #[test]
    fn a_block_parting_a_paragraph_is_one_undo_step() {
        // The split went to twig without leaf counting it, so the first Undo
        // took back the block, left the paragraph parted, and reported nothing
        // more to undo.
        let mut rule = wysiwyg_doc("part_undo_rule", "before after\n");
        rule.caret = 7;
        rule.insert_thematic_break();
        let mut table = wysiwyg_doc("part_undo_table", "before after\n");
        table.caret = 7;
        table.insert_table(1, 1);
        let mut page = wysiwyg_doc("part_undo_page", "before after\n");
        page.caret = 7;
        page.insert_page_break();
        for (name, d) in [
            ("rule", &mut rule),
            ("table", &mut table),
            ("page break", &mut page),
        ] {
            assert_ne!(d.source, "before after\n", "{name}");
            d.undo();
            assert_eq!(d.source, "before after\n", "{name}: one Undo");
            assert_eq!(d.caret, 7, "{name}: the caret back where it was");
            assert!(!d.can_undo(), "{name}");
            assert!(d.can_redo(), "{name}");
            d.redo();
            assert!(!d.source.starts_with("before after"), "{name}: one Redo");
            assert!(!d.can_redo(), "{name}");
        }
        // A table twig would refuse parts nothing: the refusal comes first.
        let mut d = wysiwyg_doc("part_undo_zero", "before after\n");
        d.caret = 7;
        d.insert_table(0, 1);
        assert_eq!(d.source, "before after\n");
        assert!(!d.can_undo());
    }

    #[test]
    fn enter_at_a_paragraph_s_start_opens_an_empty_paragraph_above_it() {
        // twig's split at a paragraph's start writes one newline ahead of it,
        // which Fold draws as one more gap, so the first Enter showed nothing.
        // A paragraph break opens a line above, the caret staying before the
        // text. Past a mark's opening delimiters — where a tap before the
        // first letter lands — the split used to cut the mark in two
        // (`**\n\nb**`); the break goes in front of them, as it goes in
        // front of a heading's `#`, where the heading's own Enter left an
        // empty heading over its text as a paragraph. A paragraph that
        // opens a div gets its line outside the div, where it draws.
        for (body, caret, want, want_caret) in [
            ("a\n\nb\n", 3, "a\n\n\n\nb\n", 5),
            ("# H\n\nb\n", 5, "# H\n\n\n\nb\n", 7),
            ("a\n\n# H\n", 5, "a\n\n\n\n# H\n", 7),
            ("a\n\n**b** c\n", 3, "a\n\n\n\n**b** c\n", 5),
            ("a\n\n**b** c\n", 5, "a\n\n\n\n**b** c\n", 7),
            (
                "a\n\n<span data-color=\"orange\">b</span>\n",
                29,
                "a\n\n\n\n<span data-color=\"orange\">b</span>\n",
                31,
            ),
            (
                "a\n\n<div class=\"center\">\n\nb\n\n</div>\n",
                25,
                "a\n\n\n\n<div class=\"center\">\n\nb\n\n</div>\n",
                27,
            ),
        ] {
            let mut d = wysiwyg_doc("enter_para_start", body);
            d.build_visual(80);
            d.caret = caret;
            d.newline();
            assert_eq!(d.source, want, "{body:?}");
            assert_eq!(d.caret, want_caret, "{body:?}");
            d.build_visual(80);
            let (row, _) = d.vmap.pos_of_offset(d.caret);
            assert!(
                d.vmap.rows[row - 2].glyphs.is_empty(),
                "{body:?}: an empty line above"
            );
            assert!(
                !d.vmap.rows[row - 2].decoration,
                "{body:?}: one the caret can stand on"
            );
        }
        // Past the first letter it is an ordinary split.
        let mut d = wysiwyg_doc("enter_para_mid", "a\n\n**b** c\n");
        d.caret = 8;
        d.newline();
        assert_eq!(d.source, "a\n\n**b**\n\nc\n");
    }

    #[test]
    fn enter_again_at_a_paragraph_s_start_opens_one_more_line() {
        // The first Enter's paragraph break leaves a gap either side of the
        // empty line. A second break drew two lines more; a newline draws one.
        let mut d = wysiwyg_doc("enter_para_again", "a\n\nb\n");
        d.build_visual(80);
        d.caret = 3;
        for (want, lines) in [("a\n\n\n\nb\n", 1), ("a\n\n\n\n\nb\n", 2)] {
            d.newline();
            assert_eq!(d.source, want);
            assert_eq!(&d.source[d.caret..], "b\n");
            d.build_visual(80);
            let empty = d
                .vmap
                .rows
                .iter()
                .filter(|r| r.glyphs.is_empty() && !r.decoration)
                .count();
            assert_eq!(empty, lines, "{want:?}");
        }
    }

    #[test]
    fn enter_at_the_first_block_s_start_pushes_it_down() {
        // No row was drawn for a blank line above the first block, so Enter
        // there wrote a line and the text stayed where it was. Each Enter now
        // draws one empty line above it, the caret staying with the text;
        // past frontmatter, the line conventionally under it still draws
        // nothing until Enter adds to it.
        for (body, caret, wants) in [
            ("b\n", 0, ["\n\nb\n", "\n\n\nb\n"]),
            ("**b** c\n", 2, ["\n\n**b** c\n", "\n\n\n**b** c\n"]),
            ("# H\n", 2, ["\n\n# H\n", "\n\n\n# H\n"]),
            (
                "---\nt: x\n---\n\nb\n",
                14,
                ["---\nt: x\n---\n\n\nb\n", "---\nt: x\n---\n\n\n\nb\n"],
            ),
        ] {
            let mut d = wysiwyg_doc("enter_first_block", body);
            d.build_visual(80);
            let empty = |d: &Doc| {
                d.vmap
                    .rows
                    .iter()
                    .filter(|r| r.glyphs.is_empty() && !r.decoration)
                    .count()
            };
            assert_eq!(empty(&d), 0, "{body:?}: nothing above the text yet");
            d.caret = caret;
            for (lines, want) in wants.into_iter().enumerate() {
                d.newline();
                assert_eq!(d.source, want, "{body:?}");
                assert!(d.source[d.caret..].starts_with(&body[caret..]), "{body:?}");
                d.build_visual(80);
                assert_eq!(empty(&d), lines + 1, "{want:?}");
            }
            // The caret can go up onto the lines it opened.
            d.move_up(false);
            let (row, _) = d.vmap.pos_of_offset(d.caret);
            assert!(d.vmap.rows[row].glyphs.is_empty(), "{body:?}");
            assert!(!d.vmap.rows[row].decoration, "{body:?}");
        }
    }

    /// What a reader sees of a map: each row's text, whether it is a gap,
    /// and the presentation it is drawn with. Offsets are left out, since an
    /// edit shifts them without anything on screen changing.
    fn drawn(d: &Doc) -> Vec<String> {
        d.vmap
            .rows
            .iter()
            .map(|r| {
                let text: String = r.glyphs.iter().map(|g| g.ch).collect();
                format!(
                    "{}{text:?} h={:?} lh={:?} a={:?}",
                    if r.decoration { "~" } else { "" },
                    r.heading,
                    r.line_height,
                    r.align
                )
            })
            .collect()
    }

    #[test]
    fn enter_and_backspace_anywhere_show_what_they_write_and_undo_in_one_step() {
        // Enter that writes what the view does not draw looks like a key that
        // did nothing, so it gets pressed again, and the lines pile up unseen
        // until something redraws them all at once. The rule, the page break,
        // a paragraph's start, the first block's start and the blank page
        // were each that bug once. So: at every place the caret can stand, in
        // both flows, Enter either changes what is drawn or writes nothing,
        // and one Undo takes back whatever it wrote. Enter and Backspace both
        // leave the view a fresh open of the text would draw, so nothing
        // moves when the view is next rebuilt from scratch.
        let cases: &[(Format, &str)] = &[
            (
                Format::Markdown,
                "I talk to myself\n\nI laugh\n\nI cry\n\nknees to nose\n\nlet me go\n",
            ),
            (Format::Markdown, "I talk to myself\nI laugh\nI cry\n"),
            (
                Format::Markdown,
                "<div data-line-height=\"1.5\">\n\nI laugh\n\nI cry\n\nknees to nose\n\n</div>\n",
            ),
            (
                Format::Markdown,
                "<div data-line-height=\"1.15\">\n\nI cry\n\n</div>\n\n<div data-line-height=\"1.15\">\n\nknees\n\n</div>\n",
            ),
            (
                Format::Markdown,
                "<div class=\"center\">\n\nmiddle\n\n</div>\n\nafter\n",
            ),
            (
                Format::Markdown,
                "---\ntitle: x\n---\n\n# Title\n\nbody **bold** and <span data-color=\"red\">red</span>\n",
            ),
            (Format::Markdown, "\n\n\nafter blank lines\n\n\n\nmid\n\n\n"),
            (
                Format::Markdown,
                "- one\n- two\n\n1. a\n2. b\n\n- [ ] task\n",
            ),
            (Format::Markdown, "> quoted\n> more\n\n> - in a quote\n"),
            (Format::Markdown, "a\n\n---\n\nb\n\n::page-break\n\nc\n"),
            (Format::Markdown, "Title\n=====\n\nSub\n---\n\ntext\n"),
            (
                Format::Markdown,
                "```\ncode\n```\n\n| a | b |\n|---|---|\n| 1 | 2 |\n",
            ),
            (Format::Djot, "I laugh\n\nI cry\n\nknees to nose\n"),
            (
                Format::Djot,
                "{data-line-height=\"2\"}\nI cry\n\n{data-line-height=\"2\"}\nknees to nose\n",
            ),
            (Format::Djot, "# Title\n\na\n\n* * *\n\nb\n"),
            // HTML is not here: Enter writes blank lines there, which the
            // view draws and HTML does not keep. See
            // docs/tasks/enter-in-html-writes-whitespace.md.
        ];
        let mut failures = Vec::new();
        for &(format, body) in cases {
            for flow in [LineFlow::Fold, LineFlow::Preserve] {
                let open = || {
                    let mut d = Doc::from_source(body.into(), format).unwrap();
                    d.view = View::Wysiwyg;
                    d.set_line_flow(flow);
                    d.build_visual_unwrapped();
                    d
                };
                // The map a fresh open of the same text would draw. The
                // edited map differing from it is a view that changes on the
                // next full rebuild, seconds after the key, with no key.
                let fresh = |source: &str| {
                    let mut ed =
                        twig::Editor::new_ext(source.as_bytes(), format, parse_extensions())
                            .unwrap();
                    let nodes = ed.nodes().unwrap();
                    crate::wysiwyg::build(
                        &nodes,
                        source,
                        None,
                        flow == LineFlow::Preserve,
                        &wysiwyg::Surface::default(),
                        None,
                    )
                };
                let probe = open();
                let stops: Vec<usize> = (0..=body.len())
                    .filter(|&o| probe.vmap.is_stop(o))
                    .collect();
                for at in stops {
                    for enter in [true, false] {
                        let mut d = open();
                        d.caret = at;
                        d.anchor = None;
                        let before = drawn(&d);
                        if enter {
                            d.newline();
                        } else {
                            d.backspace();
                        }
                        d.build_visual_unwrapped();
                        let key = if enter { "enter" } else { "backspace" };
                        let what = format!(
                            "{key} {format:?} {flow:?} caret {at} in {body:?} -> {:?}",
                            d.source
                        );
                        if maps_differ(&d.vmap, &fresh(&d.source)) {
                            failures.push(format!("redraws differently: {what}"));
                        }
                        if d.source == body {
                            continue;
                        }
                        if enter && drawn(&d) == before {
                            failures.push(format!("drew nothing: {what}"));
                        }
                        d.undo();
                        if d.source != body {
                            failures.push(format!("undo left {:?}: {what}", d.source));
                        }
                    }
                }
            }
        }
        assert!(
            failures.is_empty(),
            "{} failures:\n{}",
            failures.len(),
            failures.join("\n")
        );
    }

    #[test]
    fn enter_opens_a_line_that_draws_inside_a_div_a_quote_and_preserve_flow() {
        // Each of these wrote a line the view did not draw, so Enter looked
        // dead and every press left one more.
        let enter = |format, flow, body: &str, at: usize| {
            let mut d = Doc::from_source(body.into(), format).unwrap();
            d.view = View::Wysiwyg;
            d.set_line_flow(flow);
            d.build_visual(80);
            d.caret = at;
            d.newline();
            d.build_visual(80);
            d
        };
        let div = "<div data-line-height=\"1.5\">\n\nI cry\n\nknees\n\n</div>\n";
        let end = div.find("knees").unwrap() + "knees".len();

        // At the end of a div's last paragraph, the empty paragraph opens
        // inside the div, drawn with its line height, and what is typed
        // there stays in it.
        let mut d = enter(Format::Markdown, LineFlow::Fold, div, end);
        let (row, _) = d.vmap.pos_of_offset(d.caret);
        let r = &d.vmap.rows[row];
        assert!(r.glyphs.is_empty() && !r.decoration, "{:?}", drawn(&d));
        assert!(
            r.line_height.is_some(),
            "the new line keeps the div's spacing"
        );
        d.insert("x");
        assert_eq!(
            d.source,
            "<div data-line-height=\"1.5\">\n\nI cry\n\nknees\n\nx\n\n</div>\n"
        );
        // Preserve flow's soft line there draws too.
        let d = enter(Format::Markdown, LineFlow::Preserve, div, end);
        assert_eq!(
            d.source,
            "<div data-line-height=\"1.5\">\n\nI cry\n\nknees\n\n\n</div>\n"
        );
        assert_eq!(d.vmap.rows.len(), 4, "{:?}", drawn(&d));
        // Between two of the div's paragraphs, the empty line takes its
        // spacing as well.
        let d = enter(
            Format::Markdown,
            LineFlow::Fold,
            div,
            div.find("knees").unwrap(),
        );
        let (row, _) = d.vmap.pos_of_offset(d.caret);
        assert!(
            d.vmap.rows[row - 2].line_height.is_some(),
            "{:?}",
            drawn(&d)
        );

        // Under `</div>` the blank line is the one Markdown needs to end the
        // div. Preserve flow drew it as a line to type on, and Backspace
        // there glued the next paragraph to the tag (`</div>after`).
        let below = "<div class=\"center\">\n\nmiddle\n\n</div>\n\nafter\n";
        let mut d = Doc::from_source(below.into(), Format::Markdown).unwrap();
        d.view = View::Wysiwyg;
        d.set_line_flow(LineFlow::Preserve);
        d.build_visual(80);
        assert_eq!(d.vmap.rows.len(), 2, "{:?}", drawn(&d));
        d.caret = below.find("after").unwrap();
        d.backspace();
        assert!(!d.source.contains("</div>after"), "{:?}", d.source);

        // At a quote's first line, the line opens above the quote; twig's
        // split wrote a quoted blank line above the text, which drew nothing.
        let d = enter(
            Format::Markdown,
            LineFlow::Fold,
            "a\n\n> quoted\n> more\n",
            5,
        );
        assert_eq!(d.source, "a\n\n\n\n> quoted\n> more\n");
        assert_eq!(&d.source[d.caret..], "quoted\n> more\n");

        // Preserve flow at a paragraph's start: past a mark's delimiters its
        // newline cut the mark (`**\nb**`), and in djot a heading ran on
        // over it unseen.
        let d = enter(Format::Markdown, LineFlow::Preserve, "a\n\n**b** c\n", 5);
        assert_eq!(d.source, "a\n\n\n**b** c\n");
        let d = enter(Format::Djot, LineFlow::Preserve, "a\n\n# Title\n", 5);
        assert_eq!(d.source, "a\n\n\n# Title\n");
        // Inside a heading it parts the heading, as in Fold flow.
        let d = enter(Format::Djot, LineFlow::Preserve, "a\n\n# Title\n", 7);
        assert_eq!(d.source, "a\n\n# Ti\n\ntle\n");
    }

    #[test]
    fn enter_on_a_blank_page_writes_nothing() {
        // With no block to push down, Fold flow draws no line for Enter to
        // open, and the newline it wrote was an edit and an undo step that
        // showed nothing. Preserve flow draws every line, so there it writes.
        for body in ["", "\n", "\n\n  \n", "---\nt: x\n---\n"] {
            let mut d = wysiwyg_doc("enter_blank_page", body);
            d.build_visual(80);
            d.caret = d.source.len();
            d.newline();
            assert_eq!(d.source, body);
            assert!(!d.can_undo(), "{body:?}");
            assert!(!d.dirty, "{body:?}");
        }
        let mut d = wysiwyg_doc("enter_blank_page_preserve", "");
        d.set_line_flow(LineFlow::Preserve);
        d.build_visual(80);
        d.newline();
        assert_eq!(d.source, "\n");
    }

    #[test]
    fn enter_beside_a_rule_opens_a_line_under_it() {
        // Beside a rule the caret stands at the home past it, which in source is
        // the start of the blank line under the rule, and Enter took it for an
        // empty paragraph: a lone newline, drawn as one more gap, and the caret
        // put on the next block's first line — where typing joined that block.
        // It goes on under the rule instead, as the rule button leaves it.
        let mut d = wysiwyg_doc("enter_rule", "a\n\n---\n\nb\n");
        d.build_visual(80);
        d.caret = "a\n\n---\n".len();
        d.newline();
        assert_eq!(d.source, "a\n\n---\n\n\n\nb\n");
        assert_eq!(d.caret, "a\n\n---\n\n".len());
        d.insert("X");
        assert_eq!(d.source, "a\n\n---\n\nX\n\nb\n");

        // Closing the document.
        let mut d = wysiwyg_doc("enter_rule_end", "a\n\n---\n");
        d.caret = d.source.len();
        d.newline();
        assert_eq!(d.source, "a\n\n---\n\n");
        assert_eq!(d.caret, d.source.len());

        // In a quote, where the lines it writes keep the quote's prefix.
        let mut d = wysiwyg_doc("enter_rule_quote", "> a\n>\n> ---\n>\n> b\n");
        d.caret = "> a\n>\n> ---\n".len();
        d.newline();
        assert_eq!(d.source, "> a\n>\n> ---\n>\n> \n>\n> b\n");
        d.insert("X");
        assert_eq!(d.source, "> a\n>\n> ---\n>\n> X\n>\n> b\n");

        // Preserve draws the blank line under a rule as a line, and the caret
        // on it: Enter there is a blank line's, one line down.
        let mut d = wysiwyg_doc("enter_rule_preserve", "a\n\n---\n\nb\n");
        d.set_line_flow(LineFlow::Preserve);
        d.caret = "a\n\n---\n".len();
        d.newline();
        assert_eq!(d.source, "a\n\n---\n\n\nb\n");
        assert_eq!(d.caret, "a\n\n---\n\n".len());
    }

    #[test]
    fn insert_table_at_a_paragraph_s_end_splits_nothing() {
        // The same door as the rule's, through the placement they share.
        let mut d = Doc::from_source("para\n".into(), Format::Djot).unwrap();
        d.caret = 4;
        d.insert_table(1, 1);
        assert_eq!(d.source, "para\n\n|  |\n|---|\n|  |\n");
        let mut d = doc_with("table_end_typed", "para");
        d.caret = 4;
        d.insert_table(1, 1);
        assert_eq!(d.source, "para\n\n|  |\n| --- |\n|  |\n");
        assert!(d.caret_in_table());
    }

    #[test]
    fn insert_thematic_break_spells_the_rule_the_format_s_own_way() {
        // The whole point of delegating: `---` is Markdown's, `* * *` is djot's,
        // and leaf wrote the first into both until twig started spelling it.
        let mut md = doc_with("hr_md", "para\n");
        md.caret = 2;
        md.insert_thematic_break();
        assert_eq!(md.source, "pa\n\n---\n\nra\n");

        let mut dj = Doc::from_source("para\n".into(), Format::Djot).unwrap();
        dj.caret = 2;
        dj.insert_thematic_break();
        assert_eq!(dj.source, "pa\n\n* * *\n\nra\n");
    }

    #[test]
    fn insert_table_parts_the_paragraph_and_lands_in_the_first_header_cell() {
        // The table goes *at* the caret the way the rule does: the paragraph is
        // parted first, and twig writes the grid after its first half. The
        // caret then sits in the first header cell — selected, as Tab would
        // leave it — so the next keystroke is the heading.
        let mut d = doc_with("table_mid", "before after\n");
        d.caret = 7;
        d.insert_table(2, 3);
        assert_eq!(
            d.source,
            "before \n\n|  |  |  |\n| --- | --- | --- |\n|  |  |  |\n|  |  |  |\n\nafter\n"
        );
        assert!(d.caret_in_table());
        let first_bar = d.source.find('|').unwrap();
        assert!(
            d.caret > first_bar && d.caret < d.source.find("| ---").unwrap(),
            "caret {} is not in the header row",
            d.caret
        );
        d.insert("Name");
        assert!(d.source.starts_with("before \n\n| Name |  |  |\n"));
        // And the grid the table was written into is one the table keys walk
        // (over the map a frontend rebuilds after every edit).
        d.build_visual(80);
        assert!(d.cell_tab(true));
        d.insert("Qty");
        assert!(d.source.starts_with("before \n\n| Name | Qty |  |\n"));
    }

    #[test]
    fn insert_table_spells_the_grid_the_format_s_own_way() {
        // Djot's delimiter row is unpadded, and leaf never has to know that.
        let mut dj = Doc::from_source("para\n".into(), Format::Djot).unwrap();
        dj.caret = 2;
        dj.insert_table(1, 2);
        assert_eq!(dj.source, "pa\n\n|  |  |\n|---|---|\n|  |  |\n\nra\n");
        assert!(dj.caret_in_table());
    }

    #[test]
    fn insert_table_refuses_where_the_format_spells_no_table() {
        let mut d = Doc::from_source("<p>ab</p>\n".into(), Format::Html).unwrap();
        d.caret = 4;
        d.insert_table(1, 1);
        assert_eq!(d.source, "<p>ab</p>\n");
        assert!(d.status.as_deref().unwrap_or("").contains("not supported"));
        assert!(!d.capabilities().table);
    }

    #[test]
    fn insert_table_reports_a_zero_shape_and_writes_nothing() {
        let mut d = doc_with("table_zero", "para\n");
        d.caret = 2;
        d.insert_table(0, 2);
        assert_eq!(d.source, "para\n");
        assert!(d.status.as_deref().unwrap_or("").starts_with("table:"));
    }

    #[test]
    fn clicking_below_a_final_thematic_break_can_type_after_it() {
        let mut d = wysiwyg_doc("hr_final_click", "---\n");
        d.build_visual(80);
        d.click(d.vmap.num_rows() + 2, 0, false);
        assert_eq!(d.caret, d.source.len(), "the caret belongs after the rule");
        d.insert("after");
        assert_eq!(d.source, "---\nafter");
    }

    #[test]
    fn enter_in_a_nested_list_item_keeps_the_new_item_nested() {
        // The same bytes are two documents. In Markdown `  - b` is a nested item
        // and the next one belongs beside it, at its indent. In Djot a list
        // marker can't interrupt a paragraph, so those bytes are literal text in
        // item `a` and there is only one item — writing `  - ` under it would add
        // no item at all, just more text, and the new sibling has to go to
        // column zero. Both spellings come out of the *enclosing item's* line.
        let mut md = wysiwyg_doc("enter_nested_md", "- a\n  - b\n");
        md.caret = "- a\n  - b".len();
        md.newline();
        assert_eq!(md.source, "- a\n  - b\n  - \n");
        assert_eq!(list_items(&mut md), 3);

        let mut dj = Doc::from_source("- a\n  - b\n".into(), Format::Djot).unwrap();
        dj.view = View::Wysiwyg;
        dj.build_visual(80);
        dj.caret = "- a\n  - b".len();
        dj.newline();
        assert_eq!(dj.source, "- a\n  - b\n- \n");
        assert_eq!(list_items(&mut dj), 2);

        // Where Djot's nesting is real — opened by a blank line — the indent is
        // reproduced there too, and the two formats agree again.
        let mut dj = Doc::from_source("- a\n\n  - b\n".into(), Format::Djot).unwrap();
        dj.view = View::Wysiwyg;
        dj.build_visual(80);
        dj.caret = "- a\n\n  - b".len();
        dj.newline();
        assert_eq!(dj.source, "- a\n\n  - b\n  - \n");
        assert_eq!(list_items(&mut dj), 3);
    }

    #[test]
    fn tab_nests_an_item_at_the_column_its_own_marker_asks_for() {
        // Tab replaces the line's whole prefix with the one twig spells, so the
        // quote markers, the parent's indent and an ordered marker's extra
        // column are all its answer rather than leaf's arithmetic.
        for (name, body, caret, want) in [
            ("bullet", "- a\n- b\n", 6, "- a\n  - b\n"),
            ("ordered", "1. a\n2. b\n", 8, "1. a\n   1. b\n"),
            ("quoted", "> - a\n> - b\n", 10, "> - a\n>   - b\n"),
            // A checkbox is markup the item's own text wraps past, but a nested
            // list may only open at the *list* marker's column — four in from
            // there is a paragraph continuation, and `- [ ] a\n      - [ ] b`
            // parses as one item, not two.
            ("task", "- [ ] a\n- [ ] b\n", 14, "- [ ] a\n  - [ ] b\n"),
            (
                "quoted task",
                "> - [ ] a\n> - [ ] b\n",
                18,
                "> - [ ] a\n>   - [ ] b\n",
            ),
        ] {
            let mut doc = wysiwyg_doc(name, body);
            doc.caret = caret;
            doc.indent();
            assert_eq!(doc.source, want, "{name}");
            // The nesting is real, not just indented text.
            assert_eq!(list_items(&mut doc), 2, "{name}");
        }
    }

    #[test]
    fn backspace_only_outdents_where_the_format_says_there_is_an_item() {
        // The same bytes, the two formats disagreeing, and a gesture that used
        // to read the bytes. `  - b` is a nested item in Markdown, so Backspace
        // at its marker outdents. In Djot a marker can't interrupt a paragraph,
        // so those bytes are literal text inside item `a` — there is nothing to
        // outdent, and treating them as a marker turned one item into two, a
        // structural edit from a keystroke that should delete one character.
        //
        // twig's `line_prefix` is what tells them apart: it reports the marker
        // on the Markdown line and nothing on the Djot one, which is a
        // continuation. No byte scan can reach that answer.
        let src = "- a\n  - b\n";
        let at = "- a\n  - ".len();

        let mut md = Doc::from_source(src.into(), Format::Markdown).unwrap();
        md.view = View::Wysiwyg;
        md.build_visual(80);
        md.caret = at;
        md.backspace();
        assert_eq!(md.source, "- a\n- b\n");
        assert_eq!(list_items(&mut md), 2);

        let mut dj = Doc::from_source(src.into(), Format::Djot).unwrap();
        dj.view = View::Wysiwyg;
        dj.build_visual(80);
        dj.caret = at;
        dj.backspace();
        assert_eq!(dj.source, "- a\n  -b\n"); // an ordinary character delete
        assert_eq!(list_items(&mut dj), 1); // and the structure is untouched
    }

    #[test]
    fn enter_in_a_checklist_item_starts_another_unchecked_one() {
        // Leaf used to spell the next item from the marker bytes it scanned, and
        // its scanner stopped at the bullet — so Enter in a checklist wrote `- `
        // and dropped out of the checklist. twig reproduces the whole
        // continuation, and a fresh item is always unticked however the one above
        // it stands.
        for (name, body, want) in [
            ("unchecked", "- [ ] a\n", "- [ ] a\n- [ ] \n"),
            ("checked", "- [x] a\n", "- [x] a\n- [ ] \n"),
        ] {
            let mut doc = wysiwyg_doc(name, body);
            doc.caret = body.trim_end_matches('\n').len();
            doc.newline();
            assert_eq!(doc.source, want, "{name}");
            // Both items are checklist items — the new one is a box, not the
            // plain bullet the old marker scan left behind — and it is unticked
            // whichever way the one above it faces.
            let boxes: Vec<Option<bool>> = doc
                .nodes()
                .iter()
                .filter(|n| n.kind == Kind::TaskListItem)
                .map(|n| n.checked)
                .collect();
            assert_eq!(boxes.len(), 2, "{name}");
            assert_eq!(boxes[1], Some(false), "{name}");
        }
    }

    #[test]
    fn a_split_takes_the_space_the_caret_was_in_front_of() {
        // Splicing a break at the caret strands the space the words were parted
        // at on the head of the second block, where it reads as an indent nobody
        // typed. twig's split consumes it.
        for (name, body, caret, want) in [
            ("para", "one two\n", 3, "one\n\ntwo\n"),
            ("item", "- one two\n", 5, "- one\n- two\n"),
            ("quote", "> one two\n", 5, "> one\n>\n> two\n"),
            // A heading takes leaf's own path, which has to match.
            ("heading", "# one two\n", 5, "# one\n\ntwo\n"),
        ] {
            let mut doc = wysiwyg_doc(name, body);
            doc.caret = caret;
            doc.newline();
            assert_eq!(doc.source, want, "{name}");
        }
    }

    #[test]
    fn enter_at_the_end_of_a_heading_opens_a_paragraph() {
        // The one place leaf keeps its own break: `split_block` repeats the `#`,
        // and Enter after a title is how the body under it is asked for.
        let mut doc = wysiwyg_doc("head_enter", "# Title\n");
        doc.caret = "# Title".len();
        doc.newline();
        doc.insert("body");
        assert_eq!(doc.source, "# Title\n\nbody\n");
        assert_eq!(
            doc.nodes()
                .iter()
                .filter(|n| n.kind == Kind::Heading)
                .count(),
            1
        );
    }

    #[test]
    fn enter_in_a_quoted_list_item_starts_the_next_quoted_item() {
        // A quoted item's marker doesn't open its line, so a scan that starts at
        // column zero finds a `>` where it wanted a bullet, calls the line "not a
        // list" and hands Enter to the plain-quote branch — which writes `> ` and
        // drops the list. The next item has to carry the whole prefix.
        for (name, body, want) in [
            ("flat", "> - a\n", "> - a\n> - \n"),
            ("sibling", "> - a\n> - b\n", "> - a\n> - b\n> - \n"),
            ("nested", "> - a\n>   - b\n", "> - a\n>   - b\n>   - \n"),
            ("ordered", "> 1. a\n> 2. b\n", "> 1. a\n> 2. b\n> 3. \n"),
            ("twice quoted", "> > - a\n", "> > - a\n> > - \n"),
        ] {
            let mut doc = wysiwyg_doc(name, body);
            doc.caret = body.trim_end_matches('\n').len();
            doc.newline();
            assert_eq!(doc.source, want, "{name}");
            // The marker isn't just spelled right, it parses as an item.
            assert_eq!(list_items(&mut doc), body.lines().count() + 1, "{name}");
        }
    }

    #[test]
    fn an_empty_quoted_item_leaves_the_list_and_stays_in_the_quote() {
        // Double-Enter exits the list. Unquoted that means a blank line, but a
        // *bare* blank line would end the quote too and drop the caret out of it,
        // so the separator keeps its `>` and the caret's line keeps its `> `.
        let mut doc = wysiwyg_doc("quoted_exit", "> - a\n> - \n");
        doc.caret = "> - a\n> - ".len();
        doc.newline();
        assert_eq!(doc.source, "> - a\n>\n> \n");
        assert_eq!(list_items(&mut doc), 1);
        // What "still in the quote" means for the next keystroke: the caret sits
        // behind the prefix, and what's typed there lands inside the quote as a
        // paragraph of its own — not as more of item `a`.
        doc.insert("x");
        assert_eq!(doc.source, "> - a\n>\n> x\n");
        assert!(
            doc.editor
                .ancestors_at(doc.caret - 1)
                .is_ok_and(|c| c.into_iter().any(|m| m.kind == Kind::BlockQuote))
        );
    }

    #[test]
    fn backspace_at_a_quoted_marker_takes_the_marker_and_leaves_the_quote() {
        // The marker is hidden block markup, so Backspace over it is structural —
        // but only the marker is the list's. Splicing from the line start would
        // take the `>` with it and silently unquote the line.
        let mut doc = wysiwyg_doc("quoted_bksp", "> - a\n");
        doc.caret = "> - ".len();
        doc.backspace();
        assert_eq!(doc.source, "> a\n");
        assert_eq!(list_items(&mut doc), 0);

        // A nested one outdents instead, moving the bullet within the quote
        // rather than moving the quote.
        let mut doc = wysiwyg_doc("quoted_outdent", "> - a\n>   - b\n");
        doc.caret = "> - a\n>   - ".len();
        doc.backspace();
        assert_eq!(doc.source, "> - a\n> - b\n");
        assert_eq!(list_items(&mut doc), 2);
    }

    #[test]
    fn only_a_bare_paragraph_is_parted_around_the_caret() {
        // The split is deliberately narrow. Parting a fenced block would leave
        // two fences with a rule between them, and parting a list item would
        // mint an item nobody asked for on the way to a rule that lands after
        // the list either way — so both keep the whole block intact and take the
        // rule after it. A caret in a quote is likewise left alone.
        for (name, body, caret, want) in [
            (
                "code",
                "```\nfn x() {}\n```\n",
                8,
                "```\nfn x() {}\n```\n\n---\n\n",
            ),
            ("list", "- one two\n", 6, "- one two\n\n---\n\n"),
            ("quote", "> one two\n", 6, "> one two\n>\n> ---\n>\n> \n"),
        ] {
            let mut d = doc_with(&format!("hr_narrow_{name}"), body);
            d.caret = caret;
            d.insert_thematic_break();
            assert_eq!(d.source, want, "{name}: the block should stay whole");
        }
    }

    #[test]
    fn insert_thematic_break_replaces_the_selection() {
        // Now that the rule lands *at* the caret again, replacing the selection
        // is coherent once more: the text goes, and the rule takes its place.
        // The space the deletion left leading the second half is consumed by the
        // split rather than opening the new paragraph with it.
        let mut d = doc_with("hr_sel", "one two three\n");
        d.anchor = Some(4);
        d.caret = 7; // "two"
        d.insert_thematic_break();
        assert_eq!(d.source, "one \n\n---\n\nthree\n");
        assert_eq!(d.selection(), None);
    }

    #[test]
    fn insert_thematic_break_clears_a_code_block_and_a_table_rather_than_refusing() {
        // Both are blocks the rule lands *after*. Leaf used to refuse a fence,
        // because writing `---` into one is code, not a rule — twig now walks out
        // to the block that owns the caret's line, so there is nothing to refuse.
        let mut code = doc_with("hr_code", "```\nfn x() {}\n```\n");
        code.caret = 5; // inside the fenced code
        code.insert_thematic_break();
        assert_eq!(code.source, "```\nfn x() {}\n```\n\n---\n\n");
        assert_eq!(code.status, None, "no refusal to report any more");

        let mut table = doc_with("hr_table", "| a | b |\n|---|---|\n| 1 | 2 |\n");
        table.caret = 3; // in the header row
        table.insert_thematic_break();
        assert_eq!(table.source, "| a | b |\n|---|---|\n| 1 | 2 |\n\n---\n\n");
    }

    #[test]
    fn insert_thematic_break_in_a_list_item_ends_the_list() {
        // The un-indented rule cannot continue the list, so it closes the list
        // and lands at the top level rather than nested inside it.
        let mut d = doc_with("hr_list", "- one\n- two\n");
        d.caret = "- one\n- tw".len(); // mid "two"
        d.insert_thematic_break();
        d.build_visual(80);
        let rule_at = d.source.find("---").unwrap();
        assert_eq!(kind_at(&mut d, rule_at), Some(Kind::ThematicBreak));
        assert!(
            !d.nodes().iter().any(|n| n.kind == Kind::BulletList
                && n.span.start <= rule_at
                && rule_at < n.span.end),
            "the rule must not be nested inside the list"
        );
    }

    #[test]
    fn insert_thematic_break_in_a_blockquote_stays_in_the_quote() {
        // Leaf used to end the quote. twig gives the rule the quote's own prefix,
        // which is the document the gesture was actually asked for — and the
        // line the caret goes on to under it wears the prefix too, as the line
        // Enter opens in a quote does.
        let mut d = doc_with("hr_quote", "> hello\n");
        d.caret = 4; // inside the quoted text
        d.insert_thematic_break();
        assert_eq!(d.source, "> hello\n>\n> ---\n>\n> \n");
        assert_eq!(d.caret, "> hello\n>\n> ---\n>\n> ".len());
        d.build_visual(80);
        let rule_at = d.source.find("---").unwrap();
        assert_eq!(kind_at(&mut d, rule_at), Some(Kind::ThematicBreak));
        assert!(
            d.nodes().iter().any(|n| n.kind == Kind::BlockQuote
                && n.span.start <= rule_at
                && rule_at < n.span.end),
            "the rule belongs to the quote it was asked for"
        );
    }

    // ── typing against a block picture ────────────────────────────────────────

    /// A rendered-view document with the caret parked on one of the picture's two
    /// stops, and the map already built — the state a frontend is in between
    /// drawing a frame and the next keystroke.
    fn doc_at_picture(name: &str, src: &str, side: MediaStop) -> Doc {
        let mut d = doc_in(View::Wysiwyg, name, src);
        d.build_visual_unwrapped();
        let start = src.find("![").unwrap();
        d.caret = match side {
            MediaStop::Before => start,
            MediaStop::After => start + "![](p.png)".len(),
        };
        d
    }

    /// The block media the map publishes, after rebuilding it — "is this still a
    /// picture, or has it become a line of text with an image in it?"
    fn media_count(d: &mut Doc) -> usize {
        d.build_visual_unwrapped();
        d.vmap.media.len()
    }

    #[test]
    fn typing_past_a_block_picture_opens_a_paragraph_under_it() {
        // The accident this prevents: tap the blank page under a photo (which
        // lands on the picture's trailing stop), type, and `![](p.png)xy` is a
        // paragraph with an *inline* image — the photo stops being drawn.
        let mut d = doc_at_picture("pic_after", "hi\n\n![](p.png)\n", MediaStop::After);
        d.insert("xy");
        assert_eq!(d.source, "hi\n\n![](p.png)\n\nxy\n");
        assert_eq!(media_count(&mut d), 1, "still a picture");
    }

    #[test]
    fn typing_in_front_of_a_block_picture_opens_a_paragraph_above_it() {
        let mut d = doc_at_picture("pic_before", "hi\n\n![](p.png)\n", MediaStop::Before);
        d.insert("xy");
        assert_eq!(d.source, "hi\n\nxy\n\n![](p.png)\n");
        assert_eq!(media_count(&mut d), 1);
    }

    #[test]
    fn a_picture_that_opens_the_document_still_takes_a_paragraph_above_it() {
        let mut d = doc_at_picture("pic_first", "![](p.png)\n", MediaStop::Before);
        d.insert("x");
        assert_eq!(d.source, "x\n\n![](p.png)\n");
        assert_eq!(media_count(&mut d), 1);
    }

    #[test]
    fn one_undo_puts_the_picture_back_the_way_it_was_found() {
        // The opened paragraph is part of the keystroke, not an edit the writer
        // made — so it undoes with the character, not a step later.
        let mut d = doc_at_picture("pic_undo", "hi\n\n![](p.png)\n", MediaStop::After);
        d.insert("x");
        assert_eq!(d.source, "hi\n\n![](p.png)\n\nx\n");
        d.undo();
        assert_eq!(d.source, "hi\n\n![](p.png)\n");
    }

    #[test]
    fn pasting_against_a_block_picture_opens_a_paragraph_too() {
        // ⌘V dissolves the picture exactly as a keystroke does.
        let mut d = doc_at_picture("pic_paste", "hi\n\n![](p.png)\n", MediaStop::After);
        d.paste("pasted");
        assert_eq!(d.source, "hi\n\n![](p.png)\n\npasted\n");
        assert_eq!(media_count(&mut d), 1);
    }

    #[test]
    fn typing_beside_an_inline_image_is_ordinary_editing() {
        // An inline image has no placeholder row and no stops of its own. Opening
        // a paragraph mid-sentence would be the bug, not the fix.
        let mut d = doc_in(View::Wysiwyg, "pic_inline", "see ![](p.png) here\n");
        d.build_visual_unwrapped();
        d.caret = "see ![](p.png)".len();
        d.insert("!");
        assert_eq!(d.source, "see ![](p.png)! here\n");
    }

    #[test]
    fn source_view_types_raw_markup_against_an_image_untouched() {
        // Source view is for writing the markup itself; a break inserted behind
        // the writer's back there would be the editor arguing with them.
        let mut d = doc_in(View::Source, "pic_src", "![](p.png)\n");
        d.caret = "![](p.png)".len();
        d.insert("x");
        assert_eq!(d.source, "![](p.png)x\n");
    }

    #[test]
    fn typing_over_a_selection_that_starts_at_a_picture_stop_replaces_it() {
        // A selection is replaced, not joined into, so there is nothing to
        // protect: the range takes the picture with it.
        let mut d = doc_at_picture("pic_sel", "hi\n\n![](p.png)\n", MediaStop::Before);
        d.anchor = Some(d.caret);
        d.caret = d.source.find("![").unwrap() + "![](p.png)".len();
        d.insert("x");
        assert_eq!(d.source, "hi\n\nx\n");
    }

    #[test]
    fn backspace_past_a_block_picture_deletes_the_picture_not_its_last_byte() {
        // What this actually cost: a real vault's photo, to one stray Backspace.
        // The caret past `![](p.png)` was deleting the closing paren — invisible
        // in the rendered view — and the photo became the text `![](p.png`.
        let mut d = doc_at_picture("pic_bs", "hi\n\n![](p.png)\n", MediaStop::After);
        d.backspace();
        assert_eq!(d.source, "hi\n");
        assert_eq!(media_count(&mut d), 0, "the picture went, in one piece");
        d.undo();
        assert_eq!(
            d.source, "hi\n\n![](p.png)\n",
            "and comes back in one piece"
        );
    }

    #[test]
    fn backspace_in_front_of_a_block_picture_steps_out_instead_of_merging_it() {
        // Deleting the break here would join the picture to the paragraph above,
        // where it is an *inline* image and stops being drawn. Step over the
        // boundary; the next press deletes in the paragraph the caret reached.
        let mut d = doc_at_picture("pic_bs_before", "hi\n\n![](p.png)\n", MediaStop::Before);
        d.backspace();
        assert_eq!(d.source, "hi\n\n![](p.png)\n", "nothing deleted");
        assert_eq!(d.caret, 2, "the caret stepped up to the end of `hi`");
        d.backspace();
        assert_eq!(d.source, "h\n\n![](p.png)\n", "and now it deletes there");
        assert_eq!(media_count(&mut d), 1, "the picture was never at risk");
    }

    #[test]
    fn forward_delete_in_front_of_a_block_picture_deletes_the_picture() {
        // The mirror. A byte-step here eats the `!` and leaves a link.
        let mut d = doc_at_picture("pic_del", "hi\n\n![](p.png)\n\nbye\n", MediaStop::Before);
        d.delete_forward();
        assert_eq!(d.source, "hi\n\nbye\n");
        assert_eq!(media_count(&mut d), 0);
    }

    #[test]
    fn forward_delete_past_a_block_picture_steps_over_the_boundary() {
        let mut d = doc_at_picture(
            "pic_del_after",
            "hi\n\n![](p.png)\n\nbye\n",
            MediaStop::After,
        );
        d.delete_forward();
        assert_eq!(d.source, "hi\n\n![](p.png)\n\nbye\n", "nothing deleted");
        assert_eq!(
            d.caret,
            d.source.find("bye").unwrap(),
            "the caret stepped down to `bye`"
        );
    }

    #[test]
    fn a_picture_that_is_the_whole_document_still_deletes_cleanly() {
        let mut d = doc_at_picture("pic_only", "![](p.png)\n", MediaStop::After);
        d.backspace();
        assert_eq!(d.source, "\n");
        assert_eq!(media_count(&mut d), 0);
    }

    #[test]
    fn a_word_delete_takes_the_picture_whole_or_steps_out_of_it() {
        // ⌥⌫ past a picture would otherwise eat a "word" of its markup.
        let mut d = doc_at_picture("pic_wordbs", "hi there\n\n![](p.png)\n", MediaStop::After);
        d.delete_word_back();
        assert_eq!(d.source, "hi there\n");

        // And in front of one it runs *through* the paragraph break into the
        // prose above, which merges the picture inline — so it steps out first,
        // and the second press deletes the word it was aimed at.
        let mut d = doc_at_picture("pic_wordbs2", "hi there\n\n![](p.png)\n", MediaStop::Before);
        d.delete_word_back();
        assert_eq!(d.source, "hi there\n\n![](p.png)\n");
        d.delete_word_back();
        assert_eq!(
            d.source, "hi \n\n![](p.png)\n",
            "the word above went, the picture stayed"
        );
        assert_eq!(media_count(&mut d), 1);
    }

    #[test]
    fn source_view_deletes_raw_markup_against_an_image_untouched() {
        let mut d = doc_in(View::Source, "pic_src_del", "![](p.png)\n");
        d.caret = "![](p.png)".len();
        d.backspace();
        assert_eq!(d.source, "![](p.png\n", "raw editing, byte by byte");
    }

    #[test]
    fn image_destination_at_caret_reads_the_image_under_the_caret() {
        let mut d = doc_with("img_read", "![a cat](cat.png)\n");
        d.caret = 3; // inside the image markup
        assert_eq!(d.image_destination_at_caret(), Some("cat.png".to_string()));
        // Past the image, the caret is in no image.
        d.caret = "![a cat](cat.png)".len();
        assert_eq!(d.image_destination_at_caret(), None);
    }

    #[test]
    fn set_media_rows_reserves_blank_filler_rows_the_frontend_paints_over() {
        // The image is one placeholder row by default, and `set_media_rows` grows
        // it to the height the frontend measured: the label row plus blank
        // `decoration` fillers that hold the vertical space a raster is drawn into.
        let mut d = wysiwyg_doc("img_rows", "intro\n\n![a cat](cat.png)\n\nend\n");
        assert_eq!(d.vmap.media.len(), 1);
        let img_row = d.vmap.media[0].rows_span.start;
        assert_eq!(
            d.vmap.media[0].rows_span,
            img_row..img_row + 1,
            "default is one row"
        );

        d.set_media_rows(HashMap::from([("cat.png".to_string(), 4)]));
        d.build_visual(80);
        assert_eq!(d.vmap.media.len(), 1, "still one image, now taller");
        let span = d.vmap.media[0].rows_span.clone();
        assert_eq!(span.end - span.start, 4, "reserves the four rows asked for");
        // The label row carries the mark and its glyphs; the three below are blank
        // decoration — drawn, but no caret and no text.
        assert!(
            d.vmap.rows[span.start].media.is_some(),
            "mark rides the first row"
        );
        for r in (span.start + 1)..span.end {
            assert!(d.vmap.rows[r].decoration, "filler row {r} is decoration");
            assert!(d.vmap.rows[r].glyphs.is_empty(), "filler row {r} is blank");
            assert!(
                d.vmap.rows[r].media.is_none(),
                "only the first row is marked"
            );
        }
    }

    #[test]
    fn a_taller_image_adds_no_caret_stops_and_motion_steps_over_its_fillers() {
        // The extra rows are pure spacers: the caret's only homes stay the stop in
        // front of the image and the one just past it, so walking the document top
        // to bottom visits the same offsets whether the image is 1 row or 5.
        let body = "ab\n\n![x](p.png)\n\ncd\n";
        let stops_at = |rows: usize| -> Vec<usize> {
            let mut d = wysiwyg_doc("img_stops", body);
            if rows > 1 {
                d.set_media_rows(HashMap::from([("p.png".to_string(), rows)]));
                d.build_visual(80);
            }
            d.caret = 0;
            let mut seen = vec![d.caret];
            loop {
                d.move_right(false);
                if *seen.last().unwrap() == d.caret {
                    break;
                }
                seen.push(d.caret);
            }
            seen
        };
        assert_eq!(
            stops_at(1),
            stops_at(5),
            "reserving rows must not add stops"
        );
    }

    #[test]
    fn insert_link_repoints_the_link_at_a_bare_caret() {
        let mut d = doc_with("link_repoint", "[word](http://x.dev)\n");
        d.caret = 3; // in the link's text, nothing selected
        d.insert_link("http://y.dev");
        assert_eq!(d.source, "[word](http://y.dev)\n");
        assert_eq!(d.selected_text(), Some("word"));
    }

    #[test]
    fn insert_link_on_an_empty_range_autolinks_a_url() {
        // A link with no text of its own is an autolink, and twig spells it —
        // `<…>` is the canonical form and needs no text typed into it, so the
        // caret lands after it rather than selecting a finished link.
        let mut d = doc_with("link_empty", "\n");
        d.caret = 0;
        d.insert_link("http://x.dev");
        assert_eq!(d.source, "<http://x.dev>\n");
        assert_eq!(d.selection(), None);
        assert_eq!(d.caret, 14);
    }

    #[test]
    fn insert_link_on_an_empty_range_falls_back_for_a_non_url() {
        // `<./notes.md>` is literal text in both formats and `<foo>` is raw HTML
        // in Markdown, so a destination that can't autolink doubles as the text
        // instead — which is then selected, ready to be typed over.
        let mut d = doc_with("link_rel", "\n");
        d.caret = 0;
        d.insert_link("./notes.md");
        assert_eq!(d.source, "[./notes.md](./notes.md)\n");
        assert_eq!(d.selection(), Some((1, 11)));
        d.insert("Notes");
        assert_eq!(d.source, "[Notes](./notes.md)\n");
    }

    #[test]
    fn insert_link_repoints_the_autolink_the_caret_stands_in() {
        // The autolink's text is its URL, so re-pointing replaces the whole
        // node — the caret must not splice a second link inside the first.
        let mut d = doc_with("link_repoint_auto", "see <https://x.dev> ok\n");
        d.caret = 10;
        d.insert_link("https://y.dev");
        assert_eq!(d.source, "see <https://y.dev> ok\n");
    }

    #[test]
    fn code_language_reads_and_edits_through_the_fence() {
        let mut d = doc_with("code_lang", "```rust\nlet x = 1;\n```\n");
        d.caret = 10; // inside the code body
        assert_eq!(d.code_language_at_caret().as_deref(), Some("rust"));
        assert!(d.caret_in_fenced_code());

        d.set_code_language("python");
        assert!(
            d.source.starts_with("```python\n"),
            "source: {:?}",
            d.source
        );
        assert_eq!(d.code_language_at_caret().as_deref(), Some("python"));

        // Clearing it leaves a bare fence and no label.
        d.set_code_language("");
        assert!(d.source.starts_with("```\n"), "source: {:?}", d.source);
        assert_eq!(d.code_language_at_caret(), None);

        // A caret outside any code block edits nothing.
        let mut p = doc_with("code_lang_none", "just prose\n");
        assert!(!p.caret_in_fenced_code());
        p.set_code_language("rust");
        assert_eq!(p.source, "just prose\n");
    }

    #[test]
    fn code_language_reads_and_edits_through_a_quoted_fence() {
        let mut d = doc_with("code_lang_quote", "> ```rust\n> let x = 1;\n> ```\n");
        d.caret = d.source.find("let").unwrap();
        assert_eq!(d.code_language_at_caret().as_deref(), Some("rust"));
        assert!(d.caret_in_fenced_code());
        d.set_code_language("python");
        assert!(
            d.source.starts_with("> ```python\n"),
            "source: {:?}",
            d.source
        );
        assert_eq!(d.code_language_at_caret().as_deref(), Some("python"));
    }

    #[test]
    fn a_language_the_fence_cannot_carry_is_refused_not_written() {
        // Markdown's info string ends at whitespace, so `two words` would write
        // a fence that reads back with a different language than the one asked
        // for. twig refuses it; leaf reports that and leaves the source alone.
        // The old splice trimmed the ends and wrote whatever was left.
        let mut d = doc_with("code_lang_bad", "```rust\nx\n```\n");
        d.caret = 10;
        d.set_code_language("two words");
        assert_eq!(d.source, "```rust\nx\n```\n", "source should be untouched");
        assert!(d.status.is_some(), "the refusal should be reported");
        assert_eq!(d.code_language_at_caret().as_deref(), Some("rust"));
    }

    #[test]
    fn link_destination_at_caret_reads_both_spellings() {
        let mut d = doc_with("link_dest", "see [t](https://x.dev) ok\n");
        d.caret = 5;
        assert_eq!(
            d.link_destination_at_caret().as_deref(),
            Some("https://x.dev")
        );
        d.caret = 0;
        assert_eq!(d.link_destination_at_caret(), None);

        // An autolink has no `destination`; its text is the URL.
        let mut a = doc_with("link_dest_auto", "see <https://x.dev> ok\n");
        a.caret = 10;
        assert_eq!(
            a.link_destination_at_caret().as_deref(),
            Some("https://x.dev")
        );
        a.caret = 21;
        assert_eq!(a.link_destination_at_caret(), None);
    }

    #[test]
    fn heading_at_caret_is_the_nearest_heading_above() {
        let src = "intro\n\n# Title\n\n## The *Second* Part\n\nbody here\n";
        let mut d = doc_with("heading_at", src);
        // Under no heading at all.
        d.caret = 2;
        assert_eq!(d.heading_at_caret(), None);
        // In a paragraph under a level-2 heading: that heading, its markup
        // stripped for the slug, and its own span.
        d.caret = src.find("body").unwrap() + 2;
        let h = d.heading_at_caret().expect("under `## The Second Part`");
        assert_eq!(h.text, "The Second Part");
        assert_eq!(h.level, 2);
        assert_eq!(&src[h.span.clone()].trim_end(), &"## The *Second* Part");
        // Standing in a heading answers with that heading.
        let h = d.heading_at(src.find("Title").unwrap()).unwrap();
        assert_eq!((h.text.as_str(), h.level), ("Title", 1));
        // And the text is what `locate` lands a slug of.
        let landing = d.locate("the-second-part").unwrap();
        assert_eq!(landing.start, d.heading_at_caret().unwrap().span.start);
    }

    #[test]
    fn locate_finds_the_block_a_declared_id_names() {
        // The Book of Mormon shape: one document per chapter, one `{#v…}` per
        // verse. The locator has to land on the *verse*, which is the whole
        // reason a link carries one.
        let src = "{#v1}\nI, Nephi, having been born of goodly parents.\n\n\
                   {#v2}\nYea, I make a record in the language of my father.\n";
        let mut d = Doc::from_source(src.to_string(), Format::Djot).unwrap();
        let v2 = d.locate("v2").expect("the document declares `{#v2}`");
        assert_eq!(
            d.source[v2.start..v2.end].trim_end(),
            "Yea, I make a record in the language of my father."
        );
        // The attribute line is not part of it: `start` is a place to put a
        // caret, and `{#v2}` is markup the caret has no business landing in.
        assert!(d.source[..v2.start].ends_with("{#v2}\n"));
        assert_eq!(d.locate("v99"), None);
    }

    #[test]
    fn locate_reads_a_heading_by_its_words_when_the_format_mints_no_ids() {
        // Markdown has no ids at all — twig mints none, and `{#custom}` in a
        // Markdown heading is literal text. So `#the-second-part` can only be
        // the heading's own words, which is the rule every Markdown renderer
        // already follows and therefore the one a link was authored against.
        let src = "# Title\n\nintro\n\n## The Second Part\n\nbody\n\n## Third\n\nmore\n";
        let mut d = doc_with("locate_md", src);
        let hit = d.locate("the-second-part").expect("the heading's slug");
        assert!(d.source[hit.start..].starts_with("## The Second Part"));
        // Bounded by the next heading that isn't under it, so a peek shows the
        // section rather than only its title.
        assert_eq!(
            &d.source[hit.start..hit.end],
            "## The Second Part\n\nbody\n\n"
        );

        // A subsection does not end its parent: `# Title` runs to `## Third`'s
        // sibling only because there is no other `#`, so it covers the lot.
        let title = d.locate("title").expect("the top heading");
        assert_eq!(title.end, d.source.len());
    }

    #[test]
    fn locate_reads_a_djot_auto_id_however_the_link_spelled_it() {
        // djot mints `Some-Heading-Here`; a link to it is written
        // `#some-heading-here` by nearly everything that writes links. Both
        // spellings are one question.
        let src = "## Some Heading Here\n\nbody\n";
        let mut d = Doc::from_source(src.to_string(), Format::Djot).unwrap();
        let exact = d.locate("Some-Heading-Here").expect("djot's own spelling");
        let slugged = d.locate("some-heading-here").expect("the link's spelling");
        assert_eq!(exact, slugged);
        // The section, not the heading line — there is more to show than a title.
        assert_eq!(&d.source[exact.start..exact.end], src);
    }

    #[test]
    fn locate_ignores_an_empty_locator_and_one_that_slugs_to_nothing() {
        let mut d = doc_with("locate_empty", "# Title\n\nbody\n");
        assert_eq!(d.locate(""), None);
        assert_eq!(d.locate("   "), None);
        // All punctuation: it names nothing, and must not be read as "match the
        // first heading whose slug is also empty".
        assert_eq!(d.locate("!!!"), None);
    }

    #[test]
    fn locate_gives_a_duplicated_id_to_the_first_block_that_claims_it() {
        // The document's mistake, and the answer every other anchor
        // implementation gives — the alternative is for a link to mean whichever
        // of the two a walk happened to reach first.
        let src = "{#dup}\nfirst.\n\n{#dup}\nsecond.\n";
        let mut d = Doc::from_source(src.to_string(), Format::Djot).unwrap();
        let hit = d.locate("dup").expect("the first `{#dup}`");
        assert_eq!(d.source[hit.start..hit.end].trim_end(), "first.");
    }

    #[test]
    fn insert_footnote_writes_both_halves_and_lands_the_caret_in_the_note() {
        // The button's whole job: a reference where the caret was, a definition
        // to give it meaning, and the caret waiting in the empty note so the
        // next keystroke is the note's first word.
        let mut d = doc_with("fn_insert", "A claim and more.\n");
        d.caret = 7; // just past "A claim"
        d.insert_footnote();
        assert!(
            d.source.starts_with("A claim[^1] and more."),
            "{:?}",
            d.source
        );
        assert!(
            d.source.contains("[^1]:"),
            "the definition too: {:?}",
            d.source
        );
        assert_eq!(d.status, None);

        let reference = d.source.find("[^1]").unwrap();
        let note = d
            .footnote_at(reference + 2)
            .expect("the reference just written");
        assert_eq!(note.label, "1");
        assert_eq!(note.text.as_deref(), Some(""), "the note starts empty");
        assert_eq!(Some(d.caret), note.offset, "the caret waits in the note");
        // …and typing there is typing into the note, not near it.
        d.insert("the note");
        assert_eq!(
            d.footnote_at(reference + 2).and_then(|f| f.text),
            Some("the note".to_string())
        );
    }

    #[test]
    fn insert_footnote_numbers_past_the_notes_already_written() {
        // A second press must not hand back a label somebody else is using: twig
        // reuses a defined label rather than appending a rival definition, so a
        // repeat of `1` would quietly point the new reference at the old note.
        let mut d = doc_with("fn_insert_number", "One[^1] two.\n\n[^1]: first\n");
        d.caret = 7; // past `[^1]`, before " two."
        d.insert_footnote();
        assert!(d.source.starts_with("One[^1][^2] two."), "{:?}", d.source);
        assert_eq!(d.source.matches("[^2]:").count(), 1);
    }

    #[test]
    fn insert_footnote_counts_a_dangling_reference_and_ignores_a_named_one() {
        // `[^2]` with no definition is still a 2 that means something to whoever
        // wrote it — stepping over it would mint a note for their reference. A
        // word label takes no number, so it blocks none.
        let mut d = doc_with("fn_insert_dangling", "a[^2] b[^why] c\n\n[^why]: named\n");
        d.caret = d.source.find(" c").unwrap();
        d.insert_footnote();
        assert!(d.source.contains("[^1]:"), "1 is free: {:?}", d.source);
        assert!(
            d.source.starts_with("a[^2] b[^why][^1] c"),
            "{:?}",
            d.source
        );
    }

    #[test]
    fn insert_footnote_marks_the_selection_rather_than_replacing_it() {
        // A reference annotates the words before it. Consuming the selection —
        // which is what an insert normally does — would delete the very claim
        // the author selected in order to footnote.
        let mut d = doc_with("fn_insert_sel", "A claim and more.\n");
        d.anchor = Some(2);
        d.caret = 7; // "claim" selected
        d.insert_footnote();
        assert!(
            d.source.starts_with("A claim[^1] and more."),
            "{:?}",
            d.source
        );
    }

    #[test]
    fn a_note_just_written_still_knows_where_its_reference_is() {
        // The authoring loop in one test: press the button, type the note, ask to
        // go back. The caret ends at the note's last byte — which is the *end* of
        // the definition's span, the one offset the query used to exclude — so
        // this is where the round trip either works or doesn't.
        let mut d = doc_with("fn_insert_return", "A claim and more.\n");
        d.caret = 7;
        d.insert_footnote();
        d.insert("the note");
        assert_eq!(d.source, "A claim[^1] and more.\n\n[^1]: the note\n");
        let back = d
            .footnote_definition_at_caret()
            .expect("still in the note we just typed");
        assert_eq!(back.label, "1");
        // …and following it lands on the reference's label, where a reader's
        // return leg lands.
        assert_eq!(back.offset, Some(9));
        assert_eq!(&d.source[9..10], "1");
    }

    #[test]
    fn insert_footnote_takes_one_undo_for_both_halves() {
        // twig writes the pair as a single edit; the point of that is here.
        let before = "A claim and more.\n";
        let mut d = doc_with("fn_insert_undo", before);
        d.caret = 7;
        d.insert_footnote();
        assert_ne!(d.source, before);
        d.undo();
        assert_eq!(d.source, before, "one undo takes back both halves");
    }

    #[test]
    fn insert_footnote_refuses_a_format_that_cannot_spell_one() {
        // HTML is authorable — it spells the inline marks — and has no footnote.
        // The refusal says so rather than writing brackets that would render as
        // brackets.
        let src = "<p>A claim.</p>\n";
        let mut d = Doc::from_source(src.to_string(), Format::Html).unwrap();
        assert!(!Capabilities::of(Format::Html).footnote);
        d.caret = 5;
        d.insert_footnote();
        assert_eq!(d.source, src, "nothing written");
        assert!(d.status.is_some_and(|s| s.starts_with("footnote:")));
    }

    #[test]
    fn insert_footnote_leaves_the_caret_on_a_real_stop_in_the_rich_view() {
        // The empty body is the one place this could go wrong: the definition
        // renders as a `[1] ` marker the caret cannot occupy, so a caret aimed a
        // byte early would draw up in the paragraph above the note it belongs to.
        let mut d = doc_in(View::Wysiwyg, "fn_insert_stop", "A claim and more.\n");
        d.place_caret(7, false);
        d.insert_footnote();
        d.build_visual(80); // the frame a frontend draws after the edit
        assert_eq!(
            d.vmap.snap_to_stop(d.caret),
            d.caret,
            "the caret sits on a stop"
        );
        let (row, _) = d.caret_pos();
        assert!(
            drawn_rows(&d)[row].contains("[1]"),
            "the caret is on the note's row, not above it: {:?}",
            drawn_rows(&d)
        );
    }

    #[test]
    fn footnote_at_caret_resolves_a_reference_to_its_note() {
        // `[^1]` spans 7..11; its label byte is at 9. The definition follows a
        // blank line, as one has to.
        let mut d = doc_with("fn_at_caret", "A claim[^1] and more.\n\n[^1]: the note\n");
        d.caret = 9;
        let f = d
            .footnote_at_caret()
            .expect("the caret stands in a reference");
        assert_eq!(f.label, "1");
        assert_eq!(f.text.as_deref(), Some("the note"));
        // The offset points at the note's first word, not at the definition's
        // `[` — the marker is decoration with no caret stop on it.
        assert_eq!(f.offset, Some(29));
        assert_eq!(&d.source[29..37], "the note");
        // …and `end` closes the range, so a frontend can ask which rendered rows
        // the note occupies rather than re-deriving them from the text.
        assert_eq!(f.end, Some(37));
        assert_eq!(&d.source[f.offset.unwrap()..f.end.unwrap()], "the note");
    }

    /// Two definitions in a row: each is its own note, and neither reaches into
    /// the other.
    ///
    /// A djot definition's span used to run past the blank line into the first
    /// byte of whatever followed, so this answered `"first note.\n\n["` — and the
    /// offsets named the *next* note's rows too, showing a reader two footnotes
    /// when they had asked about one. twig 3.1 ends the span after the block's
    /// own last line; the test outlives the workaround leaf carried for it.
    #[test]
    fn footnote_at_stops_a_note_at_the_definition_after_it() {
        let src = "Claim[^2a] and [^2b].\n\n[^2a]: first note.\n\n[^2b]: second note.\n";
        for format in [Format::Markdown, Format::Djot] {
            let mut d = Doc::from_source(src.to_string(), format).unwrap();
            d.caret = 7;
            let f = d.footnote_at_caret().expect("a reference");
            assert_eq!(f.text.as_deref(), Some("first note."), "in {format:?}");
            assert_eq!(
                &src[f.offset.unwrap()..f.end.unwrap()],
                "first note.",
                "in {format:?}"
            );
        }
    }

    /// The other side of that boundary: a blank line *inside* a definition is
    /// interior to it, and the note keeps its second paragraph.
    ///
    /// This is what the old body scan cost. It stopped at the first line not
    /// indented under the note — a blank line is not — so a two-paragraph note
    /// came back as its first paragraph, and "go to note" framed half of it.
    /// Reading the span twig gives is both simpler and right.
    #[test]
    fn footnote_at_keeps_a_notes_second_paragraph() {
        let src = "Claim[^1].\n\n[^1]: first para.\n\n    second para.\n\nAfter.\n";
        let mut d = Doc::from_source(src.to_string(), Format::Djot).unwrap();
        d.caret = 7;
        let f = d.footnote_at_caret().expect("a reference");
        assert_eq!(f.text.as_deref(), Some("first para.\n\n    second para."));
        // And it stops there — `After.` is the next block, not more note.
        assert_eq!(
            &src[f.offset.unwrap()..f.end.unwrap()],
            f.text.as_deref().unwrap()
        );
        assert!(!f.text.as_deref().unwrap().contains("After"));
    }

    #[test]
    fn footnote_at_bounds_a_note_whose_body_is_empty() {
        // `[^1]:` with nothing after it. The range is empty rather than
        // inverted, and still points inside the definition — which is what keeps
        // a frontend's row lookup from walking off into the block above.
        let src = "A claim[^1].\n\n[^1]:\n";
        let mut d = doc_with("fn_empty_body", src);
        d.caret = 9;
        let f = d.footnote_at_caret().expect("a reference");
        assert_eq!(f.text.as_deref(), Some(""));
        assert_eq!(f.offset, f.end, "an empty note is an empty range");
        assert!(f.offset.unwrap() >= src.find("[^1]:").unwrap());
    }

    #[test]
    fn footnote_at_caret_ignores_a_caret_that_stands_in_no_reference() {
        let mut d = doc_with(
            "fn_at_caret_none",
            "A claim[^1] and more.\n\n[^1]: the note\n",
        );
        d.caret = 2; // in the prose
        assert_eq!(d.footnote_at_caret(), None);
    }

    #[test]
    fn footnote_at_caret_is_not_a_link_query_and_vice_versa() {
        // The two are deliberately separate: a reference names a note in this
        // document, a link names somewhere to leave for, and answering one with
        // the other is what made a reference click do nothing at all.
        let mut d = doc_with("fn_vs_link", "a[^1] b [t](https://x.dev)\n\n[^1]: note\n");
        d.caret = 3; // the `1` of `[^1]`
        assert!(d.footnote_at_caret().is_some());
        assert_eq!(
            d.link_destination_at_caret(),
            None,
            "a reference is not a link"
        );

        d.caret = 10; // inside the link's label
        assert_eq!(d.footnote_at_caret(), None, "a link is not a reference");
        assert_eq!(
            d.link_destination_at_caret().as_deref(),
            Some("https://x.dev")
        );
    }

    #[test]
    fn footnote_at_caret_reports_an_undefined_reference_rather_than_nothing() {
        // A `[^99]` the document never defines is a real state — a note deleted
        // out from under its reference — and the label is what lets a frontend
        // say so. `None` here would be indistinguishable from "not on a
        // reference", which is the wrong thing to tell a reader.
        let mut d = doc_with("fn_undefined", "A claim[^99] and more.\n");
        d.caret = 9;
        let f = d
            .footnote_at_caret()
            .expect("the reference is still a reference");
        assert_eq!(f.label, "99");
        assert_eq!(f.text, None);
        assert_eq!(f.offset, None);
    }

    #[test]
    fn footnote_at_caret_reads_a_word_label_and_a_multiline_note() {
        // Labels are not always numbers, and a note's body runs past its first
        // line — the indented continuation belongs to the note, so it comes back
        // with it (source bytes, verbatim, as documented).
        let src = "see[^note] here\n\n[^note]: first line\n    second line\n";
        let mut d = doc_with("fn_word_label", src);
        d.caret = 6;
        let f = d
            .footnote_at_caret()
            .expect("the caret stands in a reference");
        assert_eq!(f.label, "note");
        assert_eq!(f.text.as_deref(), Some("first line\n    second line"));
    }

    #[test]
    fn footnote_at_answers_for_an_offset_the_caret_is_nowhere_near() {
        // The point of the offset form: a pointer hovering a reference asks what
        // note it names, and must not drag the caret along to ask.
        let mut d = doc_with("fn_at_off", "A claim[^1] and more.\n\n[^1]: the note\n");
        d.caret = 0;
        let f = d.footnote_at(9).expect("offset 9 stands in the reference");
        assert_eq!(f.label, "1");
        assert_eq!(f.text.as_deref(), Some("the note"));
        assert_eq!(d.caret, 0, "asking must not move the caret");
        assert_eq!(d.footnote_at(2), None, "offset 2 is prose");
    }

    #[test]
    fn footnote_definition_at_caret_points_back_at_the_reference() {
        // The return leg. `[^1]` spans 7..11, so its label — the only byte of it
        // the caret can rest on — is at 9.
        let mut d = doc_with("fn_def", "A claim[^1] and more.\n\n[^1]: the note\n");
        d.caret = 30; // inside the note's body
        let f = d
            .footnote_definition_at_caret()
            .expect("the caret stands in a definition");
        assert_eq!(f.label, "1");
        assert_eq!(f.offset, Some(9));
        assert_eq!(&d.source[7..11], "[^1]");
    }

    #[test]
    fn footnote_definition_at_covers_where_a_go_to_note_actually_lands() {
        // The two legs have to meet: wherever `footnote_at` sends the caret, the
        // definition query must answer for — otherwise arriving at a note leaves
        // the reader somewhere the way back isn't offered.
        let src = "A claim[^1] and more.\n\n[^1]: the note\n";
        let mut d = doc_with("fn_def_marker", src);
        let landed = d.footnote_at(9).unwrap().offset.unwrap();
        assert_eq!(
            d.footnote_definition_at(landed).and_then(|f| f.offset),
            Some(9),
            "the note a reference sends you to offers the way back"
        );
    }

    #[test]
    fn footnote_definition_at_caret_ignores_prose_and_the_reference_itself() {
        // The two queries answer for disjoint places, which is what lets one
        // gesture mean "down to the note" in one and "back up" in the other
        // without either having to remember which way the reader is going.
        let mut d = doc_with("fn_def_none", "A claim[^1] and more.\n\n[^1]: the note\n");
        d.caret = 2; // prose
        assert_eq!(d.footnote_definition_at_caret(), None);
        d.caret = 9; // the reference
        assert_eq!(d.footnote_definition_at_caret(), None);
        assert!(
            d.footnote_at_caret().is_some(),
            "which is the reference's own query"
        );
    }

    #[test]
    fn footnote_definition_at_caret_reports_an_orphan_note_rather_than_nothing() {
        // Nothing cites `[^2]`. Answering `None` would say "you are not in a
        // note", which is false and leaves a frontend unable to explain why the
        // way back is missing.
        let src = "A claim[^1].\n\n[^1]: cited\n\n[^2]: orphan\n";
        let mut d = doc_with("fn_def_orphan", src);
        d.caret = src.find("orphan").unwrap();
        let f = d
            .footnote_definition_at_caret()
            .expect("an orphan is still a definition");
        assert_eq!(f.label, "2");
        assert_eq!(f.offset, None);
    }

    #[test]
    fn footnote_definition_at_caret_returns_to_the_first_of_repeated_references() {
        // One label, cited twice. The first is where the reader most likely came
        // from, and the only answer that doesn't depend on how they got here.
        let src = "One[^a] and two[^a].\n\n[^a]: the note\n";
        let mut d = doc_with("fn_def_repeat", src);
        d.caret = src.find("the note").unwrap();
        let f = d.footnote_definition_at_caret().expect("a definition");
        assert_eq!(
            f.offset,
            Some(5),
            "the first `[^a]`'s label, not the second's"
        );
        assert_eq!(&src[3..7], "[^a]");
    }

    #[test]
    fn footnote_navigation_is_a_round_trip_through_placed_carets() {
        // Down and back up, each leg found from the document rather than from a
        // memory of the other — so it still works for a reader who scrolled to
        // the notes instead of jumping there.
        //
        // `place_caret` rather than assigning `caret`, because that is what a
        // frontend calls: it snaps to a real caret stop, and a jump that lands
        // on a byte the caret can't rest on would arrive somewhere the return
        // leg no longer answers for. `build_map` first, since snapping is a
        // no-op until the map exists — which is exactly how this went unnoticed
        // when the offsets pointed at the `[^` markers.
        let mut d = doc_with("fn_round", "A claim[^1] and more.\n\n[^1]: the note\n");
        d.build_map(None);
        d.place_caret(9, false);
        let down = d
            .footnote_at_caret()
            .expect("a reference")
            .offset
            .expect("a note");
        d.place_caret(down, false);
        let up = d
            .footnote_definition_at_caret()
            .expect("a definition")
            .offset
            .expect("a reference");
        d.place_caret(up, false);
        assert_eq!(d.caret, up, "the way back is a stop the caret can occupy");
        assert_eq!(
            d.footnote_at_caret().expect("back on the reference").label,
            "1"
        );
    }

    #[test]
    fn insert_link_hands_the_destination_to_twig_raw() {
        // Escaping is twig's, and format-specific: Markdown ends a destination
        // at the first space and needs the `<…>` form, where djot would read
        // those angle brackets as part of the URL.
        let mut d = doc_with("link_space", "word\n");
        d.anchor = Some(0);
        d.caret = 4;
        d.insert_link("a b");
        assert_eq!(d.source, "[word](<a b>)\n");
    }

    #[test]
    fn insert_link_reports_a_destination_no_format_can_carry() {
        let mut d = doc_with("link_bad", "word\n");
        d.anchor = Some(0);
        d.caret = 4;
        d.insert_link("a\nb");
        assert_eq!(d.source, "word\n"); // untouched, not quietly rewritten
        assert!(
            d.status.is_some(),
            "InvalidArgument should reach the status line"
        );
        assert!(!d.dirty);
    }

    #[test]
    fn insert_link_works_in_wysiwyg_view() {
        let mut d = wysiwyg_doc("link_wys", "word here\n");
        d.anchor = Some(0);
        d.caret = 4;
        d.insert_link("http://x.dev");
        assert_eq!(d.source, "[word](http://x.dev) here\n");
        assert_eq!(d.selected_text(), Some("word"));
        // The map the caret has to keep riding is rebuilt each frame; motion
        // over the fresh one must still land on a real stop (the debug_assert).
        d.build_visual(80);
        d.move_right(false);
        d.move_left(false);
    }

    #[test]
    fn click_maps_a_row_col_to_a_byte_offset() {
        let mut d = doc_with("click", "ab\ncd\n");
        d.click(1, 1, false); // row 1 ("cd"), col 1 -> the 'd'
        assert_eq!(d.caret, 4);
    }

    // A pixel-hit-test placement (the GUI's `place_caret`) must land on a caret
    // stop just as the `(row, col)` click path does, so the caret can never come
    // to rest in the blank gap between two paragraphs — where it would draw in one
    // place and type in another.
    #[test]
    fn place_caret_snaps_out_of_the_blank_gap_between_paragraphs() {
        // "A\n\nB": offset 2 is the gap the paragraph break is drawn with, not a
        // caret stop (stops are 0,1,3,4).
        let mut d = wysiwyg_doc("place_gap", "A\n\nB");
        assert!(!d.vmap.is_stop(2), "offset 2 should be an unreachable gap");
        d.place_caret(2, false);
        assert!(d.vmap.is_stop(d.caret), "caret {} is not a stop", d.caret);
        assert_eq!(d.caret, 1, "should snap to the end of the paragraph above");
    }

    #[test]
    fn place_caret_dragging_through_the_gap_keeps_selection_on_stops() {
        let mut d = wysiwyg_doc("place_gap_drag", "A\n\nB");
        d.place_caret(0, false); // anchor at the start of "A"
        d.place_caret(2, true); // drag into the gap
        assert!(d.vmap.is_stop(d.caret), "caret {} is not a stop", d.caret);
        let (s, e) = d.selection().expect("a selection");
        assert!(
            d.vmap.is_stop(s) && d.vmap.is_stop(e),
            "selection {s}..{e} off a stop"
        );
    }

    #[test]
    fn place_caret_on_a_real_stop_is_left_untouched() {
        let mut d = wysiwyg_doc("place_stop", "A\n\nB");
        d.place_caret(3, false); // the start of "B" — a genuine stop
        assert_eq!(d.caret, 3);
    }

    // An *empty paragraph* (two blank lines, an intentional blank line the user
    // opened) is a real caret stop, unlike the gap — a click into it must stay.
    #[test]
    fn place_caret_rests_in_an_empty_paragraph() {
        let mut d = wysiwyg_doc("place_empty_para", "A\n\n\n\nB");
        let empty = 3; // the navigable empty row's offset (stops: 0,1,3,5,6)
        assert!(d.vmap.is_stop(empty));
        d.place_caret(empty, false);
        assert_eq!(d.caret, empty);
    }

    // The content end of a hidden mark is a home too (`VisualMap::mark_ends`):
    // a drag over the word `bold` ends there, and a caret placed there stays.
    #[test]
    fn place_caret_rests_at_the_end_of_a_hidden_marks_content() {
        let src = "| A | B |\n| --- | --- |\n| **bold** | other |\n";
        let mut d = wysiwyg_doc("place_mark_end", src);
        let start = src.find("bold").unwrap();
        d.place_caret(start, false);
        d.place_caret(start + 4, true);
        assert_eq!(d.selection(), Some((start, start + 4)), "the whole word");
        d.toggle(InlineKind::Strong);
        assert_eq!(d.source, src.replace("**bold**", "bold"));
    }

    #[test]
    fn right_steps_onto_the_end_of_a_mark_and_then_past_its_delimiter() {
        let mut d = wysiwyg_doc("right_mark_end", "a **bold** b");
        d.caret = 7; // before the `d`
        d.move_right(false);
        assert_eq!(d.caret, 8, "onto the end of the bold");
        assert!(d.active_inline_marks().contains(InlineKind::Strong));
        d.move_right(false);
        assert_eq!(d.caret, 10, "past the closing `**`");
        assert!(!d.active_inline_marks().contains(InlineKind::Strong));
        d.move_left(false);
        assert_eq!(d.caret, 8);
        d.move_left(false);
        assert_eq!(d.caret, 7);
        // Typing at the inner home extends the bold.
        d.caret = 8;
        d.insert("!");
        assert_eq!(d.source, "a **bold!** b");
    }

    #[test]
    fn a_marks_end_home_follows_an_edit_through_the_incremental_map() {
        // The splice path shifts the home with the block it is in, and the
        // re-rendered block finds its own again.
        let mut d = wysiwyg_doc("mark_end_splice", "x\n\na **bold** b\n\ny\n");
        d.build_visual_unwrapped();
        d.edit(0, 0, "zz");
        d.build_visual_unwrapped();
        wysiwyg::assert_maps_eq(&d.vmap, &reference_map(&d.source), "after a shift");
        assert!(d.vmap.is_stop(d.source.find("bold").unwrap() + 4));
        let at = d.source.find("bold").unwrap();
        d.edit(at, at, "very ");
        d.build_visual_unwrapped();
        wysiwyg::assert_maps_eq(&d.vmap, &reference_map(&d.source), "after a re-render");
        assert!(d.vmap.is_stop(d.source.find("bold").unwrap() + 4));
    }

    fn wysiwyg_doc(name: &str, body: &str) -> Doc {
        doc_in(View::Wysiwyg, name, body)
    }

    /// How many list items the source actually parses into — the check that a
    /// marker Leaf wrote is a marker the format agrees is one.
    fn list_items(doc: &mut Doc) -> usize {
        doc.editor
            .nodes()
            .unwrap()
            .iter()
            .filter(|n| n.kind == Kind::ListItem || n.kind == Kind::TaskListItem)
            .count()
    }

    /// A from-scratch, cache-free WYSIWYG map for `source` — the ground truth the
    /// incremental (`build_spliced` / `build_cached`) path must always match.
    fn reference_map(source: &str) -> crate::wysiwyg::VisualMap {
        reference_map_revealing(source, None)
    }

    /// [`reference_map`] with a reveal line — the ground truth for the
    /// `MarkupMode::Full` builds, where the map is a function of the caret's
    /// line as well as the text.
    fn reference_map_revealing(source: &str, reveal: Option<Reveal>) -> crate::wysiwyg::VisualMap {
        // The same parse `Doc` uses. With twig's plain defaults instead, the two
        // sides disagree on what the *document* is before the renderer is even
        // reached — a bare `:word` is a text directive to one and prose to the
        // other — and the mismatch reads as a splice bug that isn't one.
        let mut ed =
            twig::Editor::new_ext(source.as_bytes(), Format::Markdown, parse_extensions()).unwrap();
        let nodes = ed.nodes().unwrap();
        crate::wysiwyg::build(
            &nodes,
            source,
            None,
            false,
            &wysiwyg::Surface::default(),
            reveal,
        )
    }

    fn maps_differ(a: &crate::wysiwyg::VisualMap, b: &crate::wysiwyg::VisualMap) -> bool {
        if a.rows.len() != b.rows.len() {
            return true;
        }
        for (ra, rb) in a.rows.iter().zip(&b.rows) {
            if ra.end_src != rb.end_src || ra.glyphs.len() != rb.glyphs.len() {
                return true;
            }
            for (ga, gb) in ra.glyphs.iter().zip(&rb.glyphs) {
                if ga.ch != gb.ch || ga.src != gb.src {
                    return true;
                }
            }
        }
        false
    }

    #[test]
    fn incremental_build_matches_a_fresh_build_across_edits() {
        // Every `Doc` edit rebuilds through `build_spliced` (the single-block
        // fast path, gated on twig's `dirty_range`) or falls back to
        // `build_cached`. After each edit the map must be byte-identical to a
        // from-scratch build — this is the correctness net under the splice.
        let docs = [
            "# Title\n\nThe quick brown fox jumps.\n\nAnother paragraph here.\n\n- a\n- b\n",
            "para one\n\n> quote **bold** text\n> continued line\n\ntail paragraph\n",
            "alpha\n\nbeta\n\ngamma\n\ndelta\n\nepsilon\n\nzeta\n",
            // A footnote definition is a root beside `doc`, merged back into the
            // top-level list by `wysiwyg::top_blocks`. The random edits below
            // make and unmake definitions as they go (a deleted `:` turns one
            // back into a paragraph, and vice versa), which is exactly the
            // structural churn the splice path has to notice and bail out of.
            "text[^1] here\n\n[^1]: the note\n\nmore text[^b]\n\n[^b]: second\n",
            // A comment is a top-level block that draws no rows — a layout entry
            // at zero rows either side of blocks that do. The edits below type
            // into the blocks around it (a splice past a hidden block), and
            // break the comment open into prose and back (a structural change).
            "intro\n\n<!-- exec -->\n```\ncode\n```\n\nafter the comment\n\n<!-- trail -->\n",
            // Link reference definitions: a hidden block that an edit can turn
            // into a paragraph (a deleted `:`) and back, and whose own bytes an
            // edit can land in.
            "see [a] and [b]\n\n[a]: /a\n\nmid text\n\n[b]: /b\n",
            // Blank lines above the first block draw as rows, counted as its
            // separator; above it past frontmatter and past a comment too.
            "\n\nfirst pushed down\n\nsecond\n",
            "---\ntitle: x\n---\n\n\nafter frontmatter\n\nmore\n",
            "<!-- lead -->\n\n\nafter a comment\n\nmore\n",
            // No `<div>` here yet: an edit that takes the blank line under
            // `</div>` leaves a map no fresh build draws. See
            // docs/tasks/text-glued-under-a-closing-div.md.
        ];
        // A deterministic mix: mostly single characters (which stay inside one
        // block → splice), plus edits that reshape structure (a paragraph break,
        // a heading marker, a code fence → fallback), so both paths are exercised.
        let inserts = ["x", "y", "\n\n", "#", "`", " ", "z"];
        for src in docs {
            let mut d = wysiwyg_doc("diff", src);
            d.build_visual_unwrapped();
            wysiwyg::assert_maps_eq(&d.vmap, &reference_map(&d.source), "initial");

            for step in 0..60usize {
                let len = d.source.len();
                let raw = (step * 13 + 5) % (len + 1);
                let pos = (raw..=len).find(|&i| d.source.is_char_boundary(i)).unwrap();
                let pre = d.source.clone();
                let action;
                if step % 3 == 0 && pos < len {
                    let end = (pos + 1..=len)
                        .find(|&i| d.source.is_char_boundary(i))
                        .unwrap();
                    action = format!("delete [{pos},{end})");
                    d.edit(pos, end, "");
                } else {
                    let ins = inserts[step % inserts.len()];
                    action = format!("insert {ins:?} @ {pos}");
                    d.edit(pos, pos, ins);
                }
                d.build_visual_unwrapped();
                if maps_differ(&d.vmap, &reference_map(&d.source)) {
                    panic!(
                        "FIRST MISMATCH at step {step}: {action}\n  pre  = {pre:?}\n  post = {:?}",
                        d.source
                    );
                }
            }
        }
    }

    /// A frontend is handed [`Doc::vmap`] and may present it differently:
    /// leaf-ratatui splices blank filler rows under an oversized heading so the
    /// raster it paints there has somewhere to stand, and leaves them in the map
    /// because the caret and the mouse both read it between frames. The splice
    /// path addresses that map by *row index*, against the block layout the last
    /// build recorded — so handed a map with rows in it that no block owns, it
    /// laid the re-rendered block over one of the fillers and carried the rows
    /// the block really occupied into the suffix. One stranded copy of the
    /// edited line, and everything below it a row further down, per keystroke.
    ///
    /// A map that isn't the one the layout describes is a map this path can't
    /// patch, whoever changed it and for whatever reason. It rebuilds instead.
    #[test]
    fn an_edit_over_a_map_a_frontend_reshaped_rebuilds_it_whole() {
        let mut d = wysiwyg_doc("reshaped", "# Title\n\nThe quick brown fox jumps.\n");
        d.build_visual_unwrapped();

        // Stand in for the heading filler rows: two blank rows past the heading
        // that no block accounts for. Cloning a real row keeps every field
        // plausible — it is the row *count* the splice can't survive.
        let filler = d.vmap.rows[0].clone();
        d.vmap.rows.insert(1, filler.clone());
        d.vmap.rows.insert(1, filler);

        // An edit inside the last block: the single-block case the splice path
        // is for, and the one the frontend hits on every keystroke.
        let at = d.source.len() - 1;
        d.edit(at, at, "!");
        d.build_visual_unwrapped();

        wysiwyg::assert_maps_eq(&d.vmap, &reference_map(&d.source), "after the edit");
    }

    /// A glyph's [`FaceId`] has to mean the same thing however its row was
    /// built. A row comes three ways — a fresh walk, a [`BlockCache`] hit
    /// cloned at a shifted offset, and a previous map's rows a splice kept
    /// untouched — and only the first of those walks a `data-font` at all. An
    /// index into a per-build table would have had the same glyph naming two
    /// families the moment a second one appeared; the id is the name's own
    /// hash, so nothing is remapped and the table is merged rather than rebuilt.
    ///
    /// Two families, because one cannot tell a wrong id from a right one.
    ///
    /// [`FaceId`]: crate::style::FaceId
    /// [`BlockCache`]: crate::wysiwyg::BlockCache
    #[test]
    fn a_spliced_rebuild_still_says_which_family_each_glyph_is_set_in() {
        use crate::style::{FaceId, FaceRef};
        let garamond = FaceId::of("Garamond");
        let futura = FaceId::of("Futura");
        let mut d = wysiwyg_doc(
            "two_faces",
            "x <span data-font=\"Garamond\">alpha</span>\n\ny <span data-font=\"Futura\">beta</span>\n",
        );
        d.build_visual_unwrapped();

        // What the map has to keep saying, whichever path built it.
        let check = |d: &Doc, ctx: &str| {
            let face_of = |ch: char| {
                d.vmap
                    .rows
                    .iter()
                    .flat_map(|r| r.glyphs.iter())
                    .find(|g| g.ch == ch)
                    .map(|g| g.style.font)
            };
            assert_eq!(face_of('a'), Some(Some(FaceRef::Named(garamond))), "{ctx}");
            assert_eq!(face_of('b'), Some(Some(FaceRef::Named(futura))), "{ctx}");
            assert_eq!(d.vmap.face_name(garamond), Some("Garamond"), "{ctx}");
            assert_eq!(d.vmap.face_name(futura), Some("Futura"), "{ctx}");
            assert_eq!(d.vmap.face_name(FaceId::of("Bodoni")), None, "{ctx}");
        };
        check(&d, "fresh");

        // An edit inside the second block: the single-block case the splice
        // path is for. The first block's rows are carried over untouched, so
        // its glyphs' ids are the previous build's and the table has to be too.
        let at = d.source.find("beta").unwrap();
        d.edit(at, at, "z");
        d.build_visual_unwrapped();
        check(&d, "after an edit in the second block");
        wysiwyg::assert_maps_eq(&d.vmap, &reference_map(&d.source), "spliced");

        // And the other way round, so the block that was kept is the one that
        // is now re-rendered.
        let at = d.source.find("alpha").unwrap();
        d.edit(at, at, "z");
        d.build_visual_unwrapped();
        check(&d, "after an edit in the first block");
        wysiwyg::assert_maps_eq(&d.vmap, &reference_map(&d.source), "spliced again");

        // A structural edit is one the splice bails out of, so the map is
        // reassembled by `build_cached` — where an untouched block is a *cache
        // hit* and its rows are cloned without a `data-font` being walked
        // again. The names the entry stored are what keeps the table honest
        // there.
        let at = d.source.find("\n\ny ").unwrap();
        d.edit(at, at, "\n\nmiddle");
        d.build_visual_unwrapped();
        wysiwyg::assert_maps_eq(&d.vmap, &reference_map(&d.source), "cached");
        // Twice, because that first `build_cached` is what stores the entries:
        // this one is the build where the Garamond block is a *hit*, its rows
        // cloned with their ids and no attribute walked to explain them.
        let at = d.source.len() - 1;
        d.edit(at, at, "\n\ntail");
        d.build_visual_unwrapped();
        check(&d, "after a structural edit, through the block cache");
        wysiwyg::assert_maps_eq(&d.vmap, &reference_map(&d.source), "cached again");

        // A family the edit took the last glyph of leaves the glyphs with no
        // face and the table with a name nothing asks for — harmless, and the
        // price of not walking the rows the splice exists to avoid walking.
        let span = d.source.find("<span data-font=\"Futura\">").unwrap();
        let end = d.source.rfind("</span>").unwrap() + "</span>".len();
        d.edit(span, end, "beta");
        d.build_visual_unwrapped();
        assert!(!d.source.contains("Futura"), "{:?}", d.source);
        wysiwyg::assert_maps_eq(&d.vmap, &reference_map(&d.source), "the face removed");
    }

    #[test]
    fn incremental_build_matches_a_fresh_build_under_full_reveal() {
        // The same correctness net as `incremental_build_matches_a_fresh_build_
        // across_edits`, under `MarkupMode::Full` — where the map depends on
        // the caret's *line* as well as the text, so the two caches have a new
        // way to be wrong. Both are exercised: the block cache can hand back
        // rows built for a line that is no longer the revealed one, and the
        // splice path can reuse a suffix that still has yesterday's line raw.
        //
        // Caret motion is interleaved with the edits deliberately, because a
        // caret that only ever moved with the edit would never cross a line
        // without also dirtying it — the case where a stale reveal survives.
        let docs = [
            "# Title\n\n*one* and **two**\n\n[lk](http://x) and `code`\n\n- a *b*\n",
            "para *em* one\n\n> quote **bold** text\n\ntail ~~del~~ paragraph\n",
        ];
        let inserts = ["x", "*", "\n\n", "#", "`", " ", "_"];
        for src in docs {
            let mut d = wysiwyg_doc("reveal_diff", src);
            d.set_markup_mode(MarkupMode::Full);

            for step in 0..60usize {
                let len = d.source.len();
                let raw = (step * 13 + 5) % (len + 1);
                let pos = (raw..=len).find(|&i| d.source.is_char_boundary(i)).unwrap();
                let pre = d.source.clone();
                let action;
                if step % 3 == 0 && pos < len {
                    let end = (pos + 1..=len)
                        .find(|&i| d.source.is_char_boundary(i))
                        .unwrap();
                    action = format!("delete [{pos},{end})");
                    d.edit(pos, end, "");
                } else {
                    let ins = inserts[step % inserts.len()];
                    action = format!("insert {ins:?} @ {pos}");
                    d.edit(pos, pos, ins);
                }
                // Walk the caret somewhere else in the document, independently
                // of where the edit landed.
                let want = (step * 29 + 11) % (d.source.len() + 1);
                d.caret = (want..=d.source.len())
                    .find(|&i| d.source.is_char_boundary(i))
                    .unwrap();
                d.build_visual_unwrapped();

                let want = reference_map_revealing(&d.source, d.reveal_line());
                if maps_differ(&d.vmap, &want) {
                    panic!(
                        "FIRST MISMATCH at step {step}: {action}, caret {}\n  pre  = {pre:?}\n  post = {:?}",
                        d.caret, d.source
                    );
                }
            }
        }
    }

    #[test]
    fn caret_motion_across_lines_rebuilds_only_under_full() {
        // The cache-key change has to earn its keep in both directions: `Full`
        // must rebuild when the caret changes line (or the reveal would never
        // move), and the hidden modes must *not* (or every arrow key would pay
        // for a feature they don't use). The existing `cache_motion` test pins
        // the second for the default mode; this pins the pair against a mode
        // change alone.
        let body = "*one* here\n\n*two* there\n";

        let mut full = doc_in(View::Wysiwyg, "motion_full", body);
        full.set_markup_mode(MarkupMode::Full);
        caret_at(&mut full, "one");
        let before = full.revision();
        caret_at(&mut full, "two");
        assert_eq!(full.revision(), before, "motion is not an edit");
        assert!(
            drawn_rows(&full).iter().any(|r| r == "*two* there"),
            "the map followed the caret: {:?}",
            drawn_rows(&full)
        );

        let mut hidden = doc_in(View::Wysiwyg, "motion_hidden", body);
        caret_at(&mut hidden, "one");
        let key = hidden.vmap_key.clone();
        caret_at(&mut hidden, "two");
        assert_eq!(
            hidden.vmap_key, key,
            "a hidden mode rebuilds nothing on motion"
        );
    }

    #[test]
    fn wysiwyg_down_crosses_a_paragraph_boundary() {
        // Regression: the blank separator row used to share the previous
        // paragraph's end offset, so Down got pinned at the boundary (while Up
        // still crossed). Both directions must step through it symmetrically.
        //
        // It's now stepped *over* rather than onto: the blank line between two
        // paragraphs is the boundary being drawn, not a line of the document, so
        // one press of Down crosses it. The goal column survives the crossing —
        // col 3 at the end of "abc" is col 3 at the end of "def".
        let mut d = wysiwyg_doc("wys_down", "abc\n\ndef\n");
        d.caret = 3; // end of "abc" (row 0)
        d.move_down(false);
        assert_eq!(d.caret_pos().0, 2, "Down should reach the second paragraph");
        assert_eq!(d.caret, 8); // end of "def", col 3 kept
        d.move_up(false);
        assert_eq!(d.caret_pos().0, 0, "Up should come back symmetrically");
        assert_eq!(d.caret, 3);
    }

    #[test]
    fn wysiwyg_up_and_down_are_inverse_across_paragraphs() {
        // The second Up and the second Down here run off the ends of the
        // document, which is no longer a place a press is swallowed: they carry
        // the caret to the start and the end of the text. The claim in the
        // middle — that a Down retraces the Up that crossed the paragraph gap —
        // is the one this test is for, and it is asserted where it is made.
        let mut d = wysiwyg_doc("wys_updown", "abc\n\ndef\n");
        d.caret = 5; // start of "def"
        let start = d.caret_pos();
        d.move_up(false);
        assert_eq!(d.caret_pos().0, 0, "Up reaches the first paragraph");
        d.move_up(false);
        assert_eq!(d.caret, 0, "a second Up runs on to the document's start");
        d.move_down(false);
        assert_eq!(d.caret_pos(), start, "Down retraces Up exactly");
        d.move_down(false);
        assert_eq!(d.caret, 8, "a second Down runs on to the document's end");
    }

    #[test]
    fn wysiwyg_new_paragraph_shows_before_typing() {
        // Regression: two Enters at the end of a paragraph produced trailing
        // newlines with no AST node, so the caret appeared stuck on the old line
        // until a character was typed. It must ride down onto the new line now.
        let mut d = doc_with("wys_newpara", "abc\n");
        d.view = View::Wysiwyg;
        d.caret = 3;
        d.insert("\n");
        d.insert("\n"); // source is now "abc\n\n\n", caret at 5
        assert_eq!(d.source, "abc\n\n\n");
        d.build_visual(80);
        let (row, _) = d.caret_pos();
        assert!(
            row >= 2,
            "caret should have moved down to the new line, got row {row}"
        );
        assert!(
            d.vmap.num_rows() >= 3,
            "the blank lines should render as rows"
        );
    }

    #[test]
    fn wysiwyg_enter_between_paragraphs_lands_on_an_empty_line() {
        // The reported bug: Enter at the end of a paragraph that has another
        // paragraph below put the caret at the *start of the next paragraph* —
        // the empty paragraph it opened had no row, so the caret snapped onto
        // "World". It must now sit on its own empty line, with a blank spacer
        // above it (the paragraph gap).
        let mut d = wysiwyg_doc("wys_gap_mid", "Hello\n\nWorld\n");
        d.caret = 5; // end of "Hello"
        d.newline();
        d.build_visual(80);
        let (row, col) = d.caret_pos();
        assert_eq!(col, 0, "caret should start an empty line, not sit in text");
        assert_eq!(
            d.vmap.row_width(row),
            0,
            "caret's row must be empty, not 'World'"
        );
        assert!(
            row >= 2,
            "a blank spacer row should sit above the caret, got row {row}"
        );
        // The row above the caret is a real (empty) gap, and "Hello" stays put.
        assert_eq!(
            d.vmap.row_width(row - 1),
            0,
            "the row above the caret is a gap"
        );
        let row0: String = d.vmap.rows[0].glyphs.iter().map(|g| g.ch).collect();
        assert_eq!(row0, "Hello", "the paragraph above the caret must not move");
    }

    #[test]
    fn wysiwyg_enter_at_eof_shows_a_gap_before_typing() {
        // At the document end a single Enter must also show the paragraph gap —
        // a blank spacer row above the caret — so the layout already matches how
        // it will look once the new paragraph has text.
        let mut d = wysiwyg_doc("wys_gap_eof", "Hello");
        d.caret = 5; // end of "Hello", no trailing newline
        d.newline(); // source becomes "Hello\n\n"
        d.build_visual(80);
        let (row, col) = d.caret_pos();
        assert_eq!(col, 0);
        assert!(
            row >= 2,
            "caret should sit below a blank spacer, got row {row}"
        );
        assert_eq!(
            d.vmap.row_width(row - 1),
            0,
            "the row above the caret is a gap"
        );
    }

    #[test]
    fn wysiwyg_typing_after_enter_does_not_shift_the_caret_row() {
        // The spacer is view-only: typing the new paragraph must not reflow the
        // caret onto a different row — the transient view already matched the
        // settled one.
        let mut d = wysiwyg_doc("wys_no_reflow", "Hello\n\nWorld\n");
        d.caret = 5;
        d.newline();
        d.build_visual(80);
        let before = d.caret_pos();
        d.insert("New");
        d.build_visual(80);
        let after = d.caret_pos();
        assert_eq!(
            after.0, before.0,
            "typing must not move the caret to another row ({before:?} -> {after:?})"
        );
    }

    #[test]
    fn wysiwyg_return_on_the_last_code_line_keeps_the_caret_in_the_block() {
        // Return at the end of the block's last line writes an empty line the
        // map used to drop, so the caret landed on `after` and the next
        // keystroke went into the paragraph below instead of into the code.
        let mut d = wysiwyg_doc("code_return", "prose\n\n```\nalpha\nbeta\n```\n\nafter\n");
        d.caret = d.source.find("beta").unwrap() + "beta".len();
        d.build_visual(80);
        let before = d.caret_pos().0;

        d.newline();
        d.build_visual(80);
        assert_eq!(d.source, "prose\n\n```\nalpha\nbeta\n\n```\n\nafter\n");

        let (row, col) = d.caret_pos();
        assert_eq!(row, before + 1, "the caret moves down one row");
        assert_eq!(col, 0, "onto the head of the empty line");
        let span = d.vmap.code_blocks[0].rows_span.clone();
        assert!(
            span.contains(&row),
            "caret row {row} is outside the block's rows {span:?}"
        );

        // The whole point: what is typed next is code.
        d.insert("gamma");
        assert_eq!(d.source, "prose\n\n```\nalpha\nbeta\ngamma\n```\n\nafter\n");
    }

    #[test]
    fn wysiwyg_hides_frontmatter_from_the_caret_and_copy() {
        let fm = "---\ntitle: hi\n---\n";
        let body = format!("{fm}# leaf\n\nbody\n");
        let mut d = wysiwyg_doc("wys_fm", &body);
        // Opening lifts the caret out of the now-hidden frontmatter.
        assert_eq!(
            d.caret,
            fm.len(),
            "caret should start at the first real block"
        );
        // Left at the content start can't step back into frontmatter.
        d.move_left(false);
        assert_eq!(d.caret, fm.len(), "left must not enter frontmatter");
        // Doc-start lands on the content floor, not offset 0.
        d.move_doc_start(false);
        assert_eq!(d.caret, fm.len());
        // Select-all + copy never include the frontmatter bytes.
        d.select_all();
        let sel = d.selected_text().unwrap().to_string();
        assert!(!sel.contains("title"), "copy leaked frontmatter: {sel:?}");
        assert!(
            sel.starts_with("# leaf"),
            "selection should begin at content: {sel:?}"
        );
    }

    #[test]
    fn typing_in_a_frontmatter_only_document_lands_after_the_frontmatter() {
        // A fresh note is frontmatter and nothing else. With no rendered block
        // to floor the caret it opened at offset 0 — before the opening `---` —
        // so the first keystroke wrote itself in front of the metadata and the
        // file came out as `This---\ntitle: …`.
        let fm = "---\ntitle: 2026-08-29\nid: f8s32cd\n---\n";
        let mut d = wysiwyg_doc("wys_fm_only", fm);
        assert_eq!(d.caret, fm.len(), "caret must open past the frontmatter");
        // Nothing is rendered, so the caret draws at the origin of an empty view
        // — the same place an empty document puts it.
        assert_eq!(d.caret_pos(), (0, 0));
        d.insert("This");
        assert_eq!(d.source, format!("{fm}This"));
    }

    /// `select_range` is the verb for a range a host already knows the bytes of,
    /// so it must not snap — and must still hold every invariant `place_caret`
    /// holds, the frontmatter floor above all.
    #[test]
    fn select_range_takes_the_range_as_given_but_still_floors_it() {
        let fm = "---\ntitle: foo\n---\n\n";
        let body = format!("{fm}body foo here\n");
        let mut d = wysiwyg_doc("wys_select_range", &body);

        // The `foo` in the body: taken exactly, not snapped to a caret stop.
        let at = body.rfind("foo").unwrap();
        d.select_range(at, at + 3);
        assert_eq!(d.selection(), Some((at, at + 3)));
        assert_eq!(d.selected_text(), Some("foo"));

        // The `foo` in the hidden frontmatter: below the floor, so both ends
        // come up to it rather than parking the caret in the metadata, where a
        // later keystroke would rewrite `title:`.
        let hidden = body.find("foo").unwrap();
        assert!(hidden < d.vmap.content_start);
        d.select_range(hidden, hidden + 3);
        assert!(
            d.caret >= d.vmap.content_start && d.anchor.unwrap() >= d.vmap.content_start,
            "a range under the floor must not leave the caret in the frontmatter"
        );

        // Past the end, and mid-character, are both brought back to something
        // sliceable rather than panicking the next reader of the range.
        let multi = wysiwyg_doc("wys_select_range_utf8", "héllo\n");
        let mut d = multi;
        d.select_range(2, 9_999);
        assert_eq!(d.caret, d.source.len());
        assert!(d.source.is_char_boundary(d.anchor.unwrap()));
        assert!(d.source.is_char_boundary(d.caret));
    }

    /// The bug `select_range` exists for: a match butting up against a hidden
    /// delimiter. `place_caret` snaps to the nearest *visible* stop, which is
    /// the one before the `**`.
    #[test]
    fn select_range_does_not_snap_off_a_hidden_delimiter() {
        let mut d = wysiwyg_doc("wys_select_range_bold", "a **needle** in it\n");
        let at = d.source.find("needle").unwrap();
        d.select_range(at, at + 6);
        assert_eq!(d.selected_text(), Some("needle"), "not \"needl\"");
    }

    #[test]
    fn wysiwyg_backspace_at_content_start_leaves_frontmatter_intact() {
        // Backspace deletes `prev_boundary..caret` directly; at the first real
        // block that boundary is inside the hidden frontmatter, so it must be a
        // no-op rather than eating the closing `---`.
        let fm = "---\ntitle: hi\n---\n";
        let body = format!("{fm}leaf\n");
        let mut d = wysiwyg_doc("wys_fm_bs", &body);
        assert_eq!(d.caret, fm.len());
        d.backspace();
        assert_eq!(d.source, body, "backspace must not touch frontmatter");
        d.delete_word_back();
        assert_eq!(
            d.source, body,
            "word-delete must not touch frontmatter either"
        );
    }

    #[test]
    fn wysiwyg_edits_inside_a_vis_directive_block_without_disturbing_its_fences() {
        // diaryx's `:::vis{.audience}` visibility block — any `:::name{.class}`
        // fenced div, really, since core parses these on for every document
        // now (`parse_extensions`). The container is a `directive` node, an
        // `is_block_container` kind like `block_quote`, so the caret works
        // inside its child paragraph exactly as it would inside a quote: typing
        // edits the paragraph, and the `:::vis{...}` / `:::` fences round-trip
        // untouched.
        let body = ":::vis{.public .family}\nhello\n:::\nafter\n";
        let mut d = wysiwyg_doc("wys_vis", body);
        d.caret = body.find("hello").unwrap() + "hello".len();
        d.insert("!");
        assert_eq!(
            d.source, ":::vis{.public .family}\nhello!\n:::\nafter\n",
            "typing inside the block edits its content in place"
        );
        assert!(
            d.source.contains(":::vis{.public .family}"),
            "opening fence survives"
        );
        assert!(d.source.contains(":::\nafter"), "closing fence survives");
    }

    #[test]
    fn source_view_still_reaches_frontmatter() {
        // The metadata is only *hidden*, never lost: the source view edits and
        // selects it in full, and it's always preserved on save.
        let fm = "---\ntitle: hi\n---\n";
        let body = format!("{fm}# leaf\n");
        let mut d = doc_with("src_fm", &body);
        d.select_all();
        let sel = d.selected_text().unwrap();
        assert!(
            sel.contains("title"),
            "source view should select everything"
        );
        d.move_doc_start(false);
        assert_eq!(d.caret, 0, "source view can reach offset 0");
    }

    const TABLE: &str = "| Name | Qty |\n|:-----|----:|\n| Pear | 3 |\n| Fig | 12 |\n";

    #[test]
    fn wysiwyg_right_crosses_a_cell_border_without_stalling() {
        // The border and padding between two cells all share one source offset,
        // so a column-stepping caret would sit on `│` and then stall there
        // forever. Right must step: end of "Name" -> start of "Qty".
        let mut d = wysiwyg_doc("tbl_right", TABLE);
        d.caret = TABLE.find("Name").unwrap() + 4; // just after "Name"
        d.move_right(false);
        assert_eq!(
            d.caret,
            TABLE.find("Qty").unwrap(),
            "should land in the next cell"
        );
        let (r, c) = d.caret_pos();
        assert_eq!(d.vmap.rows[r].glyphs[c].ch, 'Q');
    }

    #[test]
    fn wysiwyg_left_crosses_back_to_the_previous_cell() {
        let mut d = wysiwyg_doc("tbl_left", TABLE);
        d.caret = TABLE.find("Qty").unwrap();
        d.move_left(false);
        assert_eq!(
            d.caret,
            TABLE.find("Name").unwrap() + 4,
            "end of the previous cell"
        );
    }

    #[test]
    fn wysiwyg_down_steps_over_a_table_rule() {
        // Between the header and the first body row sits a `├───┼───┤` rule.
        // It's drawn but holds no caret, so one Down must reach "Pear".
        let mut d = wysiwyg_doc("tbl_down", TABLE);
        d.caret = TABLE.find("Name").unwrap();
        d.move_down(false);
        assert_eq!(
            d.caret,
            TABLE.find("Pear").unwrap(),
            "one Down reaches the body row"
        );
        d.move_down(false);
        assert_eq!(d.caret, TABLE.find("Fig").unwrap());
    }

    #[test]
    fn wysiwyg_tab_walks_the_cells_and_shift_tab_walks_back() {
        let mut d = wysiwyg_doc("tbl_tab", TABLE);
        d.caret = TABLE.find("Name").unwrap();
        // A hop lands with the destination cell's whole content selected, the
        // caret at its end — so typing replaces the cell like a form field.
        assert!(d.cell_hop(true));
        assert_eq!(
            d.selected_text(),
            Some("Qty"),
            "the target cell comes up selected"
        );
        assert_eq!(d.caret, TABLE.find("Qty").unwrap() + "Qty".len());
        assert!(d.cell_hop(true), "Tab wraps onto the next row's first cell");
        assert_eq!(d.selected_text(), Some("Pear"));
        assert!(d.cell_hop(false));
        assert_eq!(d.selected_text(), Some("Qty"));
    }

    #[test]
    fn tab_outside_a_table_is_not_a_cell_hop() {
        // `cell_hop` reports false so the frontend can indent as usual.
        let mut d = wysiwyg_doc("tbl_none", "just a paragraph\n");
        d.caret = 4;
        assert!(!d.cell_hop(true));
        assert_eq!(d.caret, 4, "a refused hop leaves the caret alone");
    }

    #[test]
    fn tab_at_the_last_cell_declines_rather_than_leaving_the_table() {
        let mut d = wysiwyg_doc("tbl_edge", TABLE);
        d.caret = TABLE.rfind("12").unwrap(); // the final cell
        assert!(!d.cell_hop(true), "no cell after the last one");
        d.caret = TABLE.find("Name").unwrap();
        assert!(!d.cell_hop(false), "no cell before the first one");
    }

    #[test]
    fn wysiwyg_vertical_cell_motion_holds_the_column() {
        // Down/Up step to the cell above/below in the *same column*, not back to
        // the top-left the way a naive row/col motion over the picture would.
        let mut d = wysiwyg_doc("tbl_vert", TABLE);
        d.caret = TABLE.find("Qty").unwrap();
        // Each vertical hop selects the destination cell, holding the column.
        assert!(d.cell_move_vertical(true));
        assert_eq!(d.selected_text(), Some("3"), "Down holds column 1");
        assert!(d.cell_move_vertical(true));
        assert_eq!(d.selected_text(), Some("12"), "Down again, still column 1");
        assert!(!d.cell_move_vertical(true), "no row below the last");
        assert!(d.cell_move_vertical(false));
        assert_eq!(d.selected_text(), Some("3"), "Up holds column 1");
        assert!(d.cell_move_vertical(false));
        assert_eq!(d.selected_text(), Some("Qty"), "Up onto the header");
        assert!(!d.cell_move_vertical(false), "no row above the header");
    }

    #[test]
    fn tab_off_the_last_cell_grows_a_row_and_enters_it() {
        let mut d = wysiwyg_doc("tbl_grow", TABLE);
        d.caret = TABLE.rfind("12").unwrap();
        let rows_before = d.source.matches('\n').count();
        assert!(d.cell_tab(true), "acts as a table key");
        assert_eq!(
            d.source.matches('\n').count(),
            rows_before + 1,
            "a fresh row was appended"
        );
        assert!(d.caret_in_table(), "the caret entered the new row");
        // The caret sits in the new row's first cell — past the old last cell.
        assert!(d.caret > TABLE.rfind("12").unwrap());
    }

    #[test]
    fn return_in_a_table_drops_a_cell_and_grows_a_row_at_the_bottom() {
        let mut d = wysiwyg_doc("tbl_ret", TABLE);
        d.caret = TABLE.find("Name").unwrap();
        assert!(d.cell_return(), "acts as a table key");
        assert_eq!(
            d.selected_text(),
            Some("Pear"),
            "Return drops one cell, selecting it"
        );
        // From the last row, Return appends a row and enters it.
        d.caret = TABLE.rfind("Fig").unwrap();
        let rows_before = d.source.matches('\n').count();
        assert!(d.cell_return());
        assert_eq!(d.source.matches('\n').count(), rows_before + 1);
        assert!(d.caret_in_table());
    }

    #[test]
    fn return_and_tab_outside_a_table_decline() {
        let mut d = wysiwyg_doc("tbl_decline", "just a paragraph\n");
        d.caret = 4;
        assert!(!d.cell_return(), "no table: the frontend inserts a newline");
        assert!(!d.cell_tab(true), "no table: the frontend indents");
        assert!(
            !d.cell_line_break(),
            "no table: the frontend breaks the line"
        );
    }

    #[test]
    fn a_click_under_a_trailing_table_lands_past_it_and_enter_opens_a_line() {
        // A document that ends in a table used to end *inside* it: nothing
        // past the last cell was a caret stop, so a click in the blank space
        // under the grid snapped back into the table and there was no way to
        // write a line after it. The bottom border's end is that stop now.
        let mut d = wysiwyg_doc("tbl_trail", TABLE);
        let rows = d.vmap.num_rows();
        d.click(rows + 3, 0, false);
        let end = TABLE.trim_end_matches('\n').len();
        assert_eq!(d.caret, end, "the caret stands just past the table");
        assert!(!d.caret_in_table(), "past the table is outside it");
        assert!(!d.cell_return(), "Return there is the frontend's newline");
        d.newline();
        d.insert("after");
        assert_eq!(
            d.source,
            format!("{TABLE}\nafter\n"),
            "Enter opens a paragraph under the table"
        );
    }

    #[test]
    fn typing_at_a_table_s_trailing_stop_opens_a_paragraph_first() {
        // The stop sits at the end of the table's last source line, and a
        // line glued under a table is a row of it — `| Fig | 12 |x` would be a
        // three-cell row. So the text gets a paragraph of its own, as it does
        // beside a block picture.
        let mut d = wysiwyg_doc("tbl_type", TABLE);
        d.caret = TABLE.trim_end_matches('\n').len();
        d.insert("x");
        assert_eq!(d.source, format!("{TABLE}\nx\n"));
        assert_eq!(d.caret, TABLE.len() + 2, "the caret follows the text");
        // And a paste, which joins the block exactly as typing would.
        let mut d = wysiwyg_doc("tbl_paste", TABLE);
        d.caret = TABLE.trim_end_matches('\n').len();
        d.paste("pasted");
        assert_eq!(d.source, format!("{TABLE}\npasted\n"));
    }

    #[test]
    fn right_leaves_a_table_by_its_trailing_stop_and_backspace_steps_back_in() {
        let mut d = wysiwyg_doc("tbl_edge", TABLE);
        let last_cell_end = TABLE.rfind("12").unwrap() + 2;
        let end = TABLE.trim_end_matches('\n').len();
        d.caret = last_cell_end;
        d.move_right(false);
        assert_eq!(d.caret, end, "Right from the last cell leaves the table");
        // Backspace there takes no byte: the one behind the caret is the row's
        // closing `|`, which the rich view never drew. It steps back instead.
        d.backspace();
        assert_eq!(d.source, TABLE, "nothing deleted");
        assert_eq!(d.caret, last_cell_end, "back into the last cell");
        // Down from the last row lands on the same stop, and Up returns.
        d.move_down(false);
        assert_eq!(d.caret, end, "Down from the last row leaves the table");
        d.move_up(false);
        assert_eq!(d.caret, last_cell_end);
    }

    #[test]
    fn a_table_s_trailing_stop_sits_between_it_and_the_text_below() {
        // With prose under the table, the stop is one hop between the last
        // cell and the paragraph — the shape a block picture's second stop has.
        let src = format!("{TABLE}\nafter\n");
        let mut d = wysiwyg_doc("tbl_mid", &src);
        d.caret = TABLE.rfind("12").unwrap() + 2;
        d.move_right(false);
        assert_eq!(d.caret, TABLE.trim_end_matches('\n').len());
        d.move_right(false);
        assert_eq!(d.caret, src.find("after").unwrap());
        // Typing at the stop still opens a paragraph, and the text below keeps
        // its own.
        d.move_left(false);
        d.insert("x");
        assert_eq!(d.source, format!("{TABLE}\nx\n\nafter\n"));
    }

    #[test]
    fn shift_return_inserts_an_in_cell_break_the_renderer_reads_as_a_line() {
        let mut d = wysiwyg_doc("tbl_break", TABLE);
        d.caret = TABLE.find("Pear").unwrap() + 4; // just after "Pear"
        assert!(d.cell_line_break(), "acts as a table key");
        assert!(
            d.source.contains("Pear<br>"),
            "spelled as an inline <br>: {}",
            d.source
        );
        assert!(d.caret_in_table(), "still in the cell, past the break");
        // The break renders as a real line: the "Pear" cell now draws two lines,
        // so the table's picture is one row taller than a single-line table.
        d.build_visual(80);
        let table = &d.vmap.tables[0];
        let cell = &table.grid[1].cells[0]; // first body row, first column
        assert!(
            cell.glyphs.iter().any(|g| g.ch == '\n'),
            "the cell carries the break as a newline glyph for the frontend to split"
        );
    }

    #[test]
    fn shift_return_in_a_markdown_cell_leaves_a_semantic_hard_break_not_raw_html() {
        // twig promotes the in-cell `<br>` to a `hard_break`, so the break reads
        // back as structure — the whole point of routing through insert_line_break
        // instead of splicing raw `<br>` bytes.
        let mut d = wysiwyg_doc("tbl_break_semantic", TABLE);
        d.caret = TABLE.find("Pear").unwrap() + 4;
        assert!(d.cell_line_break());
        let kinds: Vec<Kind> = d
            .editor
            .nodes()
            .unwrap()
            .iter()
            .map(|n| n.kind.clone())
            .collect();
        assert!(kinds.contains(&Kind::HardBreak), "got {kinds:?}");
        assert!(
            !kinds.contains(&Kind::RawInline),
            "still raw HTML: {kinds:?}"
        );
    }

    #[test]
    fn backspace_over_an_in_cell_break_deletes_the_whole_br_not_a_byte() {
        // The `<br>` draws as one newline glyph, so Backspace over it must take
        // all four bytes — a one-byte delete would strand a visible `<br` in the
        // cell (the reported bug).
        let mut d = wysiwyg_doc("tbl_break_bs", TABLE);
        d.caret = TABLE.find("Pear").unwrap() + 4;
        assert!(d.cell_line_break());
        assert!(d.source.contains("Pear<br>"), "precondition: {}", d.source);
        d.backspace(); // caret sits just past the break
        assert!(
            !d.source.contains("<br"),
            "no half-deleted <br left: {}",
            d.source
        );
        assert!(
            d.source.contains("| Pear |"),
            "the cell is back to one line: {}",
            d.source
        );
    }

    #[test]
    fn backspace_at_a_cell_start_is_a_wall() {
        // The task's repro: two presses used to take the padding and then the
        // `|`, merging `d` into `c`'s cell and leaving the row a column short.
        let src = "| a | b |\n| - | - |\n| c | d |\n";
        let mut d = wysiwyg_doc("tbl_wall_bs", src);
        d.place_caret(src.find("d |").unwrap(), false);
        for _ in 0..3 {
            d.backspace();
        }
        assert_eq!(d.source, src);
        // Inside the padding too — right after the `|`, before the space. Set
        // directly: `place_caret` would snap it onto the stop past the space.
        d.caret = src.find("| d").unwrap() + 1;
        d.backspace();
        assert_eq!(d.source, src);
        // The row's first cell, and an empty one, are walls the same.
        d.place_caret(src.find("c |").unwrap(), false);
        d.backspace();
        assert_eq!(d.source, src);
        // A cell that opens on hidden markup: the wall is the first letter
        // drawn, not the `**` in front of it.
        let bold = "| a | b |\n| - | - |\n| c | **d** |\n";
        let mut d = wysiwyg_doc("tbl_wall_bs_bold", bold);
        d.place_caret(bold.find("d**").unwrap(), false);
        d.backspace();
        assert_eq!(d.source, bold);
        let empty = "| a | b |\n| - | - |\n| c |   |\n";
        let mut d = wysiwyg_doc("tbl_wall_bs_empty", empty);
        d.place_caret(empty.find("|   |").unwrap() + 2, false);
        d.backspace();
        assert_eq!(d.source, empty);
    }

    #[test]
    fn backspace_inside_a_cell_still_deletes_a_character() {
        let src = "| a | b |\n| - | - |\n| c | de |\n";
        let mut d = wysiwyg_doc("tbl_wall_bs_mid", src);
        d.place_caret(src.find("e |").unwrap(), false);
        d.backspace();
        assert_eq!(d.source, "| a | b |\n| - | - |\n| c | e |\n");
    }

    #[test]
    fn delete_at_a_cell_end_is_a_wall() {
        // The mirror: Delete at `c`'s end would take the padding, then the `|`.
        let src = "| a | b |\n| - | - |\n| c | d |\n";
        let mut d = wysiwyg_doc("tbl_wall_del", src);
        d.place_caret(src.find("c |").unwrap() + 1, false);
        for _ in 0..3 {
            d.delete_forward();
        }
        assert_eq!(d.source, src);
        // And inside the trailing padding, set directly past the snap.
        d.caret = src.find("c |").unwrap() + 2;
        d.delete_forward();
        assert_eq!(d.source, src);
        // The row's last cell is a wall on its closing `|` as well.
        d.place_caret(src.find("d |").unwrap() + 1, false);
        d.delete_forward();
        assert_eq!(d.source, src);
        // Mid-cell Delete is untouched.
        d.place_caret(src.find("c |").unwrap(), false);
        d.delete_forward();
        assert_eq!(d.source, "| a | b |\n| - | - |\n|  | d |\n");
    }

    #[test]
    fn delete_forward_over_an_in_cell_break_deletes_the_whole_br() {
        let mut d = wysiwyg_doc("tbl_break_del", TABLE);
        d.caret = TABLE.find("Pear").unwrap() + 4;
        assert!(d.cell_line_break());
        d.caret = TABLE.find("Pear").unwrap() + 4; // back onto the break's start
        d.delete_forward();
        assert!(
            !d.source.contains("<br"),
            "no half-deleted <br: {}",
            d.source
        );
        assert!(
            d.source.contains("| Pear |"),
            "cell back to one line: {}",
            d.source
        );
    }

    #[test]
    fn shift_return_in_a_djot_cell_is_swallowed_and_leaves_the_row_intact() {
        // Djot has no idiomatic in-cell break, so twig refuses it. The gesture is
        // still consumed (a real newline would split the one-line row), but the
        // cell must be left exactly as it was — no non-idiomatic `<br>` spliced in.
        let src = "| Name | Qty |\n|:-----|----:|\n| Pear | 3 |\n";
        let mut d = Doc::from_source(src.to_string(), Format::Djot).unwrap();
        d.caret = src.find("Pear").unwrap() + 4;
        assert!(d.caret_in_table(), "caret should be inside the djot table");
        assert!(
            d.cell_line_break(),
            "the key is consumed, not passed to the frontend"
        );
        assert_eq!(d.source, src, "the djot cell is left untouched");
        assert!(
            !d.source.contains("<br>"),
            "no non-idiomatic <br> spliced into djot"
        );
        assert!(
            d.status.is_some(),
            "the refusal is surfaced on the status line"
        );
    }

    #[test]
    fn typing_in_a_cell_edits_that_cell() {
        // Editing comes free once offsets map correctly: the caret is a source
        // offset, so a normal splice lands inside the pipe table.
        let mut d = wysiwyg_doc("tbl_type", TABLE);
        d.caret = TABLE.find("Pear").unwrap() + 4;
        d.insert("s");
        assert!(d.source.contains("| Pears | 3 |"), "got {:?}", d.source);
    }

    #[test]
    fn motion_and_delete_treat_an_emoji_as_one_character() {
        // 👨‍👩‍👧 is a single grapheme built from three emoji joined by ZWJ — 18
        // bytes, several codepoints. Right-arrow must clear it in one step, and
        // backspace must remove the whole cluster, not a stray joiner.
        let family = "👨‍👩‍👧";
        let mut d = doc_with("emoji", &format!("a{family}b\n"));
        d.caret = 1; // just after 'a', before the emoji
        d.move_right(false);
        assert_eq!(
            d.caret,
            1 + family.len(),
            "one step clears the whole cluster"
        );
        assert_eq!(&d.source[d.caret..d.caret + 1], "b");

        d.backspace(); // delete the emoji as a unit
        assert_eq!(d.source, "ab\n");
        assert_eq!(d.caret, 1);
    }

    #[test]
    fn motion_handles_a_combining_accent_as_one_character() {
        // "e" + U+0301 (combining acute) renders as one é.
        let mut d = doc_with("combining", "e\u{0301}x\n");
        d.caret = 0;
        d.move_right(false);
        assert_eq!(
            d.caret,
            "e\u{0301}".len(),
            "steps past base + combining mark"
        );
    }

    #[test]
    fn undo_then_redo_round_trips_an_edit() {
        let mut d = doc_with("undo", "hello\n");
        d.caret = 5;
        d.insert("!");
        assert_eq!(d.source, "hello!\n");
        d.undo();
        assert_eq!(d.source, "hello\n");
        assert_eq!(d.caret, 5, "undo restores the caret");
        d.redo();
        assert_eq!(d.source, "hello!\n");
    }

    #[test]
    fn a_run_of_typing_undoes_as_one_step() {
        let mut d = doc_with("coalesce", "\n");
        d.caret = 0;
        d.insert("a");
        d.insert("b");
        d.insert("c");
        assert_eq!(d.source, "abc\n");
        d.undo(); // the whole typed run, not just "c"
        assert_eq!(d.source, "\n");
        d.undo(); // nothing left — the run was one step
        assert_eq!(d.source, "\n");
        assert_eq!(d.status.as_deref(), Some("nothing to undo"));
    }

    // ── IME composition ──────────────────────────────────────────────────────

    #[test]
    fn a_composition_run_undoes_as_one_step() {
        let mut d = doc_with("compose", "\n");
        d.caret = 0;
        // What an IME does: each step replaces the last one's provisional bytes.
        d.edit_composing(0, 0, "k");
        d.edit_composing(0, 1, "か");
        d.edit_composing(0, 3, "かん");
        d.edit_composing(0, 6, "感"); // the commit
        d.end_composition();
        assert_eq!(d.source, "感\n");
        d.undo(); // the whole composition, not its last keystroke
        assert_eq!(d.source, "\n");
        assert_eq!(d.status.as_deref(), None, "the run was a single step");
    }

    /// A Replace All the way a host does one: every match, last to first so
    /// the offsets still to go stay put, each a selection replaced by an
    /// insert, inside one undo group. `select_range` rather than the host's
    /// snapping `place_caret`, which would snap against a map these tests do
    /// not rebuild between edits.
    fn replace_all_in(d: &mut Doc, needle: &str, with: &str) {
        let hits: Vec<usize> = d.source.match_indices(needle).map(|(i, _)| i).collect();
        d.begin_undo_group();
        for at in hits.into_iter().rev() {
            d.select_range(at, at + needle.len());
            d.insert(with);
        }
        d.end_undo_group();
    }

    #[test]
    fn a_replace_all_undoes_and_redoes_as_one_step() {
        let mut d = wysiwyg_doc("group", "a cat, a **cat**, a cat\n");
        d.caret = 0;
        d.insert("x");
        replace_all_in(&mut d, "cat", "dog");
        assert_eq!(d.source, "xa dog, a **dog**, a dog\n");
        assert!(d.undo(), "one undo");
        assert_eq!(
            d.source, "xa cat, a **cat**, a cat\n",
            "puts back every match"
        );
        assert!(d.redo(), "one redo");
        assert_eq!(
            d.source, "xa dog, a **dog**, a dog\n",
            "replaces them all again"
        );
        assert!(d.undo());
        assert!(d.undo(), "the typing before is its own step");
        assert_eq!(d.source, "a cat, a **cat**, a cat\n");
        assert!(!d.can_undo());
    }

    #[test]
    fn typing_after_a_group_is_a_step_of_its_own() {
        // A one-character replacement is an insert like a keystroke, and a
        // keystroke after it would coalesce with it outside a group.
        let mut d = wysiwyg_doc("group", "a b a\n");
        replace_all_in(&mut d, "a", "c");
        d.caret = d.source.len() - 1;
        d.insert("d");
        assert_eq!(d.source, "c b cd\n");
        assert!(d.undo());
        assert_eq!(d.source, "c b c\n", "the typing alone");
        assert!(d.undo());
        assert_eq!(d.source, "a b a\n", "then the replacement, whole");
    }

    #[test]
    fn a_group_of_more_edits_than_the_history_holds_is_still_one_step() {
        // twig keeps 200 steps; a group folds as it goes, so it never holds
        // more than one of them.
        let src = format!("start\n{}\n", "x ".repeat(300));
        let mut d = wysiwyg_doc("group", &src);
        d.caret = 5;
        d.insert("!");
        replace_all_in(&mut d, "x", "yy");
        assert_eq!(d.undo_steps, 2);
        assert!(d.undo());
        assert_eq!(d.source, src.replacen("start", "start!", 1));
        assert!(d.undo());
        assert_eq!(d.source, src);
        assert!(!d.can_undo());
    }

    #[test]
    fn an_undo_closes_an_open_group() {
        let mut d = wysiwyg_doc("group", "one\n");
        d.caret = 3;
        d.begin_undo_group();
        d.begin_undo_group();
        d.insert("x");
        assert!(d.in_undo_group());
        assert!(d.undo());
        assert!(
            !d.in_undo_group(),
            "the history moved, so the group is over"
        );
        d.end_undo_group();
        d.end_undo_group();
        assert_eq!(d.source, "one\n");
        assert!(d.redo());
        assert_eq!(d.source, "onex\n");
    }

    #[test]
    fn a_substitution_behind_the_caret_leaves_the_caret_and_is_its_own_step() {
        let mut d = wysiwyg_doc("substitute", "\n");
        d.caret = 0;
        for c in ["I", " ", "s", "a", "w", " ", "t", "e", "h", " "] {
            d.insert(c);
        }
        assert_eq!(d.source, "I saw teh \n");
        let at = d.source.find("teh").unwrap();
        assert!(d.substitute(at, at + 3, "the"));
        assert_eq!(d.source, "I saw the \n");
        assert_eq!(d.caret, 10, "still after the space");
        assert!(d.selection().is_none());
        d.insert("c");
        assert_eq!(d.source, "I saw the c\n");
        assert!(d.undo(), "the typing after it is a step of its own");
        assert_eq!(d.source, "I saw the \n");
        assert!(d.undo(), "the correction is one step");
        assert_eq!(d.source, "I saw teh \n", "what was typed");
        assert_eq!(d.caret, 10, "with the caret where it stood");
        assert!(d.redo());
        assert_eq!(d.source, "I saw the \n");
        assert!(d.undo());
        assert!(d.undo(), "and the typing before it is its own");
        assert_eq!(d.source, "\n");
        assert!(!d.can_undo());
    }

    #[test]
    fn a_substitution_keeps_the_markup_it_stands_beside() {
        // A quote typed just past a bold run: only its own byte changes.
        let mut d = wysiwyg_doc("substitute", "A **bold** word\n");
        let at = d.source.find(" word").unwrap();
        d.caret = at;
        d.insert("\"");
        assert_eq!(d.source, "A **bold**\" word\n");
        assert!(d.substitute(at, at + 1, "\u{201d}"));
        assert_eq!(d.source, "A **bold**\u{201d} word\n");
        assert_eq!(d.caret, at + "\u{201d}".len());
        assert!(
            d.marks_at(d.source.find("bold").unwrap())
                .iter()
                .any(|(k, _)| *k == InlineKind::Strong)
        );
        // A range that is not one is refused, and changes nothing.
        let src = d.source.clone();
        assert!(!d.substitute(3, 2, "x"));
        assert!(!d.substitute(0, src.len() + 1, "x"));
        assert!(!d.substitute(d.source.find('\u{201d}').unwrap() + 1, d.source.len(), "x"));
        assert_eq!(d.source, src);
    }

    #[test]
    fn a_substitution_that_half_lands_takes_back_only_its_own_deletion() {
        // The deletion lands and twig refuses the literal text: the deleted
        // bytes go back, and nothing else moves — not an earlier substitution
        // of the same keystroke, not the group it is in, and no redo is left
        // to delete them again.
        let mut d = wysiwyg_doc("substitute", "\n");
        d.set_markup_mode(MarkupMode::None);
        d.caret = 0;
        for c in ["a", "\"", "b", " ", "\""] {
            d.insert(c);
        }
        assert_eq!(d.source, "a\"b \"\n");

        d.refuse_next_literal = true;
        assert!(!d.substitute(1, 2, "\u{201c}"));
        assert_eq!(d.source, "a\"b \"\n");
        assert_eq!(d.caret, 5, "the caret where it stood");
        assert!(!d.can_redo(), "no redo step left behind");

        d.begin_undo_group();
        assert!(d.substitute(4, 5, "\u{201d}"));
        d.refuse_next_literal = true;
        assert!(!d.substitute(1, 2, "\u{201c}"));
        assert!(d.in_undo_group(), "the group is still open");
        assert_eq!(d.source, "a\"b \u{201d}\n", "the first substitution stands");
        d.end_undo_group();
        assert!(!d.can_redo());
        assert!(d.undo());
        assert_eq!(d.source, "a\"b \"\n", "one step takes the group back");
        assert!(d.undo());
        assert_eq!(d.source, "\n", "and the typing is still one step");
        assert!(!d.can_undo());

        // A group whose first edit is the one that half lands has no step.
        let mut d = wysiwyg_doc("substitute", "\n");
        d.set_markup_mode(MarkupMode::None);
        d.caret = 0;
        d.insert("\"");
        d.begin_undo_group();
        d.refuse_next_literal = true;
        assert!(!d.substitute(0, 1, "\u{201c}"));
        assert!(d.substitute(0, 1, "\u{201c}"));
        d.end_undo_group();
        assert!(d.undo());
        assert_eq!(
            d.source, "\"\n",
            "the substitution that landed is the group's step"
        );
        assert!(d.undo());
        assert_eq!(d.source, "\n");
    }

    #[test]
    fn a_substitution_is_written_the_way_typing_writes() {
        // Where typing is literal, so is a replacement's text.
        let mut d = wysiwyg_doc("substitute", "say hi\n");
        d.set_markup_mode(MarkupMode::None);
        assert!(d.substitute(4, 6, "*hi*"));
        assert_eq!(d.source, "say \\*hi\\*\n");
        assert!(d.undo());
        assert_eq!(d.source, "say hi\n", "one step, escapes and all");
    }

    #[test]
    fn can_undo_is_false_once_a_coalesced_run_is_undone() {
        // The task's repro: five characters typed are one twig step, and the
        // counter used to say five.
        let mut d = wysiwyg_doc("undo_truth", "\n");
        d.caret = 0;
        for c in ["h", "e", "l", "l", "o"] {
            d.insert(c);
        }
        assert!(d.can_undo());
        assert!(d.undo(), "the run undoes");
        assert_eq!(d.source, "\n");
        assert!(!d.can_undo(), "and nothing is left behind it");
        let rev = d.revision();
        assert!(!d.undo(), "an undo that does not move says so");
        assert_eq!(d.revision(), rev);
        assert!(d.can_redo());
        assert!(d.redo());
        assert_eq!(d.source, "hello\n");
        assert!(!d.can_redo());
        assert!(!d.redo());
    }

    #[test]
    fn can_undo_is_exact_across_the_gestures_that_coalesce() {
        // Each gesture below folds, or may fold, one twig step into another.
        // After it, the mirror must name exactly the number of undos twig
        // takes — and the same number of redos back.
        type Gesture = fn(&mut Doc);
        let cases: &[(&str, &str, Gesture)] = &[
            ("typing", "\n", |d| {
                d.caret = 0;
                d.insert("ab");
                d.insert("c");
                d.move_left(false);
                d.insert("x");
            }),
            ("backspaces", "abcdef\n", |d| {
                d.caret = 6;
                d.backspace();
                d.backspace();
                d.insert("z");
                d.backspace();
            }),
            ("ordered list item", "1. one\n2. two\n", |d| {
                d.caret = 6;
                d.newline();
                d.insert("x");
            }),
            ("list indent", "- one\n- two\n", |d| {
                d.caret = d.source.find("two").unwrap();
                d.indent();
                d.outdent();
            }),
            ("bold then type", "one two\n", |d| {
                d.select_range(0, 3);
                d.toggle(InlineKind::Strong);
                d.caret = d.source.len() - 1;
                d.insert("!");
            }),
            ("highlight", "one two\n", |d| {
                d.select_range(0, 3);
                d.highlight(None);
                d.highlight(Some(MarkColor::Green));
            }),
            ("heading and quote", "one\n", |d| {
                d.caret = 1;
                d.toggle_heading(2);
                d.toggle_blockquote();
            }),
            ("footnote", "one\n", |d| {
                d.caret = 3;
                d.insert_footnote();
                d.insert("note");
            }),
            ("blocks", "one\n\ntwo\n\nthree\n", |d| {
                d.caret = 1;
                d.move_block_down();
                d.toggle_list(true);
                d.set_alignment(Some(Align::Center));
                d.insert_page_break();
                d.insert_table(2, 2);
            }),
            ("paste and compose", "\n", |d| {
                d.caret = 0;
                d.paste("one two");
                d.delete_word_back();
                d.edit_composing(d.caret, d.caret, "か");
                d.edit_composing(d.caret - 3, d.caret, "蚊");
                d.end_composition();
                d.insert("z");
            }),
            ("table", "| a | b |\n| - | - |\n| c | d |\n", |d| {
                d.place_caret(d.source.find("d |").unwrap(), false);
                d.cell_tab(true);
                d.insert("e");
                d.cell_return();
            }),
            ("replace all", "cat cat cat\n", |d| {
                d.caret = 0;
                d.insert("a ");
                replace_all_in(d, "cat", "dog");
                d.insert("!");
            }),
            ("nested group", "one\n\ntwo\n", |d| {
                d.begin_undo_group();
                d.caret = 3;
                d.insert("x");
                d.begin_undo_group();
                d.toggle_heading(1);
                d.end_undo_group();
                d.insert("y");
                d.end_undo_group();
                d.backspace();
            }),
            ("group around a list", "1. one\n2. two\n", |d| {
                d.begin_undo_group();
                d.caret = 6;
                d.newline();
                d.insert("x");
                d.end_undo_group();
            }),
            ("substitution", "\n", |d| {
                d.caret = 0;
                d.insert("t");
                d.insert("e");
                d.insert("h");
                d.insert(" ");
                d.substitute(0, 3, "the");
                d.insert("c");
            }),
            ("empty group", "one\n", |d| {
                d.caret = 3;
                d.insert("x");
                d.begin_undo_group();
                d.end_undo_group();
                d.insert("y");
            }),
            ("undo inside a group", "one\n", |d| {
                d.caret = 3;
                d.begin_undo_group();
                d.insert("x");
                d.undo();
                d.insert("y");
                d.insert("z");
                d.end_undo_group();
            }),
        ];
        for (name, src, gesture) in cases {
            let mut d = wysiwyg_doc("undo_exact", src);
            gesture(&mut d);
            let (steps, after) = (d.undo_steps, d.source.clone());
            let mut undone = 0;
            while d.can_undo() {
                assert!(d.undo(), "{name}: can_undo said yes, undo moved nothing");
                undone += 1;
            }
            assert!(!d.undo(), "{name}: can_undo said no, undo moved");
            assert_eq!(undone, steps, "{name}");
            assert_eq!(d.source, *src, "{name}: back to the start");
            let mut redone = 0;
            while d.can_redo() {
                assert!(d.redo(), "{name}: can_redo said yes, redo moved nothing");
                redone += 1;
            }
            assert!(!d.redo(), "{name}: can_redo said no, redo moved");
            assert_eq!(redone, steps, "{name}");
            assert_eq!(d.source, after, "{name}: forward to the end");
        }
    }

    #[test]
    fn two_compositions_are_two_undo_steps() {
        let mut d = doc_with("compose_two", "\n");
        d.caret = 0;
        d.edit_composing(0, 0, "か");
        d.edit_composing(0, 3, "蚊");
        d.end_composition();
        d.edit_composing(3, 3, "き");
        d.edit_composing(3, 6, "木");
        d.end_composition();
        assert_eq!(d.source, "蚊木\n");
        d.undo();
        assert_eq!(d.source, "蚊\n", "only the second composition");
        d.undo();
        assert_eq!(d.source, "\n");
    }

    #[test]
    fn a_composition_does_not_fold_into_the_typing_around_it() {
        let mut d = doc_with("compose_typing", "\n");
        d.caret = 0;
        d.insert("a");
        d.insert("b");
        d.edit_composing(2, 2, "か");
        d.edit_composing(2, 5, "蚊");
        d.end_composition();
        d.insert("c");
        assert_eq!(d.source, "ab蚊c\n");
        d.undo();
        assert_eq!(d.source, "ab蚊\n");
        d.undo();
        assert_eq!(d.source, "ab\n");
        d.undo();
        assert_eq!(d.source, "\n");
    }

    #[test]
    fn ending_a_composition_that_never_began_leaves_a_typing_run_alone() {
        let mut d = doc_with("compose_spurious", "\n");
        d.caret = 0;
        d.insert("a");
        d.end_composition(); // an IME unmarking unprompted
        d.insert("b");
        assert_eq!(d.source, "ab\n");
        d.undo();
        assert_eq!(d.source, "\n", "still one typed run");
    }

    // ── the clipboard's rich flavor ──────────────────────────────────────────

    #[test]
    fn an_inline_selection_publishes_html_without_a_paragraph_wrapper() {
        let mut d = doc_with("sel_inline", "a **bold** c\n");
        d.anchor = Some(2);
        d.caret = 10; // `**bold**`, inside the paragraph
        assert_eq!(d.selection_html().as_deref(), Some("<strong>bold</strong>"));
    }

    #[test]
    fn a_whole_block_selection_keeps_its_paragraph() {
        let mut d = doc_with("sel_block", "a **bold** c\n");
        d.anchor = Some(0);
        d.caret = 12; // the entire paragraph
        assert_eq!(
            d.selection_html().as_deref(),
            Some("<p>a <strong>bold</strong> c</p>")
        );
    }

    #[test]
    fn a_multi_block_selection_keeps_its_structure() {
        let mut d = doc_with("sel_multi", "para\n\n- one\n- two\n");
        d.select_all();
        let html = d.selection_html().expect("renders");
        assert!(html.contains("<p>para</p>"), "{html:?}");
        assert!(html.contains("<li>one</li>"), "{html:?}");
    }

    #[test]
    fn a_word_inside_a_heading_publishes_as_text_not_a_heading() {
        // The fragment `Head` is a paragraph standalone; the *document* says it
        // sits inside one block, so the wrapper is an artifact either way.
        let mut d = doc_with("sel_heading", "# Head line\n");
        d.anchor = Some(2);
        d.caret = 6;
        assert_eq!(d.selection_html().as_deref(), Some("Head"));
    }

    #[test]
    fn no_selection_publishes_no_html() {
        let mut d = doc_with("sel_none", "a b\n");
        d.caret = 1;
        assert_eq!(d.selection_html(), None);
    }

    #[test]
    fn pasting_html_converts_it_and_is_one_undo_step() {
        let mut d = doc_with("paste_html", "x\n");
        d.caret = 1;
        assert!(d.paste_html("<p>a <strong>b</strong> c</p>"));
        assert_eq!(d.source, "xa **b** c\n");
        d.undo();
        assert_eq!(d.source, "x\n", "the whole paste, in one step");
    }

    #[test]
    fn pasting_html_replaces_the_selection() {
        let mut d = doc_with("paste_html_sel", "keep drop\n");
        d.anchor = Some(5);
        d.caret = 9;
        assert!(d.paste_html("<em>new</em>"));
        assert_eq!(d.source, "keep *new*\n");
    }

    #[test]
    fn html_that_would_paste_garbage_declines_so_the_caller_falls_back() {
        let mut d = doc_with("paste_html_bad", "x\n");
        d.caret = 1;
        // twig builds no table from HTML; raw `<table>` in prose is worse than
        // the plain flavor the caller still holds.
        assert!(!d.paste_html("<table><tr><td>a</td></tr></table>"));
        assert_eq!(d.source, "x\n", "declined edits nothing");
    }

    #[test]
    fn copy_then_paste_round_trips_through_the_html_flavor() {
        let mut d = doc_with("clip_round", "a **b** and [l](https://x.dev)\n");
        d.select_all();
        let html = d.selection_html().expect("renders");
        let mut into = doc_with("clip_round_dst", "\n");
        into.caret = 0;
        assert!(into.paste_html(&html));
        assert_eq!(into.source, "a **b** and [l](https://x.dev)\n");
    }

    #[test]
    fn moving_the_caret_starts_a_new_undo_group() {
        let mut d = doc_with("break", "\n");
        d.caret = 0;
        d.insert("a");
        d.insert("b"); // "ab\n", caret at 2
        d.move_left(false); // breaks the run
        d.insert("X"); // "aXb\n"
        assert_eq!(d.source, "aXb\n");
        d.undo();
        assert_eq!(
            d.source, "ab\n",
            "first undo removes only the post-move insert"
        );
        d.undo();
        assert_eq!(d.source, "\n", "second undo removes the earlier run");
    }

    #[test]
    fn undo_reverses_a_format_toggle() {
        let mut d = doc_with("fmt_undo", "a word b\n");
        d.anchor = Some(2);
        d.caret = 6;
        d.toggle(InlineKind::Strong);
        assert_eq!(d.source, "a **word** b\n");
        d.undo();
        assert_eq!(d.source, "a word b\n");
    }

    #[test]
    fn undo_back_to_the_saved_state_clears_dirty() {
        let mut d = doc_with("dirty_undo", "hello\n");
        assert!(!d.dirty);
        d.caret = 5;
        d.insert("!");
        assert!(d.dirty);
        d.undo();
        assert!(
            !d.dirty,
            "undoing to the saved source is not a modification"
        );
    }

    #[test]
    fn marking_saved_as_what_was_written_keeps_later_typing_dirty() {
        let mut d = doc_with("saved_as", "Does this\n");
        d.caret = 9;
        d.insert(" ");
        // The host reads the source, and its write takes a while…
        let written = d.source.clone();
        // …during which the rest of the sentence is typed.
        d.insert("work?");
        d.mark_saved_as(&written);
        assert!(d.dirty, "what was typed during the write is not saved");
        d.undo();
        assert!(!d.dirty, "undoing to what was written is the saved state");

        d.redo();
        let written = d.source.clone();
        d.mark_saved_as(&written);
        assert!(!d.dirty, "nothing typed meanwhile: saved");
    }

    #[test]
    fn a_new_edit_invalidates_redo() {
        let mut d = doc_with("redo_inv", "\n");
        d.caret = 0;
        d.insert("a");
        d.undo();
        d.insert("b"); // diverges — the redo of "a" is now gone
        d.redo();
        assert_eq!(d.source, "b\n");
    }

    #[test]
    fn can_undo_and_can_redo_follow_the_history_a_menu_would_enable_by() {
        let mut d = doc_with("can_undo", "hello\n");
        assert!(
            !d.can_undo() && !d.can_redo(),
            "a fresh document has no history"
        );
        d.caret = 5;
        d.insert("!");
        assert!(
            d.can_undo() && !d.can_redo(),
            "an edit is a step to take back"
        );
        d.undo();
        assert!(!d.can_undo() && d.can_redo(), "undone: only redo remains");
        d.redo();
        assert!(d.can_undo() && !d.can_redo(), "redone: back to undoable");
        d.undo();
        d.insert("?");
        assert!(
            d.can_undo() && !d.can_redo(),
            "a fresh edit ends the redo chain"
        );
        // A coalesced run over-counts steps — the bound is what a menu needs,
        // and it reconciles the moment twig reports the history empty.
        d.insert("a");
        d.insert("b");
        while d.can_undo() {
            d.undo();
        }
        assert_eq!(d.source, "hello\n");
        assert!(!d.can_undo());
        // A reading surface has nothing to undo, whatever the history holds.
        d.redo();
        d.set_read_only(true);
        assert!(!d.can_undo() && !d.can_redo());
    }

    #[test]
    fn undo_on_empty_history_is_a_no_op() {
        let mut d = doc_with("undo_empty", "hi\n");
        d.undo();
        assert_eq!(d.source, "hi\n");
        assert_eq!(d.status.as_deref(), Some("nothing to undo"));
    }

    #[test]
    fn a_one_character_paste_is_its_own_undo_step() {
        for view in [View::Source, View::Wysiwyg] {
            let mut d = doc_in(view, "paste_step", "ab\n");
            d.caret = 0;
            d.insert("x");
            d.insert("y"); // a run of typing
            d.paste("z"); // one character, but pasted — not part of that run
            assert_eq!(d.source, "xyzab\n");
            d.undo();
            assert_eq!(d.source, "xyab\n", "the paste undoes on its own");
            assert_eq!(d.caret, 2, "and hands back the caret it found");
            d.undo();
            assert_eq!(d.source, "ab\n", "the typed run is still one step under it");
        }
    }

    #[test]
    fn the_same_character_typed_still_joins_the_run() {
        // The other half of the pair: `z` is a keystroke here and a paste above,
        // and the two undo differently. Nothing about the *string* says which —
        // which is why provenance has to come from the door the caller uses.
        for view in [View::Source, View::Wysiwyg] {
            let mut d = doc_in(view, "typed_run", "ab\n");
            d.caret = 0;
            d.insert("x");
            d.insert("y");
            d.insert("z");
            d.undo();
            assert_eq!(d.source, "ab\n", "one run, one step");
        }
    }

    #[test]
    fn undo_restores_the_caret_to_where_it_was_not_to_the_edit_site() {
        for view in [View::Source, View::Wysiwyg] {
            let mut d = doc_in(view, "undo_caret", "hello world\n");
            d.caret = 11; // standing at the end of "world", away from the edit
            d.edit(0, 5, "goodbye");
            assert_eq!(d.source, "goodbye world\n");
            d.undo();
            assert_eq!(d.source, "hello world\n");
            // The undone edit ends at offset 5; the user was at 11.
            assert_eq!(d.caret, 11, "the caret comes back with the bytes");
        }
    }

    #[test]
    fn undo_restores_the_selection_the_edit_replaced() {
        for view in [View::Source, View::Wysiwyg] {
            let mut d = doc_in(view, "undo_sel", "a word b\n");
            d.anchor = Some(2);
            d.caret = 6; // "word" selected
            d.insert("X");
            assert_eq!(d.source, "a X b\n");
            d.undo();
            assert_eq!(d.source, "a word b\n");
            assert_eq!(d.selection(), Some((2, 6)), "the selection comes back too");
        }
    }

    #[test]
    fn redo_restores_the_caret_the_edit_left_behind() {
        for view in [View::Source, View::Wysiwyg] {
            let mut d = doc_in(view, "redo_caret", "hello world\n");
            d.caret = 11;
            d.edit(0, 5, "goodbye");
            assert_eq!(d.caret, 7, "the edit left the caret after its new text");
            d.undo();
            d.redo();
            assert_eq!(d.source, "goodbye world\n");
            assert_eq!(d.caret, 7, "redo puts it back where the edit had it");
        }
    }

    #[test]
    fn undoing_a_typed_run_restores_the_caret_from_before_the_whole_run() {
        for view in [View::Source, View::Wysiwyg] {
            let mut d = doc_in(view, "run_caret", "hi\n");
            d.caret = 2;
            d.insert("a");
            d.insert("b");
            d.insert("c");
            assert_eq!(d.source, "hiabc\n");
            d.undo();
            assert_eq!(d.source, "hi\n");
            assert_eq!(d.caret, 2, "before the run, not before its last keystroke");
            d.redo();
            assert_eq!(d.caret, 5, "and redo restores the end of the whole run");
        }
    }

    #[test]
    fn undo_restores_the_caret_across_a_format_toggle() {
        // A toggle reaches twig without going through `splice`, so it has to
        // record its own step — miss it and every stack depth below it is off by
        // one, and undo starts handing back another edit's caret.
        for view in [View::Source, View::Wysiwyg] {
            let mut d = doc_in(view, "fmt_caret", "a word b\n");
            d.caret = 8;
            d.anchor = Some(2);
            d.caret = 6;
            d.toggle(InlineKind::Strong);
            assert_eq!(d.source, "a **word** b\n");
            d.undo();
            assert_eq!(d.source, "a word b\n");
            assert_eq!(
                d.selection(),
                Some((2, 6)),
                "the toggled selection comes back"
            );
        }
    }

    #[test]
    fn an_edit_after_an_undo_truncates_the_caret_history_with_twigs() {
        // The drift that would never announce itself: twig drops its redo stack
        // on any fresh edit, so a leaf redo entry that outlives it would restore
        // a caret from the timeline that edit abandoned.
        for view in [View::Source, View::Wysiwyg] {
            let mut d = doc_in(view, "redo_trunc", "hello world\n");
            d.caret = 11;
            d.edit(0, 5, "goodbye"); // step A, caret 11 → 7
            d.undo();
            assert_eq!(d.caret, 11);
            d.caret = 0;
            d.insert("X"); // diverges: A's redo is gone from twig
            assert_eq!(d.source, "Xhello world\n");

            d.redo();
            assert_eq!(d.source, "Xhello world\n", "nothing to redo onto");
            assert_eq!(d.status.as_deref(), Some("nothing to redo"));
            d.undo();
            assert_eq!(d.source, "hello world\n");
            assert_eq!(
                d.caret, 0,
                "the surviving step's caret, not the dropped one"
            );
        }
    }

    #[test]
    fn indent_and_outdent_move_the_caret_line_with_its_text() {
        for view in [View::Source, View::Wysiwyg] {
            let g = |m, f: fn(&mut Doc)| golden_in(view, "indent_line", m, f);
            assert_eq!(g("he|llo\n", |d| d.indent()), "  he|llo\n");
            assert_eq!(g("  he|llo\n", |d| d.outdent()), "he|llo\n");
            // Indentation the caret is standing *in* collapses to the line start
            // rather than dragging the caret into the text.
            assert_eq!(g("| hello\n", |d| d.outdent()), "|hello\n");
            // A line with none to give back is left exactly as it was.
            assert_eq!(g("he|llo\n", |d| d.outdent()), "he|llo\n");
            // Less than a full level gives back what it has.
            assert_eq!(g(" he|llo\n", |d| d.outdent()), "he|llo\n");
            // A tab is one level however many spaces it isn't.
            assert_eq!(g("\the|llo\n", |d| d.outdent()), "he|llo\n");
        }
    }

    #[test]
    fn one_indent_level_leaves_a_paragraph_a_paragraph() {
        // Why the level is two spaces and not the four both frontends type
        // today. Four is markdown's indented-code-block marker, so a Tab on a
        // paragraph would silently restyle it as code — a width that changes
        // what the document *means* isn't an indent. Pinned because the number
        // is the kind of thing a later list-aware pass would reach for.
        let mut d = doc_with("indent_kind", "hello\n");
        d.caret = 2;
        d.indent();
        assert_eq!(d.source, "  hello\n");
        assert!(
            d.nodes().iter().any(|n| n.kind == Kind::Para),
            "still prose after a Tab"
        );
        assert!(!d.nodes().iter().any(|n| n.kind == Kind::CodeBlock));

        // The four-space level this replaces, for contrast: same text, and twig
        // reparses the paragraph into a code block.
        let mut wide = doc_with("indent_kind_4", "    hello\n");
        wide.build_visual(80);
        assert!(
            wide.nodes().iter().any(|n| n.kind == Kind::CodeBlock),
            "four spaces is a code block, not an indented paragraph"
        );
    }

    #[test]
    fn indent_nests_a_list_item_under_its_parent() {
        // Tab indents a list item by its own marker width, landing its marker at
        // the parent's content column so twig reparses it as a nested list.
        for view in [View::Source, View::Wysiwyg] {
            let mut d = doc_in(view, "indent_nest", "- a\n- b\n");
            d.caret = 6; // on the second item
            d.indent();
            assert_eq!(d.source, "- a\n  - b\n");
            let lists = d
                .nodes()
                .iter()
                .filter(|n| n.kind == Kind::BulletList)
                .count();
            assert_eq!(lists, 2, "the indented item is a nested list");
        }
    }

    #[test]
    fn indent_nests_an_ordered_item_at_its_marker_width() {
        // An ordered marker `1. ` is three columns wide, so a two-space step
        // (which nests a bullet) leaves it flat. Regression: Tab must use the
        // marker width, three, so the item actually nests — and the source
        // renumbers so the sub-list restarts at 1 and the outer list resumes.
        for view in [View::Source, View::Wysiwyg] {
            let mut d = doc_in(view, "indent_ord", "1. a\n2. b\n3. c\n");
            d.caret = d.source.find('b').unwrap();
            d.indent();
            assert_eq!(d.source, "1. a\n   1. b\n2. c\n");
            let lists = d
                .nodes()
                .iter()
                .filter(|n| n.kind == Kind::OrderedList)
                .count();
            assert_eq!(lists, 2, "the indented item is a nested ordered list");
        }
    }

    #[test]
    fn indent_leaves_a_lists_first_item_put() {
        // The first item of a list has no sibling above it to nest under, so Tab
        // is a no-op there — the marker stays at column zero rather than being
        // shoved into indentation twig can't read as a sub-list.
        for view in [View::Source, View::Wysiwyg] {
            let mut d = doc_in(view, "indent_first", "- a\n- b\n");
            d.caret = 1; // on the FIRST item
            d.indent();
            assert_eq!(d.source, "- a\n- b\n", "the first item doesn't nest");
            // The sibling below still nests, proving the guard is per-item.
            d.caret = d.source.find('b').unwrap();
            d.indent();
            assert_eq!(d.source, "- a\n  - b\n");
        }
    }

    #[test]
    fn hidden_mode_keeps_typed_markup_literal() {
        // The Diaryx default: typing `*hi*` gives the characters, not emphasis —
        // twig escapes what would open markup, so the source is `\*hi\*` and the
        // AST is a plain string. Formatting is the commands' job in this mode.
        let mut d = doc_in(View::Wysiwyg, "hidden_literal", "");
        d.insert("*hi*");
        assert_eq!(d.source, "\\*hi\\*");
        assert!(
            d.nodes()
                .iter()
                .all(|n| n.kind != Kind::Emph && n.kind != Kind::Strong)
        );
    }

    #[test]
    fn hidden_mode_escapes_a_line_start_block_marker() {
        // A `#`/`-`/`>` at a line start would open a block, so Hidden mode keeps
        // it literal too — a Diaryx user's "# 1 idea" stays prose, not a heading.
        let mut d = doc_in(View::Wysiwyg, "hidden_block", "");
        d.insert("# hi");
        assert_eq!(d.source, "\\# hi");
        assert!(d.nodes().iter().all(|n| n.kind != Kind::Heading));
    }

    #[test]
    fn authoring_modes_keep_typed_markup_live() {
        // Both authoring rungs of the ladder: typing `*hi*` really is emphasis
        // (no escape), the same as source view — escaping is `None`'s alone, and
        // it's the axis, not the reveal, that decides.
        for (view, mode) in [
            (View::Wysiwyg, MarkupMode::Shortcuts),
            (View::Wysiwyg, MarkupMode::Full),
            (View::Source, MarkupMode::None),
        ] {
            let mut d = doc_in(view, "live_markup", "");
            d.set_markup_mode(mode);
            d.insert("*hi*");
            assert_eq!(d.source, "*hi*", "{mode:?} in {view:?} types raw markup");
        }
    }

    #[test]
    fn hidden_mode_overwrite_undoes_in_one_step() {
        // Typing over a selection escapes the replacement *and* stays a single
        // undo — the selection-delete and the literal insert fold together, so
        // one undo brings the whole selection back, like a plain overwrite.
        let mut d = doc_in(View::Wysiwyg, "hidden_overwrite", "a word b\n");
        d.anchor = Some(2);
        d.caret = 6; // "word"
        d.insert("*");
        assert_eq!(d.source, "a \\* b\n", "the replacement is escaped");
        d.undo();
        assert_eq!(d.source, "a word b\n");
        assert_eq!(d.selection(), Some((2, 6)), "one undo, selection restored");
    }

    #[test]
    fn backspace_over_an_escaped_char_takes_the_hidden_backslash_too() {
        // Type `*` in Hidden mode → `\*` (drawn as one `*`); one Backspace clears
        // the whole visual character, never stranding the hidden `\`.
        let mut d = doc_in(View::Wysiwyg, "bsp_escape", "");
        d.insert("*");
        assert_eq!(d.source, "\\*");
        d.backspace();
        assert_eq!(d.source, "", "the escape backslash went with the *");
        // A *literal* backslash (source view, no escape) is an ordinary char.
        let mut s = doc_in(View::Source, "bsp_lit", "a\\b\n");
        s.caret = 3; // after `b`
        s.backspace();
        assert_eq!(s.source, "a\\\n", "only the b is deleted, the \\ stays");
    }

    #[test]
    fn hidden_mode_leaves_structural_markup_alone() {
        // Enter continues a bullet list by writing a real `- ` marker (an
        // `insert_raw`, not the typing path), so Hidden mode's escaping never
        // touches it — the list keeps working.
        let mut d = doc_in(View::Wysiwyg, "hidden_struct", "- item\n");
        d.caret = 6;
        d.newline();
        d.insert("two");
        assert_eq!(d.source, "- item\n- two\n");
    }

    #[test]
    fn markup_mode_defaults_to_none_and_round_trips() {
        // Diaryx's default is the clean `None` surface; a markup-fluent
        // frontend can climb the ladder, and the choice sticks.
        let mut d = doc_in(View::Wysiwyg, "markup_mode", "hi\n");
        assert_eq!(d.markup_mode(), MarkupMode::None, "None by default");
        for mode in [MarkupMode::Shortcuts, MarkupMode::Full, MarkupMode::None] {
            d.set_markup_mode(mode);
            assert_eq!(d.markup_mode(), mode);
        }
    }

    #[test]
    fn full_mode_reveals_only_the_caret_line() {
        // The mode's whole claim: the caret's line shows its raw delimiters and
        // every other line stays resolved. Two paragraphs with identical markup
        // so the only difference between the rows is where the caret is.
        let mut d = doc_in(
            View::Wysiwyg,
            "reveal_caret_line",
            "*one* here\n\n*two* there\n",
        );
        d.set_markup_mode(MarkupMode::Full);

        caret_at(&mut d, "one");
        let rows = drawn_rows(&d);
        assert!(
            rows.iter().any(|r| r == "*one* here"),
            "caret's line raw: {rows:?}"
        );
        assert!(
            rows.iter().any(|r| r == "two there"),
            "other line resolved: {rows:?}"
        );

        // Move to the other paragraph: the reveal follows, and the line just
        // left goes back to being resolved.
        caret_at(&mut d, "two");
        let rows = drawn_rows(&d);
        assert!(
            rows.iter().any(|r| r == "*two* there"),
            "caret's line raw: {rows:?}"
        );
        assert!(
            rows.iter().any(|r| r == "one here"),
            "left line resolved: {rows:?}"
        );
    }

    #[test]
    fn revealing_a_coloured_highlight_shows_the_emoji_that_spelled_it() {
        // The emoji is a delimiter, not content — so `MarkupMode::Full` owes it
        // the same treatment as an emphasis's `*`: hidden while the caret is
        // elsewhere, shown in full where the caret lands. That falls out of
        // `delims` reading the bytes between the mark's span and its content
        // span, which is exactly `==🔴 ` and `==`, rather than from a table
        // of spellings — so the no-space form `==🟢green==` reveals right too.
        let mut d = doc_in(
            View::Wysiwyg,
            "reveal_coloured_mark",
            "a ==🔴 red== one\n\nb ==plain== two\n",
        );
        d.set_markup_mode(MarkupMode::Full);

        caret_at(&mut d, "red");
        let rows = drawn_rows(&d);
        assert!(
            rows.iter().any(|r| r == "a ==🔴 red== one"),
            "the caret's line shows the colour it was written with: {rows:?}"
        );
        assert!(
            rows.iter().any(|r| r == "b plain two"),
            "and every other line stays resolved: {rows:?}"
        );

        // Away from it, the emoji goes back to being markup — the reader sees
        // the words and the wash.
        caret_at(&mut d, "two");
        let rows = drawn_rows(&d);
        assert!(
            rows.iter().any(|r| r == "a red one"),
            "resolved again: {rows:?}"
        );
    }

    #[test]
    fn hidden_modes_never_reveal_wherever_the_caret_is() {
        // The two rungs below `Full` share a rendering: delimiters stay hidden
        // even under the caret. `Shortcuts` differing from `None` only in what
        // typing does is exactly the point of splitting the axes.
        for mode in [MarkupMode::None, MarkupMode::Shortcuts] {
            let mut d = doc_in(View::Wysiwyg, "reveal_hidden", "*one* here\n");
            d.set_markup_mode(mode);
            caret_at(&mut d, "one");
            let rows = drawn_rows(&d);
            assert!(
                rows.iter().any(|r| r == "one here"),
                "{mode:?} hides: {rows:?}"
            );
            assert!(
                !rows.iter().any(|r| r.contains('*')),
                "{mode:?} shows no `*`: {rows:?}"
            );
        }
    }

    #[test]
    fn revealed_delimiters_are_the_authors_own_spelling() {
        // Delimiters are re-read from the source rather than synthesized per
        // kind, so a line comes back spelled the way it was written: `_em_` does
        // not turn into `*em*`, and a two-backtick fence keeps both backticks.
        let body = "_em_ and __st__ and ``lit ` tick`` and [lk](http://x) and ~~del~~\n";
        let mut d = doc_in(View::Wysiwyg, "reveal_spelling", body);
        d.set_markup_mode(MarkupMode::Full);
        caret_at(&mut d, "em");
        let rows = drawn_rows(&d);
        assert!(
            rows.iter().any(|r| r == body.trim_end()),
            "the revealed line is its own source: {rows:?}"
        );
    }

    #[test]
    fn revealed_heading_shows_its_hashes() {
        // The `# ` marker is a block-level prefix, not an inline delimiter, so
        // it takes its own path — but it reveals on the same rule.
        let mut d = doc_in(View::Wysiwyg, "reveal_heading", "# Title\n\nbody\n");
        d.set_markup_mode(MarkupMode::Full);

        caret_at(&mut d, "Title");
        assert!(
            drawn_rows(&d).iter().any(|r| r == "# Title"),
            "{:?}",
            drawn_rows(&d)
        );

        caret_at(&mut d, "body");
        let rows = drawn_rows(&d);
        assert!(
            rows.iter().any(|r| r == "Title"),
            "hashes hidden again: {rows:?}"
        );
    }

    #[test]
    fn revealed_delimiters_are_caret_stops() {
        // A delimiter that is drawn but can't be reached is worse than one
        // that's hidden: the mode exists so the markup can be *edited*. Every
        // revealed byte must be somewhere the caret can stand.
        let mut d = doc_in(View::Wysiwyg, "reveal_stops", "*em* x\n");
        d.set_markup_mode(MarkupMode::Full);
        caret_at(&mut d, "em");
        let opener = d.source.find('*').unwrap();
        assert!(d.vmap.is_stop(opener), "the opening `*` is a caret stop");
        assert!(
            d.vmap.is_stop(opener + 3),
            "the closing `*` is a caret stop"
        );
    }

    #[test]
    fn setext_heading_reveals_nothing_across_its_newline() {
        // A setext heading's underline is on another line, so it is not the
        // caret line's to reveal — and emitting it would inject a `\n` glyph
        // that splits the row where the author wrote no break.
        let mut d = doc_in(View::Wysiwyg, "reveal_setext", "Title\n=====\n\nbody\n");
        d.set_markup_mode(MarkupMode::Full);
        caret_at(&mut d, "Title");
        let rows = drawn_rows(&d);
        assert!(
            rows.iter().any(|r| r == "Title"),
            "title renders alone: {rows:?}"
        );
        assert!(
            !rows.iter().any(|r| r.contains('=')),
            "no underline leaks in: {rows:?}"
        );
    }

    #[test]
    fn markup_mode_axes_split_the_ladder() {
        // The two behaviours the ladder spells: `Shortcuts` is the middle rung
        // that authors markup but still hides it, and it's the only rung where
        // the two axes disagree.
        assert!(!MarkupMode::None.authors());
        assert!(!MarkupMode::None.reveals_caret_line());
        assert!(MarkupMode::Shortcuts.authors());
        assert!(!MarkupMode::Shortcuts.reveals_caret_line());
        assert!(MarkupMode::Full.authors());
        assert!(MarkupMode::Full.reveals_caret_line());
    }

    #[test]
    fn indenting_an_empty_dash_item_under_text_dodges_the_setext_collapse() {
        // Tabbing an empty `- ` under a text line would spell `- hello\n  - `,
        // which twig (correctly, per CommonMark — pandoc agrees) reparses as a
        // setext H2. leaf swaps the dash for a `*` so the item stays an empty
        // nested bullet and `hello` stays prose: the file round-trips instead of
        // hiding a heading the user never asked for.
        for view in [View::Source, View::Wysiwyg] {
            let mut d = doc_in(view, "setext_guard", "- hello\n- \n");
            d.caret = d.source.find("- \n").unwrap() + 2; // after the empty marker
            d.indent();
            assert_eq!(d.source, "- hello\n  * \n");
            assert!(
                d.nodes().iter().all(|n| n.kind != Kind::Heading),
                "no heading"
            );
            // And it's genuinely a nested list, not a flat one.
            assert_eq!(
                d.nodes()
                    .iter()
                    .filter(|n| n.kind == Kind::BulletList)
                    .count(),
                2
            );
        }
    }

    #[test]
    fn indenting_a_dash_item_with_content_keeps_its_dash() {
        // With content, `- x` can't be a setext underline, so there's nothing to
        // dodge: the marker stays a dash and nests as an ordinary sub-bullet.
        let mut d = doc_in(View::Wysiwyg, "setext_ok", "- hello\n- x\n");
        d.caret = d.source.find('x').unwrap();
        d.indent();
        assert_eq!(d.source, "- hello\n  - x\n");
    }

    #[test]
    fn the_setext_swap_undoes_as_one_step_with_the_indent() {
        // The dash→`*` repair coalesces into the Tab, so a single undo restores
        // the whole pre-Tab state rather than stranding a half-collapsed doc.
        let mut d = doc_in(View::Wysiwyg, "setext_undo", "- hello\n- \n");
        d.caret = d.source.find("- \n").unwrap() + 2;
        d.indent();
        assert_eq!(d.source, "- hello\n  * \n");
        d.undo();
        assert_eq!(d.source, "- hello\n- \n", "one undo, not two");
    }

    #[test]
    fn indent_leaves_a_nested_lists_first_item_put_too() {
        // The guard is about siblings, not depth: the first item of an *inner*
        // list (already nested under `a`) still has nothing before it at its own
        // level, so Tab can't take it deeper.
        let mut d = doc_in(View::Wysiwyg, "indent_first_nested", "- a\n  - b\n  - c\n");
        d.caret = d.source.find('b').unwrap();
        d.indent();
        assert_eq!(d.source, "- a\n  - b\n  - c\n", "inner first item holds");
        // But `c` (a sibling of `b`) nests under `b`.
        d.caret = d.source.find('c').unwrap();
        d.indent();
        assert_eq!(d.source, "- a\n  - b\n    - c\n");
    }

    #[test]
    fn backspace_at_a_nested_item_start_outdents_it() {
        // Backspace with the caret right after a nested item's marker gives back
        // one level of nesting, the mirror of Tab — and renumbers the flattened
        // ordered list back to a clean run.
        let mut d = doc_in(View::Wysiwyg, "bsp_outdent", "1. a\n   1. b\n2. c\n");
        d.caret = d.source.find('b').unwrap(); // start of the nested item's content
        d.backspace();
        assert_eq!(d.source, "1. a\n2. b\n3. c\n");
    }

    #[test]
    fn backspace_at_a_top_level_item_start_strips_the_marker() {
        // At the outermost level there's no nesting left to give back, so the same
        // keystroke drops the bullet and leaves a plain paragraph.
        let mut d = doc_in(View::Wysiwyg, "bsp_strip", "- a\n- b\n");
        d.caret = d.source.find('b').unwrap(); // right after `- `
        d.backspace();
        assert_eq!(d.source, "- a\nb\n", "the marker is gone, the text stays");
    }

    #[test]
    fn backspace_mid_item_still_deletes_a_character() {
        // The list behaviour is armed only at the item's content start; anywhere
        // else Backspace is the ordinary character delete.
        let mut d = doc_in(View::Wysiwyg, "bsp_mid", "- ab\n");
        d.caret = d.source.find('b').unwrap(); // between `a` and `b`
        d.backspace();
        assert_eq!(d.source, "- b\n");
    }

    #[test]
    fn backspace_at_a_heading_start_strips_the_marker() {
        // The `# ` is markup the rich view hides, so Backspace over it takes the
        // whole marker and leaves a paragraph. Deleting a byte of it instead left
        // `#Title` — no longer a heading, with the hash now literal text the user
        // never typed and has to delete again.
        let mut d = doc_in(View::Wysiwyg, "bsp_head", "## Title\n");
        d.caret = d.source.find('T').unwrap(); // right after `## `
        d.backspace();
        assert_eq!(d.source, "Title\n");
        assert_eq!(
            d.caret, 0,
            "the caret stays with the text it was in front of"
        );
    }

    #[test]
    fn backspace_at_a_heading_start_keeps_the_block_around_it() {
        // Only the heading's own marker goes — the quote (or list) it sits in is
        // untouched, exactly as un-heading it should be.
        let mut d = doc_in(View::Wysiwyg, "bsp_head_quote", "> # Title\n");
        d.caret = d.source.find('T').unwrap();
        d.backspace();
        assert_eq!(d.source, "> Title\n");
    }

    #[test]
    fn backspace_at_a_heading_start_takes_its_closing_sequence_too() {
        // `# Title #`'s trailing hashes are hidden at the other end; leaving them
        // behind would surface the same stray hash the marker delete just avoided.
        let mut d = doc_in(View::Wysiwyg, "bsp_head_closed", "# Title #\n");
        d.caret = d.source.find('T').unwrap();
        d.backspace();
        assert_eq!(d.source, "Title\n");
        // And it's one edit: a single undo puts the whole heading back.
        d.undo();
        assert_eq!(d.source, "# Title #\n");
    }

    #[test]
    fn backspace_mid_heading_still_deletes_a_character() {
        // The heading behaviour is armed only at the content's start; anywhere
        // else Backspace is the ordinary character delete.
        let mut d = doc_in(View::Wysiwyg, "bsp_head_mid", "# ab\n");
        d.caret = d.source.find('b').unwrap();
        d.backspace();
        assert_eq!(d.source, "# b\n");
    }

    #[test]
    fn source_view_backspace_still_edits_the_heading_marker_literally() {
        // In source view the `# ` is text on the screen the user is deleting a
        // byte of, so it keeps its literal meaning — the same split the list
        // ladder and Enter draw between the two views.
        let mut d = doc_with("bsp_head_src", "# Title\n");
        d.caret = d.source.find('T').unwrap();
        d.backspace();
        assert_eq!(d.source, "#Title\n");
    }

    #[test]
    fn outdent_unnests_an_ordered_item_in_one_press() {
        // Shift+Tab gives back exactly the marker width the indent added, so a
        // nested ordered item unnests in a single press, and the flattened list
        // renumbers back to a clean 1, 2, 3.
        let mut d = doc_with("outdent_ord", "1. a\n   2. b\n3. c\n");
        d.caret = d.source.find('b').unwrap();
        d.outdent();
        assert_eq!(d.source, "1. a\n2. b\n3. c\n");
        let lists = d
            .nodes()
            .iter()
            .filter(|n| n.kind == Kind::OrderedList)
            .count();
        assert_eq!(lists, 1, "back to one flat list");
    }

    #[test]
    fn table_insert_row_adds_a_row_below_the_caret() {
        let mut d = doc_with("tbl_ins_row", "| a | b |\n| --- | --- |\n| 1 | 2 |\n");
        d.caret = d.source.find('1').unwrap(); // in the body row
        d.table_insert_row(true);
        assert_eq!(d.source, "| a | b |\n| --- | --- |\n| 1 | 2 |\n|  |  |\n");
    }

    #[test]
    fn table_insert_and_delete_column_at_the_caret() {
        let mut d = doc_with("tbl_col", "| a | b |\n| --- | --- |\n| 1 | 2 |\n");
        d.caret = d.source.find('a').unwrap(); // column 0
        d.table_insert_column(true); // add a column to the right of `a`
        assert_eq!(
            d.source,
            "| a |  | b |\n| --- | --- | --- |\n| 1 |  | 2 |\n"
        );
        d.caret = d.source.find('b').unwrap(); // now the third column
        d.table_delete_column();
        assert_eq!(d.source, "| a |  |\n| --- | --- |\n| 1 |  |\n");
    }

    // ── ragged formats ───────────────────────────────────────────────────────
    // No format spells every gesture. HTML writes the inline marks as a tag pair
    // and no heading, list, quote or link; Markdown spells five of the eight
    // marks — the highlight only because leaf parses with `highlight`, which is
    // why the question is asked with the extensions; djot spells all eight and
    // no in-cell break. leaf asks twig per
    // gesture (`Doc::supports`) and refuses at the door, rather than letting each
    // op discover the fact on its own — one of them didn't.

    /// An HTML document in the rich view, ready for a gesture.
    fn html_doc(body: &str) -> Doc {
        let mut d = Doc::from_source(body.to_string(), Format::Html).unwrap();
        d.view = View::Wysiwyg;
        d.build_visual(80);
        d
    }

    #[test]
    fn a_table_gesture_leaves_an_html_table_alone() {
        // The regression this guard exists for. twig's table editor consults no
        // `Syntax` table — it spells a grid, not a delimiter — so it rebuilt an
        // HTML `<table>` as a *pipe table* and reported success: the whole
        // element replaced by `| a | b |`, silently, on one press of a toolbar
        // button. Every grid op went the same way.
        let src = "<table><tr><td>a</td><td>b</td></tr><tr><td>c</td><td>d</td></tr></table>\n";
        // A table of named operations, which is what it looks like.
        #[allow(clippy::type_complexity)]
        let ops: [(&str, &dyn Fn(&mut Doc)); 7] = [
            ("insert row", &|d: &mut Doc| d.table_insert_row(true)),
            ("delete row", &|d: &mut Doc| d.table_delete_row()),
            ("insert column", &|d: &mut Doc| d.table_insert_column(true)),
            ("delete column", &|d: &mut Doc| d.table_delete_column()),
            ("align", &|d: &mut Doc| {
                d.table_set_alignment(Alignment::Right)
            }),
            ("move row", &|d: &mut Doc| d.table_move_row(true)),
            ("move column", &|d: &mut Doc| d.table_move_column(true)),
        ];
        for (name, op) in ops {
            let mut d = html_doc(src);
            d.caret = d.source.find('a').unwrap();
            assert!(d.caret_in_table(), "{name}: the caret really is in a table");
            op(&mut d);
            assert_eq!(d.source, src, "{name} rewrote an HTML table");
            assert!(
                !d.dirty,
                "{name} marked the document dirty without editing it"
            );
            assert!(d.status.is_some(), "{name} refused without saying why");
        }
    }

    #[test]
    fn the_block_gestures_html_cannot_spell_are_refused_with_a_reason() {
        // A task box is a form control in HTML and a footnote has no native
        // spelling at all — the two gestures twig 3.5 still spells nothing
        // for, now that a quote, a list, a link and an image print through
        // its renderer (see the test below).
        let src = "<h1>Title</h1>\n<p>Hello world</p>\n<ul><li>one</li></ul>\n";
        // A table of named operations, which is what it looks like.
        #[allow(clippy::type_complexity)]
        let ops: [(&str, &dyn Fn(&mut Doc)); 3] = [
            ("task item", &|d: &mut Doc| d.toggle_task_item()),
            ("task tick", &|d: &mut Doc| d.toggle_task_checked()),
            ("footnote", &|d: &mut Doc| d.insert_footnote()),
        ];
        for (name, op) in ops {
            let mut d = html_doc(src);
            let at = d.source.find("Hello").unwrap();
            d.caret = at;
            d.anchor = Some(at + 5); // a selection, for the ops that want one
            op(&mut d);
            assert_eq!(d.source, src, "{name} edited an HTML document");
            assert!(
                !d.dirty,
                "{name} marked the document dirty without editing it"
            );
            let status = d.status.as_deref().unwrap_or("");
            assert!(
                status.contains("html"),
                "{name}: the refusal should name the format, got {status:?}"
            );
        }
    }

    #[test]
    fn html_spells_a_quote_a_list_a_link_and_an_image_through_the_renderer() {
        // twig 3.5: where HTML has no marker alphabet it prints the fresh
        // node — a `<blockquote>` around the paragraph, a `<ul>`/`<ol>` with
        // the paragraph as its item, an `<a>` or `<img>` over the selection.
        // Until then every one of these was a refusal; now each is a real
        // edit, which is what the toolbar's capability flags say too.
        let src = "<h1>Title</h1>\n<p>Hello world</p>\n<ul><li>one</li></ul>\n";
        #[allow(clippy::type_complexity)]
        let ops: [(&str, &dyn Fn(&mut Doc), &str); 5] = [
            (
                "quote",
                &|d: &mut Doc| d.toggle_blockquote(),
                "<blockquote>",
            ),
            ("list", &|d: &mut Doc| d.toggle_list(false), "<ul>\n<li>"),
            (
                "ordered list",
                &|d: &mut Doc| d.toggle_list(true),
                "<ol>\n<li>",
            ),
            (
                "link",
                &|d: &mut Doc| d.insert_link("https://example.dev"),
                "<a href=\"https://example.dev\">Hello</a>",
            ),
            (
                "image",
                &|d: &mut Doc| d.insert_image("pic.png", "alt"),
                "<img alt=\"Hello\" src=\"pic.png\">",
            ),
        ];
        for (name, op, expect) in ops {
            let mut d = html_doc(src);
            let at = d.source.find("Hello").unwrap();
            d.caret = at;
            d.anchor = Some(at + 5);
            op(&mut d);
            assert!(d.source.contains(expect), "{name}: got {:?}", d.source);
            assert!(d.dirty, "{name}: a real edit");
            assert_eq!(
                d.status, None,
                "{name}: a supported gesture reports nothing"
            );
        }
    }

    #[test]
    fn html_spells_a_heading_as_its_tag_pair() {
        // twig 3.4 rebuilds a heading or paragraph as its tag pair, attributes
        // along — the one block gesture whose HTML shape it can write. So ⌘2
        // in an HTML document is a real edit, and ⌘0 takes it back.
        let src = "<h1>Title</h1>\n<p>Hello world</p>\n";
        let mut d = html_doc(src);
        d.caret = d.source.find("Hello").unwrap();
        d.toggle_heading(2);
        assert_eq!(d.source, "<h1>Title</h1>\n<h2>Hello world</h2>\n");
        assert!(d.dirty);
        assert_eq!(d.status, None, "a supported gesture reports nothing");
        d.toggle_heading(2);
        assert_eq!(d.source, src, "the same level again is back to a paragraph");
    }

    #[test]
    fn html_spells_the_inline_marks_and_the_rule() {
        // The other half, and why one per-document flag stopped being enough:
        // ⌘B in an HTML document writes `<strong>` — the tag the serializer
        // already emits and the parser reads straight back as the same mark —
        // and the rule button writes an `<hr>`. Refusing these on the old
        // "HTML is parse-only" reading would now be leaf's own limitation.
        let mut d = html_doc("<p>Hello world</p>\n");
        let at = d.source.find("world").unwrap();
        d.caret = at;
        d.anchor = Some(at + 5);
        d.toggle(InlineKind::Strong);
        assert_eq!(d.source, "<p>Hello <strong>world</strong></p>\n");
        assert!(d.dirty);
        assert_eq!(d.status, None, "a supported gesture reports nothing");

        // And off again — the toggle reverses, which is the property that makes
        // authoring in HTML worth offering rather than a one-way trip.
        d.toggle(InlineKind::Strong);
        assert_eq!(d.source, "<p>Hello world</p>\n");

        let mut d = html_doc("<p>Hello world</p>\n");
        d.caret = d.source.find("world").unwrap();
        d.insert_thematic_break();
        assert!(d.source.contains("<hr>"), "got {:?}", d.source);
    }

    #[test]
    fn a_mark_the_format_cannot_spell_arms_nothing() {
        // `toggle` with a collapsed caret doesn't reach twig at all — it arms a
        // sticky mark for the next text typed. Guarding only the twig call
        // leaves that path live, promising a mark the gesture will not write and
        // then swallowing the error inside `insert`.
        //
        // Markdown carries this, on the superscript now rather than on the
        // highlight: `^x^` is text there in any configuration, whereas twig
        // 3.3.1 authors `==x==` for an editor holding the `highlight` extension,
        // which every leaf document does.
        let mut d = doc_with("mark", "Hello world\n");
        d.view = View::Wysiwyg;
        d.build_visual(80);
        d.caret = d.source.find("world").unwrap();
        d.toggle(InlineKind::Superscript);
        assert!(d.pending_marks.is_empty(), "no mark should be armed");
        assert!(d.status.as_deref().unwrap_or("").contains("markdown"));
        d.insert("X");
        assert_eq!(d.source, "Hello Xworld\n");
    }

    #[test]
    fn markdown_authors_a_highlight_and_a_strikethrough() {
        // twig 3.3.1: the two marks Markdown reads and, until it, refused to
        // write. `==x==` is authorable because leaf's own `parse_extensions`
        // turns `highlight` on — twig will only mint bytes this editor's reparse
        // reads back — and `~~x~~` because GFM strikethrough is parsed by
        // default, so the refusal there was never right for any leaf document.
        // twig 3.10 adds the underline, spelled `<u>x</u>` and paired back into
        // an `insert` under the `html_elements` leaf parses with.
        for (kind, marked) in [
            (InlineKind::Mark, "a ==word== b\n"),
            (InlineKind::Delete, "a ~~word~~ b\n"),
            (InlineKind::Insert, "a <u>word</u> b\n"),
        ] {
            let mut d = doc_with("author_mark", "a word b\n");
            d.anchor = Some(2);
            d.caret = 6;
            d.toggle(kind);
            assert_eq!(d.source, marked, "{kind:?}");
            assert_eq!(d.status, None, "{kind:?}: a supported gesture is silent");
            assert!(d.dirty, "{kind:?}");
            // The region stays selected, so the second press reverses it — the
            // property that separates authoring from a one-way trip.
            d.toggle(kind);
            assert_eq!(d.source, "a word b\n", "{kind:?}");
        }
    }

    #[test]
    fn an_authored_highlight_reads_back_as_a_mark() {
        // The round trip the extension gate exists to protect: what the toggle
        // writes, the reparse must read back as a `mark` rather than as two
        // literal `=` pairs. A `Role::Mark` glyph is that answer, taken from the
        // rebuilt map rather than from the source text.
        let mut d = doc_with("mark_roundtrip", "a word b\n");
        d.view = View::Wysiwyg;
        d.build_visual(80);
        d.anchor = Some(2);
        d.caret = 6;
        d.toggle(InlineKind::Mark);
        assert_eq!(d.source, "a ==word== b\n");
        d.build_visual(80);
        let w = d
            .vmap
            .rows
            .iter()
            .flat_map(|r| r.glyphs.iter())
            .find(|g| g.ch == 'w')
            .expect("the highlighted word");
        assert_eq!(w.style.role, crate::Role::Mark(None));
    }

    #[test]
    fn an_authored_underline_reads_back_as_an_insert() {
        // The same round trip for `<u>…</u>`: under `html_elements` the tag pair
        // is one `insert`, so the word is underlined and the tags are markup the
        // caret-off line hides — not two `raw_inline` runs drawn as literal
        // HTML around plain text.
        let mut d = doc_with("underline_roundtrip", "a word b\nnext\n");
        d.view = View::Wysiwyg;
        d.build_visual(80);
        d.anchor = Some(2);
        d.caret = 6;
        d.toggle(InlineKind::Insert);
        assert_eq!(d.source, "a <u>word</u> b\nnext\n");
        d.anchor = None;
        d.caret = d.source.find("next").unwrap();
        d.build_visual(80);
        let glyphs: Vec<_> = d.vmap.rows.iter().flat_map(|r| r.glyphs.iter()).collect();
        let w = glyphs
            .iter()
            .find(|g| g.ch == 'w')
            .expect("the underlined word");
        assert!(w.style.underline);
        assert!(!glyphs.iter().any(|g| g.ch == '<'), "the tags are hidden");
    }

    #[test]
    fn a_highlight_takes_a_colour_changes_it_and_gives_it_back() {
        // The three states of one gesture, in the order a palette is pressed:
        // an uncoloured highlight takes the prefix, a coloured one has it
        // replaced, and `None` takes it away with the space that was part of the
        // spelling.
        let mut d = doc_with("mark_colour", "a ==word== b\n");
        d.caret = d.source.find("word").unwrap();
        d.set_mark_color(Some(MarkColor::Red));
        assert_eq!(d.source, "a ==🔴 word== b\n");
        assert_eq!(d.status, None);
        assert!(d.dirty);

        d.set_mark_color(Some(MarkColor::Blue));
        assert_eq!(d.source, "a ==🔵 word== b\n");

        d.set_mark_color(None);
        assert_eq!(d.source, "a ==word== b\n");
    }

    #[test]
    fn the_caret_keeps_its_place_in_the_text_across_a_colour() {
        // The prefix is written *before* the word, so an offset in the word has
        // to ride its width — a caret that stayed put would be a caret that
        // walked backwards through the text it was standing in.
        let mut d = doc_with("mark_colour_caret", "a ==word== b\n");
        let word = d.source.find("word").unwrap();
        d.caret = word + 2; // between `wo` and `rd`
        d.set_mark_color(Some(MarkColor::Red));
        assert_eq!(&d.source[d.caret..d.caret + 2], "rd", "still before `rd`");

        // And back the other way when the prefix goes.
        d.set_mark_color(None);
        assert_eq!(&d.source[d.caret..d.caret + 2], "rd");
    }

    #[test]
    fn the_colour_at_the_caret_is_what_the_palette_lights() {
        let mut d = doc_with("mark_colour_read", "a ==🔴 red== and ==plain== b\n");
        d.caret = d.source.find("red").unwrap();
        assert!(d.caret_in_mark());
        assert_eq!(d.mark_color_at_caret(), Some(MarkColor::Red));

        d.caret = d.source.find("plain").unwrap();
        assert!(d.caret_in_mark(), "a highlight with no colour is still one");
        assert_eq!(d.mark_color_at_caret(), None);

        d.caret = d.source.find(" and ").unwrap() + 2;
        assert!(!d.caret_in_mark());
        assert_eq!(d.mark_color_at_caret(), None);
    }

    #[test]
    fn a_colour_without_a_highlight_says_so_and_writes_nothing() {
        // The gesture colours a highlight that exists; it does not make one.
        // Two presses is the price of a coloured highlight from bare text, and
        // the reason is undo — one press that spliced twice would take two
        // presses to take back.
        let mut d = doc_with("mark_colour_none", "a word b\n");
        d.caret = d.source.find("word").unwrap();
        d.set_mark_color(Some(MarkColor::Red));
        assert_eq!(d.source, "a word b\n");
        assert!(d.status.is_some(), "it should say why");
        assert!(!d.dirty);

        // Clearing where there is nothing to clear is the same refusal, not a
        // quiet success — the caret is in no highlight either way.
        d.status = None;
        d.set_mark_color(None);
        assert_eq!(d.source, "a word b\n");
        assert!(d.status.is_some());
    }

    #[test]
    fn clearing_an_uncoloured_highlight_is_a_quiet_no_op() {
        // twig answers this one *successfully* with a `Change` describing some
        // earlier edit, so a caller that trusted the change would jump the caret
        // to wherever that was. Core answers it before asking.
        let mut d = doc_with("mark_colour_noop", "a ==word== b\n");
        d.toggle(InlineKind::Strong); // an earlier edit for a stale change to name
        d.caret = d.source.find("word").unwrap();
        let (source, caret) = (d.source.clone(), d.caret);
        d.set_mark_color(None);
        assert_eq!(d.source, source);
        assert_eq!(
            d.caret, caret,
            "the caret must not ride a change that isn't one"
        );
        assert_eq!(d.status, None, "and it is not an error either");
    }

    #[test]
    fn djot_spells_the_highlight_and_not_its_colour() {
        // The reason the palette is its own capability rather than the Highlight
        // button's: `{=word=}` is a highlight djot writes happily, and there is
        // no djot spelling for a colour on it.
        assert!(Capabilities::of(Format::Djot).mark);
        assert!(!Capabilities::of(Format::Djot).mark_color);
        assert!(Capabilities::of(Format::Markdown).mark_color);

        let mut d = Doc::from_source("a {=word=} b\n".into(), Format::Djot).unwrap();
        d.caret = d.source.find("word").unwrap();
        assert!(
            d.caret_in_mark(),
            "the caret is in a highlight all the same"
        );
        d.set_mark_color(Some(MarkColor::Red));
        assert_eq!(d.source, "a {=word=} b\n");
        assert!(
            d.status.as_deref().unwrap_or("").contains("djot"),
            "and the refusal names the document's format: {:?}",
            d.status
        );
    }

    #[test]
    fn a_coloured_highlight_is_one_undo_step_and_reads_back_as_its_colour() {
        // The round trip that matters for a palette: the bytes twig writes are
        // bytes its own reparse reads back as a colour, so the swatch that was
        // pressed is the swatch that lights afterwards.
        let mut d = doc_with("mark_colour_undo", "a word b\n");
        d.anchor = Some(2);
        d.caret = 6;
        d.toggle(InlineKind::Mark);
        d.caret = d.source.find("word").unwrap();
        d.set_mark_color(Some(MarkColor::Green));
        assert_eq!(d.source, "a ==🟢 word== b\n");
        assert_eq!(d.mark_color_at_caret(), Some(MarkColor::Green));

        // One splice, one step: the colour comes off and the highlight stays.
        d.undo();
        assert_eq!(d.source, "a ==word== b\n");
        d.undo();
        assert_eq!(d.source, "a word b\n");
    }

    #[test]
    fn every_colour_leaf_names_is_one_twig_writes() {
        // The two enums are one vocabulary, and this is what says so: each of
        // leaf's colours writes an emoji twig's reparse reads back as *that*
        // colour, so `twig_mark_color`'s table cannot quietly pair red with
        // orange.
        for color in MarkColor::ALL {
            let mut d = doc_with("mark_colour_all", "a ==word== b\n");
            d.caret = d.source.find("word").unwrap();
            d.set_mark_color(Some(color));
            assert_eq!(d.status, None, "{color:?}");
            assert_eq!(d.mark_color_at_caret(), Some(color), "{color:?}");
        }
    }

    #[test]
    fn a_fresh_highlight_takes_a_colour_without_moving_the_caret_first() {
        // The two presses a coloured highlight is made of, in the state the
        // first one leaves: `toggle` selects the whole `==word==` and puts the
        // caret one past the closing `==`, which is *not* in the mark. Asking at
        // the caret alone would refuse to colour the highlight just written —
        // the selection's start is what answers.
        let mut d = doc_with("mark_colour_fresh", "a word b\n");
        d.anchor = Some(2);
        d.caret = 6;
        d.toggle(InlineKind::Mark);
        assert_eq!(d.source, "a ==word== b\n");
        assert_eq!(d.caret, 10, "the caret twig leaves, past the closing `==`");

        assert!(d.caret_in_mark(), "the selected highlight is the one meant");
        d.set_mark_color(Some(MarkColor::Yellow));
        assert_eq!(d.source, "a ==🟡 word== b\n");
        assert_eq!(d.status, None);
    }

    #[test]
    fn one_press_highlights_a_selection_and_colours_it() {
        // What a toolbar swatch means over a plain selection, and the undo it
        // has to have: one press, one step. Two steps would leave an uncoloured
        // highlight behind on the way back, which is a state the author never
        // asked for and never saw.
        let mut d = doc_with("highlight_one", "a word b\n");
        d.anchor = Some(2);
        d.caret = 6;
        d.highlight(Some(MarkColor::Purple));
        assert_eq!(d.source, "a ==\u{1F7E3} word== b\n");
        assert_eq!(d.status, None);

        d.undo();
        assert_eq!(d.source, "a word b\n", "one press, one undo");
    }

    #[test]
    fn one_press_on_an_existing_highlight_only_recolours_it() {
        // The other half: inside a highlight there is nothing to make, so the
        // compound is the plain gesture and the text is untouched.
        let mut d = doc_with("highlight_recolour", "a ==\u{1F534} word== b\n");
        d.caret = d.source.find("word").unwrap();
        d.highlight(Some(MarkColor::Blue));
        assert_eq!(d.source, "a ==\u{1F535} word== b\n");
        d.undo();
        assert_eq!(d.source, "a ==\u{1F534} word== b\n", "the highlight stays");
    }

    #[test]
    fn one_press_with_no_colour_over_a_selection_just_highlights_it() {
        // `None` means "no colour", and over bare text that is the Highlight
        // button's own job. The fold must not happen here — there is no second
        // splice, and folding would take the *previous* edit into this one.
        let mut d = doc_with("highlight_none", "a word b and more\n");
        d.caret = d.source.find("more").unwrap() + 4; // after "more"
        d.insert("!"); // an earlier edit for a wrong fold to swallow
        d.anchor = Some(2);
        d.caret = 6;
        d.highlight(None);
        assert_eq!(d.source, "a ==word== b and more!\n");

        d.undo();
        assert_eq!(
            d.source, "a word b and more!\n",
            "only the highlight came off"
        );
        d.undo();
        assert_eq!(
            d.source, "a word b and more\n",
            "and the edit before it survived"
        );
    }

    #[test]
    fn one_press_at_a_bare_caret_in_no_highlight_writes_nothing() {
        // `toggle` at a collapsed caret arms a mark for text not yet typed, and
        // a colour cannot be armed with it — so the compound declines rather
        // than leaving half a promise.
        let mut d = doc_with("highlight_bare", "a word b\n");
        d.caret = 4;
        d.highlight(Some(MarkColor::Red));
        assert_eq!(d.source, "a word b\n");
        assert!(d.pending_marks.is_empty(), "and nothing armed");
        assert!(d.status.is_some());
    }

    #[test]
    fn a_read_only_document_takes_no_colour() {
        let mut d = doc_with("mark_colour_ro", "a ==word== b\n");
        d.caret = d.source.find("word").unwrap();
        d.set_read_only(true);
        d.set_mark_color(Some(MarkColor::Red));
        assert_eq!(d.source, "a ==word== b\n");
    }

    #[test]
    fn a_sticky_highlight_wraps_the_next_typed_text_in_markdown() {
        // The other door into `toggle`: no selection, so nothing reaches twig
        // until `insert` realises the armed mark. It is armed now — the guard
        // above asks `Doc::supports`, which asks with the extensions — and what
        // it writes is the same `==…==`.
        let mut d = doc_with("sticky_mark", "xy\n");
        d.caret = 1;
        d.toggle(InlineKind::Mark);
        assert!(d.pending_marks.contains(InlineKind::Mark));
        d.insert("Z");
        assert_eq!(d.source, "x==Z==y\n");
    }

    #[test]
    fn html_documents_still_take_typed_text() {
        // The guard covers *markup* gestures and must not touch plain editing:
        // twig's splicer is language-neutral, and typing into an HTML document
        // is the thing that does work today.
        let mut d = html_doc("<p>Hello world</p>\n");
        d.caret = d.source.find("world").unwrap();
        d.insert("big ");
        assert_eq!(d.source, "<p>Hello big world</p>\n");
        assert!(d.dirty);
        d.backspace();
        assert_eq!(d.source, "<p>Hello bigworld</p>\n");
        d.undo();
        d.undo();
        assert_eq!(d.source, "<p>Hello world</p>\n");
    }

    #[test]
    fn authorable_is_the_coarse_question_and_capabilities_the_useful_one() {
        // `authorable` only separates "there is a door in" from "there is not",
        // and HTML is on the near side of that line — which is exactly why a
        // toolbar must not be built from it.
        let html = Doc::from_source("<p>x</p>\n".into(), Format::Html).unwrap();
        assert!(html.authorable());
        assert!(
            !Doc::from_source("<r>x</r>".into(), Format::Xml)
                .unwrap()
                .authorable()
        );

        let caps = html.capabilities();
        assert!(caps.bold && caps.italic && caps.code && caps.mark);
        assert!(caps.thematic_break && caps.cell_line_break);
        // A heading is a tag pair twig rebuilds (3.4), and since 3.5 so are a
        // quote, a list, a code block's language, a link and an image — each
        // printed as a fresh node where HTML has no marker to rewrite. A task
        // box is a form control and a footnote has no spelling, so those two
        // are what keeps the record ragged.
        assert!(caps.heading && caps.blockquote && caps.bullet_list);
        assert!(caps.link && caps.image && caps.code_language);
        assert!(!caps.task && !caps.footnote);
        // The one flag that isn't twig's answer: an HTML `<table>` is a grid
        // twig's table editor would happily re-emit as `| a | b |`.
        assert!(!caps.table);

        // The two lightweight formats spell everything leaf offers — and still
        // differ from each other, which is the other half of why one boolean
        // can't serve.
        for fmt in [Format::Markdown, Format::Djot] {
            let caps = Capabilities::of(fmt);
            assert!(
                caps.heading && caps.blockquote && caps.ordered_list,
                "{fmt:?}"
            );
            assert!(
                caps.task && caps.link && caps.image && caps.table,
                "{fmt:?}"
            );
        }
        // Both spell the highlight, the strikethrough and the underline: djot
        // natively, and Markdown because `Capabilities` asks with
        // `parse_extensions` rather than with twig's defaults — `==x==` is text
        // under those, and a mark under the `highlight` leaf always parses
        // with; `<u>x</u>` likewise pairs into an `insert` only under
        // `html_elements`.
        for fmt in [Format::Markdown, Format::Djot] {
            let caps = Capabilities::of(fmt);
            assert!(caps.mark && caps.strike && caps.underline, "{fmt:?}");
        }
        // What still separates them, now that the highlight doesn't: djot has
        // no in-cell break, and Markdown spells neither of the scripts.
        assert!(Capabilities::of(Format::Djot).superscript);
        assert!(!Capabilities::of(Format::Markdown).superscript);
        assert!(Capabilities::of(Format::Markdown).cell_line_break);
        assert!(!Capabilities::of(Format::Djot).cell_line_break);

        // A parse-only format answers no to every one of them, so the coarse
        // predicate and the record agree there.
        let caps = Capabilities::of(Format::Xml);
        assert!(!caps.bold && !caps.heading && !caps.table && !caps.thematic_break);
    }

    #[test]
    fn a_refused_gesture_says_so_where_twig_would_have_said_it() {
        // The guard exists to name the *document's* format rather than twig's
        // internals, so the message has to survive being one leaf writes itself.
        // Checked against a gesture twig also refuses, since that is the pair
        // most at risk of drifting apart — the task box, once the code
        // language stopped being one (twig 3.5).
        let mut d = html_doc("<p>Hello</p>\n");
        d.caret = d.source.find("Hello").unwrap();
        d.toggle_task_item();
        assert_eq!(d.status.as_deref(), Some("task: not supported in html"));
        assert!(!d.dirty);
    }

    #[test]
    fn table_set_alignment_respells_the_delimiter() {
        let mut d = doc_with("tbl_align", "| a | b |\n| --- | --- |\n| 1 | 2 |\n");
        d.caret = d.source.find('b').unwrap();
        d.table_set_alignment(Alignment::Right);
        assert_eq!(d.source, "| a | b |\n| --- | ---: |\n| 1 | 2 |\n");
    }

    #[test]
    fn each_empty_table_cell_has_its_own_editable_home() {
        // Regression: an empty cell has no twig content_span, so both cells of a
        // `|  |  |` row collapsed onto the row's start (before the first `│`).
        // Typing there inserted *before* the table (`hello|  |  |`); nav couldn't
        // tell the cells apart. Each empty cell must now have a distinct home
        // inside it.
        let mut d = wysiwyg_doc("tbl_empty", "| a | b |\n| --- | --- |\n|  |  |\n");
        let (c0, c1) = {
            let cells = &d.vmap.tables[0].grid[1].cells;
            (cells[0].start, cells[1].start)
        };
        assert!(
            c0 < c1,
            "the two empty cells have distinct homes: {c0} < {c1}"
        );
        d.caret = c0;
        d.insert("x");
        assert_eq!(
            d.source, "| a | b |\n| --- | --- |\n| x |  |\n",
            "typed inside the cell"
        );
    }

    #[test]
    fn arrows_step_into_each_empty_table_cell() {
        let mut d = wysiwyg_doc("tbl_empty_nav", "| a | b |\n| --- | --- |\n|  |  |\n");
        let (c0, c1) = {
            let cells = &d.vmap.tables[0].grid[1].cells;
            (cells[0].start, cells[1].start)
        };
        d.caret = d.source.find('b').unwrap(); // in the header's second cell
        let mut seen = std::collections::HashSet::new();
        for _ in 0..6 {
            d.move_right(false);
            seen.insert(d.caret);
        }
        assert!(
            seen.contains(&c0),
            "right arrow reaches the first empty cell"
        );
        assert!(
            seen.contains(&c1),
            "right arrow reaches the second empty cell"
        );
    }

    #[test]
    fn table_op_off_a_table_is_a_no_op_with_a_status() {
        let mut d = doc_with("tbl_none", "just text\n");
        d.caret = 3;
        d.table_insert_row(true);
        assert_eq!(d.source, "just text\n", "nothing changed");
        assert!(d.status.is_some(), "a status explains why");
        assert!(!d.caret_in_table());
    }

    #[test]
    fn enter_in_an_ordered_list_renumbers_the_following_items() {
        // Inserting an item mid-list left the source markers stale (`1. 2. 2. 3.`);
        // the renumber pass keeps them sequential, matching what the view draws.
        let mut d = wysiwyg_doc("enter_renumber", "1. a\n2. b\n3. c\n");
        d.caret = d.source.find('a').unwrap() + 1; // end of item a
        d.newline();
        d.insert("x");
        assert_eq!(d.source, "1. a\n2. x\n3. b\n4. c\n");
    }

    #[test]
    fn outdent_with_nothing_to_give_back_records_no_undo_step() {
        for view in [View::Source, View::Wysiwyg] {
            let mut d = doc_in(view, "outdent_noop", "hello\n");
            d.caret = 2;
            d.outdent();
            assert_eq!(d.source, "hello\n");
            assert!(!d.dirty, "a no-op is not a modification");
            d.undo();
            assert_eq!(
                d.status.as_deref(),
                Some("nothing to undo"),
                "spends no undo step"
            );
            assert_eq!(d.source, "hello\n");
        }
    }

    #[test]
    fn indent_shifts_every_selected_line_and_keeps_them_selected() {
        for view in [View::Source, View::Wysiwyg] {
            let mut d = doc_in(view, "indent_sel", "one\n\ntwo\n");
            d.anchor = Some(0);
            d.caret = 7; // through "two"
            d.indent();
            assert_eq!(
                d.source, "  one\n\n  two\n",
                "the blank line keeps no trailing pad"
            );
            // Selected, so a second Tab lands on the same lines rather than on
            // whatever the shifted offsets now cover.
            assert_eq!(d.selection(), Some((0, 12)));
            d.indent();
            assert_eq!(d.source, "    one\n\n    two\n");
        }
    }

    #[test]
    fn outdent_takes_what_each_line_has_and_leaves_the_rest_alone() {
        for view in [View::Source, View::Wysiwyg] {
            let mut d = doc_in(view, "outdent_sel", "  two\n one\nnone\n");
            d.anchor = Some(0);
            d.caret = 15;
            d.outdent();
            assert_eq!(d.source, "two\none\nnone\n");
        }
    }

    #[test]
    fn a_tab_undoes_as_one_step_however_many_lines_it_moved() {
        for view in [View::Source, View::Wysiwyg] {
            let mut d = doc_in(view, "indent_undo", "one\n\ntwo\n");
            d.anchor = Some(0);
            d.caret = 7;
            d.indent();
            assert_eq!(d.source, "  one\n\n  two\n");
            d.undo();
            assert_eq!(d.source, "one\n\ntwo\n", "one step, not one per line");
            assert_eq!(
                d.selection(),
                Some((0, 7)),
                "with the selection it was aimed at"
            );
            d.redo();
            assert_eq!(d.source, "  one\n\n  two\n");
            assert_eq!(
                d.selection(),
                Some((0, 12)),
                "redo replays the caret the indent placed, not the one splice left"
            );
        }
    }

    #[test]
    fn vertical_motion_keeps_the_column() {
        let mut d = doc_with("move", "abcd\nef\n");
        d.caret = 3; // "abc|d" on row 0, col 3
        d.move_down(false); // row 1 "ef" only has cols 0..2 -> clamps to end
        assert_eq!(d.caret, 7); // just after "ef"
    }

    // ── goal column ──────────────────────────────────────────────────────────

    #[test]
    fn vertical_motion_goal_column_survives_a_short_line() {
        // Regression: re-deriving the column from the clamped position on
        // every step permanently forgets it once a short line clamps it.
        // Down through "xy" (2 cols) and into "ghijkl" must return to col 4.
        let g = |m, f: fn(&mut Doc)| golden("goalcol", m, f);
        assert_eq!(
            g("abcd|ef\nxy\nghijkl\n", |d| {
                d.move_down(false); // clamps to end of "xy"
                d.move_down(false); // restores col 4 on the long line
            }),
            "abcdef\nxy\nghij|kl\n"
        );
    }

    #[test]
    fn goal_column_state_is_set_by_vertical_motion_and_cleared_by_horizontal() {
        let mut d = doc_with("goalcol_state", "abcdef\nxy\nghijkl\n");
        assert_eq!(d.goal_col, None);
        d.caret = 4; // row 0, col 4
        d.move_down(false); // clamps into "xy"; goal stays the original col
        assert_eq!(d.goal_col, Some(4));
        assert_eq!(d.caret_pos(), (1, 2));

        // A horizontal motion drops the goal column...
        d.move_left(false);
        assert_eq!(d.goal_col, None);

        // ...so the next vertical motion picks up the *new* column (1), not
        // the stale one (4).
        d.move_down(false);
        assert_eq!(d.goal_col, Some(1));
        assert_eq!(d.caret_pos(), (2, 1));
    }

    #[test]
    fn editing_clears_the_goal_column() {
        let mut d = doc_with("goalcol_edit", "abcdef\nxy\nghijkl\n");
        d.caret = 4;
        d.move_down(false);
        assert_eq!(d.goal_col, Some(4));
        d.insert("Z");
        assert_eq!(d.goal_col, None);
    }

    #[test]
    fn vertical_motion_on_an_empty_document_is_a_no_op() {
        let mut d = doc_with("empty_vert", "");
        d.move_down(false);
        assert_eq!(d.caret, 0);
        d.move_up(false);
        assert_eq!(d.caret, 0);
    }

    // ── the document's edges ─────────────────────────────────────────────────

    #[test]
    fn vertical_motion_at_the_document_edges_runs_to_them_in_both_views() {
        // The reproduction, and the disagreement: Down on the last line ran to
        // the end of the document in the source view — by accident, an
        // out-of-range row clamping to the end of the string — and did nothing
        // whatever in the view leaf opens in. One rule now, in both.
        for (view, tag) in VIEWS {
            let mut d = doc_in(view, &format!("edge_{tag}"), "abc");
            d.caret = 1;
            d.move_down(false);
            assert_eq!(d.caret, 3, "{tag}: Down on the last line runs to the end");
            d.move_up(false);
            assert_eq!(d.caret, 0, "{tag}: Up on the first line runs to the start");
        }
    }

    #[test]
    fn vertical_motion_at_the_edges_carries_the_column_across_the_lines_between() {
        // Down off the bottom is a motion like any other, so it latches a goal
        // column — and Up comes back to the column the caret left, not to the
        // one the document's end happened to be in.
        for (view, tag) in VIEWS {
            let gap = if view == View::Source { "\n" } else { "\n\n" };
            let src = format!("abcdef{gap}ghijkl");
            let mut d = doc_in(view, &format!("edge_goal_{tag}"), &src);
            d.caret = 2; // row 0, col 2
            d.move_down(false);
            assert_eq!(d.caret_pos().1, 2, "{tag}: Down keeps the column");
            d.move_down(false);
            assert_eq!(
                d.caret,
                src.len(),
                "{tag}: Down off the bottom reaches the end"
            );
            d.move_up(false);
            assert_eq!(
                d.caret_pos().1,
                2,
                "{tag}: Up returns to the column Down left"
            );
        }
    }

    #[test]
    fn vertical_motion_with_nowhere_to_go_latches_no_goal_column() {
        // `goal_col.get_or_insert` ran *before* the early return at row 0, so an
        // Up that did nothing still armed a goal column, and the next Down aimed
        // at a column the caret had never been in.
        for (view, tag) in VIEWS {
            let mut d = doc_in(view, &format!("noop_goal_{tag}"), "abc\n\ndef");
            d.caret = 0;
            d.move_up(false);
            assert_eq!(d.caret, 0, "{tag}: already at the start");
            assert_eq!(d.goal_col, None, "{tag}: a no-op Up latched a goal column");

            d.caret = d.source.len();
            d.move_down(false);
            assert_eq!(d.caret, d.source.len(), "{tag}: already at the end");
            assert_eq!(
                d.goal_col, None,
                "{tag}: a no-op Down latched a goal column"
            );
        }
    }

    // ── soft wrap ────────────────────────────────────────────────────────────
    // Every other test here builds the map at 80 columns, where no fixture is
    // long enough to fold. A wrap is where one offset belongs to two rows at
    // once, and it broke everything that asks the caret what row it is on.

    /// The wrapped fixture these cases share, folded at 12 columns into
    /// `one two ` / `three four ` / `five six ` / `seven eight`.
    fn wrapped_doc(name: &str) -> Doc {
        let mut d = wysiwyg_doc(name, "one two three four five six seven eight");
        d.build_visual(12);
        d
    }

    #[test]
    fn home_and_end_work_from_a_wrapped_row() {
        // The reproduction: offset 19 is the `f` of "five", the first character
        // of the third row — and also the offset the second row ends at. It
        // resolved to the *second* row, so End aimed at a place the caret was
        // already in and did nothing, while Home walked backwards onto a row the
        // caret had left.
        let mut d = wrapped_doc("wrap_home_end");
        d.caret = 19;
        assert_eq!(
            d.caret_pos(),
            (2, 0),
            "the wrap boundary opens the third row"
        );
        d.move_end(false);
        assert_eq!(d.caret, 27, "End stalled at the wrap boundary");
        d.move_home(false);
        assert_eq!(d.caret, 19, "Home left the row the caret was on");
    }

    #[test]
    fn end_of_a_wrapped_row_stays_put_when_pressed_again() {
        // The row's end is the last offset that is only ever its own: the offset
        // past it opens the row below, and aiming there would send a second
        // press on to *that* row's end, and a third to the next — End walking
        // down the paragraph rather than sitting where it landed.
        let mut d = wrapped_doc("wrap_end_twice");
        d.caret = 12; // inside "three", on the second row
        d.move_end(false);
        assert_eq!(
            d.caret, 18,
            "the end of `three four`, before the space the wrap ate"
        );
        assert_eq!(d.caret_pos(), (1, 10), "drawn on the row it is the end of");
        d.move_end(false);
        assert_eq!(d.caret, 18, "a second End moved the caret");
        d.move_home(false);
        assert_eq!(d.caret, 8, "Home takes the row's own start");
    }

    #[test]
    fn vertical_motion_crosses_a_soft_wrap() {
        // Down aimed at the row below's column 0, an offset that resolved *up*
        // to the row above's end — so it landed on the offset it already had and
        // the caret could never leave a paragraph's first row.
        let mut d = wrapped_doc("wrap_down");
        d.caret = 0;
        for (want, row) in [(8, 1), (19, 2), (28, 3), (39, 3)] {
            d.move_down(false);
            assert_eq!(d.caret, want, "Down stalled");
            assert_eq!(d.caret_pos().0, row, "Down landed on the wrong row");
        }
        d.move_down(false);
        assert_eq!(d.caret, 39, "the last row's Down runs to the end and stops");

        // ...and back up, one row per press. The goal column is the end of the
        // last row, past every other row's width, so each press clamps to the
        // row's own last offset rather than to the one that opens the next.
        let mut d = wrapped_doc("wrap_up");
        d.caret = 39;
        for (want, pos) in [(27, (2, 8)), (18, (1, 10)), (7, (0, 7)), (0, (0, 0))] {
            d.move_up(false);
            assert_eq!(d.caret, want, "Up stalled");
            assert_eq!(d.caret_pos(), pos, "Up landed on the wrong row");
        }
    }

    #[test]
    fn a_kill_on_a_wrapped_row_stops_at_the_row() {
        // The kills take the same line Home and End do, so in WYSIWYG they take
        // the visual row — and a soft wrap has no newline in it to delete, so
        // nothing is joined by reaching the end of one.
        let mut d = wrapped_doc("wrap_kill");
        d.caret = 19; // the `f` of "five", opening the third row
        d.delete_to_line_end();
        // The space the wrap ate goes with the row it was drawn on: sparing it
        // would leave "four  seven", two spaces where the row had been.
        assert_eq!(d.source, "one two three four seven eight");

        // Backwards from the row's last caret position — which is *before* that
        // space, so this one survives, being on the far side of the caret.
        let mut d = wrapped_doc("wrap_kill_back");
        d.caret = 27;
        d.delete_to_line_start();
        assert_eq!(d.source, "one two three four  seven eight");
    }

    // ── document start / end ────────────────────────────────────────────────

    #[test]
    fn move_doc_start_and_end_jump_to_the_edges() {
        let g = |m, f: fn(&mut Doc)| golden("doc_edges", m, f);
        assert_eq!(
            g("hello\nwor|ld\n", |d| d.move_doc_start(false)),
            "|hello\nworld\n"
        );
        assert_eq!(
            g("hel|lo\nworld\n", |d| d.move_doc_end(false)),
            "hello\nworld\n|"
        );
        // Already at the edge: a no-op.
        assert_eq!(g("|hello\n", |d| d.move_doc_start(false)), "|hello\n");
        assert_eq!(g("hello|\n", |d| d.move_doc_end(false)), "hello\n|");
    }

    #[test]
    fn move_doc_start_and_end_extend_the_selection() {
        assert_eq!(
            golden("doc_edges_ext_end", "hello wor|ld\n", |d| d
                .move_doc_end(true)),
            "hello wor[ld\n|]"
        );
        assert_eq!(
            golden("doc_edges_ext_start", "hello wor|ld\n", |d| d
                .move_doc_start(true)),
            "[|hello wor]ld\n"
        );
    }

    #[test]
    fn move_doc_start_and_end_on_an_empty_document_are_a_no_op() {
        let mut d = doc_with("empty_edges", "");
        d.move_doc_end(false);
        assert_eq!(d.caret, 0);
        d.move_doc_start(false);
        assert_eq!(d.caret, 0);
    }

    // ── arrow collapses an active selection ─────────────────────────────────

    #[test]
    fn arrow_collapses_selection_to_its_near_edge() {
        let mut d = doc_with("collapse", "hello world\n");

        // Forward selection (anchor before caret): Right -> end, Left -> start.
        d.anchor = Some(2);
        d.caret = 7;
        d.move_right(false);
        assert_eq!((d.caret, d.anchor), (7, None));

        d.anchor = Some(2);
        d.caret = 7;
        d.move_left(false);
        assert_eq!((d.caret, d.anchor), (2, None));

        // Backward selection (anchor after caret): edges are the same
        // regardless of which end the caret started on.
        d.anchor = Some(7);
        d.caret = 2;
        d.move_right(false);
        assert_eq!((d.caret, d.anchor), (7, None));

        d.anchor = Some(7);
        d.caret = 2;
        d.move_left(false);
        assert_eq!((d.caret, d.anchor), (2, None));
    }

    #[test]
    fn arrow_with_extend_keeps_growing_the_selection() {
        let mut d = doc_with("collapse_extend", "hello world\n");
        d.anchor = Some(2);
        d.caret = 7;
        d.move_right(true); // extend: no collapse, caret steps one further
        assert_eq!((d.caret, d.anchor), (8, Some(2)));
    }

    #[test]
    fn arrow_without_a_selection_moves_one_character_as_before() {
        let mut d = doc_with("no_collapse", "hello\n");
        d.caret = 2;
        d.move_right(false);
        assert_eq!(d.caret, 3);
        d.move_left(false);
        assert_eq!(d.caret, 2);
    }

    /// Press Right until it stops, collecting the offsets walked through. Every
    /// caret bug in the WYSIWYG view shows up here as a walk that ends early:
    /// two stops sharing one source offset can't be moved between, so the caret
    /// stalls on the first of them and the walk never reaches the rest.
    fn walk_right(d: &mut Doc) -> Vec<usize> {
        let mut seen = vec![d.caret];
        for _ in 0..2000 {
            let before = d.caret;
            d.move_right(false);
            if d.caret == before {
                break;
            }
            seen.push(d.caret);
        }
        seen
    }

    #[test]
    fn the_caret_crosses_a_soft_break() {
        // A newline inside a paragraph is a `soft_break`, which twig gives no
        // span of its own — the space it renders as used to borrow the offset of
        // the character before it, and a caret can't move without changing
        // offset. Right must walk clean off the end of the first line.
        let mut d = wysiwyg_doc("soft_break_walk", "one two\nthree four\n");
        d.caret = 0;
        let seen = walk_right(&mut d);
        assert_eq!(seen, (0..=18).collect::<Vec<_>>(), "walk stalled: {seen:?}");
    }

    #[test]
    fn line_flow_preserve_resplits_the_map_and_defaults_to_fold() {
        // The paragraph holds one soft break. Folded (the default) it lays out as
        // a single reflowed row; Preserve re-lays it as a row per source line.
        // The setter must invalidate the cached map for the change to show, and
        // again on the way back — so a round trip returns to the folded layout.
        let mut d = wysiwyg_doc("line_flow", "one two\nthree four\n");
        assert_eq!(d.line_flow(), LineFlow::Fold, "fold is the default");
        d.build_visual(80);
        assert_eq!(d.vmap.num_rows(), 1, "fold: one flowing row");

        d.set_line_flow(LineFlow::Preserve);
        d.build_visual(80);
        assert_eq!(d.vmap.num_rows(), 2, "preserve: a row per source line");

        d.set_line_flow(LineFlow::Fold);
        d.build_visual(80);
        assert_eq!(d.vmap.num_rows(), 1, "fold again: back to one row");
    }

    #[test]
    fn the_caret_still_crosses_a_preserved_soft_break() {
        // Preserve renders the soft break as a row boundary rather than a space,
        // but the caret must still reach every offset — the break's own offset is
        // the first row's end stop, so Right walks clean off the end of line one
        // onto line two, exactly as it does when the break is folded.
        let mut d = wysiwyg_doc("preserve_walk", "one two\nthree four\n");
        d.set_line_flow(LineFlow::Preserve);
        d.build_visual(80);
        d.caret = 0;
        let seen = walk_right(&mut d);
        assert_eq!(seen, (0..=18).collect::<Vec<_>>(), "walk stalled: {seen:?}");
    }

    #[test]
    fn the_caret_walks_a_code_block() {
        // Every glyph of a code block used to map to the block's start, so the
        // whole block was a single offset and the caret couldn't move inside it.
        let src = "```rust\nlet x = 1;\nfn f() {}\n```\n";
        let mut d = wysiwyg_doc("code_walk", src);
        d.caret = 0;
        let seen = walk_right(&mut d);
        // The fences are markup: hidden, and no caret stop. The code between
        // them is reached a character at a time.
        let code = src.find("let").unwrap()..src.find("\n```").unwrap();
        for off in code.clone() {
            assert!(seen.contains(&off), "offset {off} unreachable: {seen:?}");
        }
        assert!(seen.contains(&code.end), "no stop after the last line");
    }

    #[test]
    fn the_caret_walks_an_indented_code_block() {
        // An indented block's text has the four-space indent stripped, so it
        // isn't a verbatim slice and its lines have to be re-found. The caret
        // lands on the code, never in the indent.
        let src = "    indented\n    code\n";
        let mut d = wysiwyg_doc("indent_code_walk", src);
        d.caret = 0;
        let seen = walk_right(&mut d);
        assert!(seen.contains(&src.find("indented").unwrap()));
        assert!(seen.contains(&src.find("code").unwrap()));
        assert!(
            !seen.contains(&0) || seen[0] == 0,
            "the caret starts where it was put"
        );
        // Nothing in the stripped indent is a stop.
        for off in [1, 2, 3] {
            assert!(!seen.contains(&off), "landed in the indent at {off}");
        }
    }

    #[test]
    fn the_caret_leaves_a_tight_heading() {
        // "# H" with text directly under it: the heading row's end and the
        // separator row's end are the same offset. Right used to find the
        // separator's copy, set the caret to where it already was, and stop.
        let mut d = wysiwyg_doc("tight_heading_walk", "# H\ntext\n");
        d.caret = 2; // the "H"
        let seen = walk_right(&mut d);
        assert!(
            seen.len() > 2,
            "Right stalled at the heading's end: {seen:?}"
        );
        assert!(
            seen.contains(&8),
            "never reached the end of \"text\": {seen:?}"
        );
    }

    #[test]
    fn the_caret_skips_the_gap_between_two_paragraphs() {
        // The blank line between two paragraphs is the boundary itself. The
        // caret used to be able to sit on it, and typing there landed in the
        // previous paragraph — "A\n\nB" became "A\nx\nB", one paragraph with a
        // soft break, so the text visibly snapped back up.
        let mut d = wysiwyg_doc("gap_skip", "A\n\nB\n");
        d.caret = 1; // the end of "A"
        d.move_right(false);
        assert_eq!(d.caret, 3, "Right stopped in the gap");
        d.insert("x");
        assert_eq!(d.source, "A\n\nxB\n", "typing landed outside B");
    }

    #[test]
    fn down_from_a_paragraph_lands_on_the_next_one() {
        let mut d = wysiwyg_doc("gap_down", "A\n\nB\n");
        d.caret = 0;
        d.move_down(false);
        assert_eq!(d.caret, 3, "Down stopped in the gap");
    }

    #[test]
    fn clicking_the_gap_lands_on_real_text() {
        // A click can still *reach* the gap — it's drawn, so it's clickable.
        // It has to resolve to somewhere the caret can be.
        let mut d = wysiwyg_doc("gap_click", "A\n\nB\n");
        d.click(1, 0, false); // the gap row
        assert!(
            d.caret == 1 || d.caret == 3,
            "click left the caret in the gap at {}",
            d.caret
        );
        d.insert("x");
        // Either edge of the boundary is a fair place to land; inside it isn't.
        assert!(
            d.source == "Ax\n\nB\n" || d.source == "A\n\nxB\n",
            "click in the gap typed into the boundary: {:?}",
            d.source
        );
    }

    #[test]
    fn enter_opens_an_empty_paragraph_the_caret_can_type_into() {
        // Enter inserts a paragraph break, which leaves a blank line spare on
        // either side of a new one. That middle line is a real empty paragraph:
        // the caret lands there, and typing makes a paragraph rather than
        // extending a neighbour.
        let mut d = wysiwyg_doc("gap_enter", "A\n\nB\n");
        d.caret = 1;
        d.newline();
        assert_eq!(d.source, "A\n\n\n\nB\n");
        d.build_visual(80);
        let (row, _) = d.caret_pos();
        assert!(
            d.vmap.row_is_navigable(row),
            "the caret landed on a gap row"
        );
        d.insert("x");
        assert_eq!(
            d.source, "A\n\nx\n\nB\n",
            "the new paragraph merged into a neighbour"
        );
    }

    #[test]
    fn enter_at_the_end_of_the_document_opens_a_paragraph_too() {
        let mut d = wysiwyg_doc("gap_eof", "A\n");
        d.caret = 1;
        d.newline();
        d.build_visual(80);
        let (row, _) = d.caret_pos();
        assert!(
            d.vmap.row_is_navigable(row),
            "the caret landed on a gap row"
        );
        d.insert("x");
        assert!(
            d.source.starts_with("A\n\n") && d.source.contains('x'),
            "typing at the end merged into A: {:?}",
            d.source
        );
    }

    // ── click_past_end ───────────────────────────────────────────────────────

    /// [`Doc::click_past_end`] on `body`, and the source it left, with the caret
    /// rendered as `|`.
    fn past_end(name: &str, body: &str) -> (Doc, String) {
        let mut d = wysiwyg_doc(name, body);
        d.click_past_end();
        let out = render_caret(&d);
        (d, out)
    }

    #[test]
    fn click_past_end_opens_an_empty_paragraph_under_the_last_block() {
        let (mut d, out) = past_end("pe_para", "A\n");
        assert_eq!(out, "A\n\n|");
        d.build_visual(80);
        let (row, _) = d.caret_pos();
        assert!(
            d.vmap.row_is_navigable(row),
            "the caret landed on a gap row"
        );
        d.insert("x");
        assert_eq!(d.source, "A\n\nx", "typing merged into A");
    }

    #[test]
    fn click_past_end_writes_both_newlines_when_the_file_has_none() {
        assert_eq!(past_end("pe_bare", "A").1, "A\n\n|");
    }

    #[test]
    fn click_past_end_is_only_a_caret_move_when_the_paragraph_is_already_there() {
        let (d, out) = past_end("pe_there", "A\n\n");
        assert_eq!(out, "A\n\n|");
        assert!(!d.dirty, "a click wrote to a document it did not need to");
        assert!(
            !d.can_undo(),
            "a click that changed nothing left an undo step"
        );
    }

    #[test]
    fn click_past_end_leaves_a_closed_fence() {
        let (mut d, out) = past_end("pe_fence", "```\ncode\n```\n");
        assert_eq!(out, "```\ncode\n```\n\n|");
        d.build_visual(80);
        let (row, _) = d.caret_pos();
        assert!(!d.vmap.rows[row].code, "the caret is still on a code row");
        d.insert("x");
        assert_eq!(d.source, "```\ncode\n```\n\nx");
    }

    #[test]
    fn click_past_end_closes_an_unclosed_fence_first() {
        assert_eq!(past_end("pe_open", "```\ncode\n").1, "```\ncode\n```\n\n|");
        assert_eq!(
            past_end("pe_open2", "````\ncode").1,
            "````\ncode\n````\n\n|"
        );
        assert_eq!(past_end("pe_tilde", "~~~\ncode\n").1, "~~~\ncode\n~~~\n\n|");
        assert_eq!(
            past_end("pe_quoted", "> ```\n> code\n").1,
            "> ```\n> code\n> ```\n\n|"
        );
    }

    #[test]
    fn click_past_end_does_not_close_an_indented_block() {
        assert_eq!(past_end("pe_indent", "    code\n").1, "    code\n\n|");
    }

    #[test]
    fn click_past_end_under_a_list_and_a_table_leaves_them() {
        let (mut d, out) = past_end("pe_list", "- a\n- b\n");
        assert_eq!(out, "- a\n- b\n\n|");
        d.insert("x");
        assert_eq!(d.source, "- a\n- b\n\nx", "typed into the list");
        assert_eq!(
            past_end("pe_table", "| a |\n|---|\n| b |\n").1,
            "| a |\n|---|\n| b |\n\n|"
        );
    }

    #[test]
    fn click_past_end_on_an_empty_document_writes_nothing() {
        let (d, out) = past_end("pe_empty", "");
        assert_eq!(out, "|");
        assert!(!d.dirty);
    }

    #[test]
    fn click_past_end_is_one_undo_step() {
        let (mut d, _) = past_end("pe_undo", "A\n");
        d.undo();
        assert_eq!(d.source, "A\n");
    }

    #[test]
    fn click_past_end_in_the_source_view_only_moves_the_caret() {
        let mut d = doc_with("pe_source", "A\n");
        d.click_past_end();
        assert_eq!(render_caret(&d), "A\n|");
        assert!(!d.dirty);
    }

    #[test]
    fn click_past_end_on_a_read_only_document_only_moves_the_caret() {
        let mut d = wysiwyg_doc("pe_ro", "A\n");
        d.set_read_only(true);
        d.click_past_end();
        assert_eq!(render_caret(&d), "A\n|");
    }

    // ── toggle_code_block ────────────────────────────────────────────────────

    #[test]
    fn toggle_code_block_fences_the_paragraph_at_the_caret_and_reverses() {
        let g = |n, m| golden_in(View::Wysiwyg, n, m, |d| d.toggle_code_block());
        assert_eq!(g("cb_on", "hel|lo\n"), "```\nhel|lo\n```\n");
        assert_eq!(g("cb_off", "```\nhel|lo\n```\n"), "hel|lo\n");
        assert_eq!(g("cb_end", "hello|\n"), "```\nhello|\n```\n");
        assert_eq!(
            g("cb_wrap", "aaa\nb|bb\nccc\n"),
            "```\naaa\nb|bb\nccc\n```\n"
        );
        assert_eq!(
            g("cb_unwrap", "```\naaa\nb|bb\nccc\n```\n"),
            "aaa\nb|bb\nccc\n"
        );
        // An indented block dedents, and the lines stay one-to-one.
        assert_eq!(g("cb_indent", "    co|de\n"), "co|de\n");
        // A quote's marker is kept on every line, the fence's included.
        assert_eq!(g("cb_quote", "> hel|lo\n"), "> ```\n> hel|lo\n> ```\n");
    }

    #[test]
    fn toggle_code_block_on_a_blank_line_opens_an_empty_fence() {
        let g = |n, m| golden_in(View::Wysiwyg, n, m, |d| d.toggle_code_block());
        assert_eq!(g("cb_blank", "A\n\n|"), "A\n\n```\n|\n```");
        assert_eq!(
            g("cb_blank_mid", "A\n\n|\n\nB\n"),
            "A\n\n```\n|\n```\n\nB\n"
        );
        // Tight against a neighbour, a blank line goes in on that side.
        assert_eq!(g("cb_blank_tight", "A\n|\nB\n"), "A\n\n```\n|\n```\n\nB\n");
        // Inside a quote, on a quoted blank line.
        assert_eq!(
            g("cb_blank_quote", "> A\n>\n> |\n"),
            "> A\n>\n> ```\n> |\n> ```\n"
        );
    }

    #[test]
    fn toggle_code_block_from_a_blank_line_is_a_block_the_caret_can_type_into() {
        let mut d = wysiwyg_doc("cb_type", "A\n\n");
        d.click_past_end();
        d.toggle_code_block();
        assert!(
            d.caret_in_code_block(),
            "the caret is not in the block it opened"
        );
        d.insert("let x = 1;");
        d.newline();
        d.insert("x");
        assert_eq!(d.source, "A\n\n```\nlet x = 1;\nx\n```");
        d.build_visual(80);
        let (row, _) = d.caret_pos();
        assert!(d.vmap.rows[row].code, "typed text is not on a code row");
        // And back out: the button reverses what it did, text kept.
        d.toggle_code_block();
        assert_eq!(d.source, "A\n\nlet x = 1;\nx\n");
        assert!(!d.caret_in_code_block());
    }

    #[test]
    fn toggle_code_block_over_a_selection_fences_it_whole_and_keeps_it_selected() {
        let mut d = wysiwyg_doc("cb_sel", "one\n\ntwo\n\nthree\n");
        d.anchor = Some(1);
        d.caret = 7;
        d.toggle_code_block();
        assert_eq!(d.source, "```\none\n\ntwo\n```\n\nthree\n");
        assert!(d.selection().is_some(), "the block came back unselected");
        d.toggle_code_block();
        assert_eq!(
            d.source, "one\n\ntwo\n\nthree\n",
            "a second press did not reverse the first"
        );
    }

    #[test]
    fn toggle_code_block_inside_a_list_item_fences_and_unfences_in_place() {
        for (name, before, fenced) in [
            ("cb_list", "- it|em\n", "- ```\n  it|em\n  ```\n"),
            ("cb_olist", "1. it|em\n", "1. ```\n   it|em\n   ```\n"),
            ("cb_task", "- [ ] it|em\n", "- [ ] ```\n  it|em\n  ```\n"),
            (
                "cb_nested",
                "- a\n  - it|em\n",
                "- a\n  - ```\n    it|em\n    ```\n",
            ),
        ] {
            let mut d = wysiwyg_doc(name, &before.replace('|', ""));
            d.caret = before.find('|').unwrap();
            d.toggle_code_block();
            assert_eq!(render_caret(&d), fenced, "{name}: fencing");
            assert!(
                d.caret_in_code_block(),
                "{name}: the caret is in the new block"
            );
            d.toggle_code_block();
            assert_eq!(
                render_caret(&d),
                before,
                "{name}: a second press reverses the first"
            );
        }
    }

    #[test]
    fn toggle_code_block_in_a_list_item_is_one_undo_step() {
        let mut d = wysiwyg_doc("cb_list_undo", "- item\n");
        d.caret = 4;
        d.toggle_code_block();
        d.undo();
        assert_eq!(render_caret(&d), "- it|em\n");
    }

    #[test]
    fn caret_in_code_block_reads_fenced_and_indented_blocks() {
        let mut d = wysiwyg_doc("cb_in", "para\n\n```\ncode\n```\n\n    more\n");
        d.caret = 2;
        assert!(!d.caret_in_code_block());
        d.caret = 11;
        assert!(d.caret_in_code_block());
        d.caret = 25;
        assert!(d.caret_in_code_block(), "an indented block is a code block");
    }

    #[test]
    fn caret_in_blockquote_reads_every_line_of_a_quote_and_nothing_after() {
        let src = "para\n\n> # Head\n> body|\n\nafter\n";
        let mut d = wysiwyg_doc("bq_in", &src.replace('|', ""));
        let quote_start = src.find('>').unwrap();
        let body_end = src.find('|').unwrap();
        let after = src.find("after").unwrap() - 1;
        for (at, inside) in [
            (2, false),
            (quote_start + 4, true),
            (body_end - 2, true),
            (body_end, true),
            (after, false),
            (after + 2, false),
        ] {
            d.caret = at;
            assert_eq!(d.caret_in_blockquote(), inside, "caret at {at}");
        }
        // A heading in a quote is both: the quote is a layer over the style.
        d.caret = quote_start + 4;
        assert_eq!(d.current_heading_level(), Some(1));
    }

    #[test]
    fn caret_in_blockquote_follows_the_toggle() {
        let mut d = wysiwyg_doc("bq_toggle", "hello\n");
        d.caret = 2;
        assert!(!d.caret_in_blockquote());
        d.toggle_blockquote();
        assert!(d.caret_in_blockquote());
        d.toggle_blockquote();
        assert!(!d.caret_in_blockquote());
    }

    #[test]
    fn toggle_code_block_is_one_undo_step() {
        let mut d = wysiwyg_doc("cb_undo", "hello\n");
        d.caret = 2;
        d.toggle_code_block();
        d.undo();
        assert_eq!(render_caret(&d), "he|llo\n");
    }

    #[test]
    fn triple_click_selects_a_paragraph_across_its_soft_breaks() {
        // A paragraph broken over two source lines is one paragraph. Selecting
        // it must not stop at the newline inside it — that newline is markup the
        // rich-text view exists to hide.
        let src = "one two\nthree four\n\nnext\n";
        let mut d = wysiwyg_doc("triple_para", src);
        d.select_block_at(2);
        assert_eq!(
            d.selected_text(),
            Some("one two\nthree four"),
            "stopped at the soft break"
        );
    }

    #[test]
    fn the_wheel_can_scroll_away_from_a_caret_that_stays_put() {
        // The reader scrolls down past the caret's row. Nothing moved the
        // caret, so the view must stay where it was put — the old code revealed
        // the caret every frame, which dragged the view straight back and made
        // the document unscrollable past the caret.
        let mut d = wysiwyg_doc("scroll_free", "a\n\nb\n\nc\n\nd\n\ne\n");
        d.caret = 0;
        d.follow_caret(0, 3, 9); // first frame: the caret is at the top
        d.scroll = 4; // the wheel
        d.follow_caret(0, 3, 9);
        assert_eq!(
            d.scroll, 4,
            "the wheel was overruled by a caret that never moved"
        );
    }

    #[test]
    fn moving_the_caret_brings_the_view_back_to_it() {
        let mut d = wysiwyg_doc("scroll_follow", "a\n\nb\n\nc\n\nd\n\ne\n");
        d.caret = 0;
        d.follow_caret(0, 3, 9);
        d.scroll = 6; // scrolled away
        d.move_right(false); // ...and now the caret moves
        let (row, _) = d.caret_pos();
        d.follow_caret(row, 3, 9);
        assert!(
            d.scroll <= row && row < d.scroll + 3,
            "caret row {row} off screen at scroll {}",
            d.scroll
        );
    }

    #[test]
    fn scrolling_stops_at_the_last_row() {
        let mut d = wysiwyg_doc("scroll_clamp", "a\n\nb\n");
        d.caret = 0;
        d.follow_caret(0, 3, 3); // a first frame, so the caret isn't "new"
        d.scroll = 999; // the wheel, spun hard
        d.follow_caret(0, 3, 3);
        assert_eq!(d.scroll, 2, "scrolled into the void past the document");
    }

    #[test]
    fn every_cell_of_a_wide_table_is_reachable() {
        // A table whose cells are far wider than the surface: the columns are
        // cut to fit and the text wraps inside them, so no cell hangs off the
        // right edge where the caret can never go.
        let src = "| Ingredient | Notes |\n|---|---|\n\
                   | flour milled coarse | sift it twice before folding it in |\n";
        let mut d = wysiwyg_doc("wide_table_walk", src);
        d.build_visual(30);
        d.caret = 0;
        let seen = walk_right(&mut d);
        for word in ["Ingredient", "Notes", "coarse", "folding"] {
            let at = src.find(word).unwrap();
            assert!(seen.contains(&at), "{word:?} at {at} unreachable: {seen:?}");
        }
    }

    // ── view parity ──────────────────────────────────────────────────────────
    // `doc_with` pins the source view, so everything above tests a view users
    // never start in — `Doc::open` opens in WYSIWYG. These run the motion and
    // deletion golden cases through *both*, plus the WYSIWYG cases the two
    // can't share: where the source carries markup the rendered text is a
    // different string, and the views agreeing would itself be the bug.

    const VIEWS: [(View, &str); 2] = [(View::Source, "source"), (View::Wysiwyg, "wysiwyg")];

    /// Run `action` in both views on one `|`-marked fixture and assert they
    /// agree. Plain prose only: with no markup to hide, WYSIWYG renders the
    /// source verbatim, so the two views are looking at the same text and any
    /// disagreement is one of them having lost the plot.
    fn both_views(name: &str, marked: &str, action: fn(&mut Doc)) -> String {
        let (src, caret) = parse_caret(marked);
        let run = |view: View, tag: &str| {
            let mut d = doc_in(view, &format!("{name}_{tag}"), &src);
            d.caret = caret;
            action(&mut d);
            render_caret(&d)
        };
        let source = run(VIEWS[0].0, VIEWS[0].1);
        let wysiwyg = run(VIEWS[1].0, VIEWS[1].1);
        assert_eq!(source, wysiwyg, "the views disagree on {marked:?}");
        source
    }

    #[test]
    fn word_motion_agrees_across_the_views_on_plain_prose() {
        let g = both_views;
        assert_eq!(
            g("par_wl", "hello wor|ld", |d| d.move_word_left(false)),
            "hello |world"
        );
        assert_eq!(
            g("par_wl2", "hello| world", |d| d.move_word_left(false)),
            "|hello world"
        );
        assert_eq!(
            g("par_wr", "hel|lo world", |d| d.move_word_right(false)),
            "hello| world"
        );
        assert_eq!(
            g("par_wr2", "hello| world", |d| d.move_word_right(false)),
            "hello world|"
        );
        assert_eq!(
            g("par_punct", "|foo.bar", |d| d.move_word_right(false)),
            "foo|.bar"
        );
        assert_eq!(
            g("par_ext", "hello |world", |d| d.move_word_right(true)),
            "hello [world|]"
        );
    }

    #[test]
    fn word_deletion_agrees_across_the_views_on_plain_prose() {
        let g = both_views;
        assert_eq!(
            g("par_db", "hello world|", |d| d.delete_word_back()),
            "hello |"
        );
        assert_eq!(
            g("par_df", "hello |world", |d| d.delete_word_forward()),
            "hello |"
        );
        assert_eq!(
            g("par_db2", "foo |bar baz", |d| d.delete_word_back()),
            "|bar baz"
        );
        assert_eq!(g("par_utf8", "café |ok", |d| d.delete_word_back()), "|ok");
    }

    #[test]
    fn character_motion_and_deletion_agree_across_the_views_on_plain_prose() {
        let g = both_views;
        assert_eq!(g("par_r", "he|llo", |d| d.move_right(false)), "hel|lo");
        assert_eq!(g("par_l", "he|llo", |d| d.move_left(false)), "h|ello");
        assert_eq!(g("par_bs", "hel|lo", |d| d.backspace()), "he|lo");
        assert_eq!(g("par_del", "hel|lo", |d| d.delete_forward()), "hel|o");
    }

    #[test]
    fn wysiwyg_motion_steps_a_grapheme_cluster_the_way_the_source_view_does() {
        // The reproduction: the stop table was built one stop per `char`, so
        // Right parked the caret 4 bytes into a ZWJ sequence — a place the
        // source view, which steps by grapheme, can't reach and backspace can't
        // survive. The two views must land on the same offset.
        let family = "👨‍👩‍👧"; // three emoji strung together with joiners: one cluster
        for (view, tag) in VIEWS {
            let mut d = doc_in(view, &format!("cluster_{tag}"), &format!("a{family}b\n"));
            d.caret = 1;
            d.move_right(false);
            assert_eq!(d.caret, 1 + family.len(), "{tag} parked inside the cluster");

            // ...and the edit that used to sever a joiner off the front of it.
            d.backspace();
            assert_eq!(d.source, "ab\n", "{tag} split the cluster");
            assert_eq!(d.caret, 1);
        }
    }

    #[test]
    fn wysiwyg_motion_treats_a_combining_accent_as_one_character() {
        for (view, tag) in VIEWS {
            let mut d = doc_in(view, &format!("combining_{tag}"), "e\u{0301}x\n");
            d.caret = 0;
            d.move_right(false);
            assert_eq!(
                d.caret,
                "e\u{0301}".len(),
                "{tag} stopped on the combining mark"
            );
        }
    }

    #[test]
    fn no_wysiwyg_motion_can_park_the_caret_inside_a_cluster() {
        // The general form: whatever route the caret takes through a document
        // full of clusters, it never lands between the codepoints of one — so no
        // motion-then-backspace sequence can leave a dangling joiner behind.
        use unicode_segmentation::UnicodeSegmentation;

        let src = "a👨‍👩‍👧b e\u{0301}mo👨‍👩‍👧ji\n\nnext 👩‍🚀 line\n";
        let mut d = wysiwyg_doc("cluster_walk", src);
        d.caret = 0;
        let boundaries: Vec<usize> = src
            .grapheme_indices(true)
            .map(|(i, _)| i)
            .chain(std::iter::once(src.len()))
            .collect();
        for off in walk_right(&mut d) {
            assert!(
                boundaries.contains(&off),
                "Right stopped at {off}, inside a grapheme cluster"
            );
        }
    }

    #[test]
    fn wysiwyg_word_motion_stays_out_of_hidden_delimiters() {
        // The reproduction: ⌥→ from inside the opening `**` computed its
        // boundary over the raw source and landed on byte 8 — inside the
        // *closing* `**`, which `caret_pos` draws at column 6, immediately after
        // "bold". The caret drew past the bold word and sat inside it.
        let mut d = wysiwyg_doc("wys_word_delim", "a **bold** c\n");
        d.caret = 2;
        d.move_word_right(false);
        assert!(
            d.vmap.is_stop(d.caret),
            "landed at {}, not a caret stop",
            d.caret
        );
        assert_eq!(d.caret, 10, "should land on the space after \"bold\"");
        // The rendered row is "a bold c": column 6 is the space just past "bold",
        // and now the caret is really there rather than only drawn there.
        assert_eq!(d.caret_pos(), (0, 6));

        // ...and back again: ⌥← returns to the "b", not into the opening `**`.
        d.move_word_left(false);
        assert_eq!(d.caret, 4);
        assert_eq!(d.caret_pos(), (0, 2));
    }

    #[test]
    fn wysiwyg_word_delete_takes_the_markup_with_the_word() {
        // The reproduction: ⌥⌫ from after "bold" walked the raw source, stopped
        // inside the closing `**`, and left "a ** c\n" — delimiters with no
        // opener. Glyph space covers the word alone, which would leave
        // "a **** c": markup wrapped around nothing. The word and the styling
        // that was only ever the word's go together.
        let mut d = wysiwyg_doc("wys_word_del_back", "a **bold** c\n");
        d.caret = 10;
        d.delete_word_back();
        assert_eq!(d.source, "a  c\n");
        assert_eq!(d.caret, 2);

        let mut d = wysiwyg_doc("wys_word_del_fwd", "a **bold** c\n");
        d.caret = 4; // the "b"
        d.delete_word_forward();
        assert_eq!(d.source, "a  c\n");
    }

    #[test]
    fn wysiwyg_word_delete_empties_a_nested_mark_and_a_code_span_too() {
        let src = "a ***bold*** c\n";
        let mut d = wysiwyg_doc("wys_word_del_nest", src);
        d.caret = src.find(" c").unwrap();
        d.delete_word_back();
        assert_eq!(
            d.source, "a  c\n",
            "the emph inside the strong empties it too"
        );

        let src = "a `code` c\n";
        let mut d = wysiwyg_doc("wys_word_del_code", src);
        d.caret = src.find(" c").unwrap();
        d.delete_word_back();
        assert_eq!(d.source, "a  c\n");
    }

    #[test]
    fn wysiwyg_word_delete_keeps_a_mark_that_still_has_text() {
        // Only an *emptied* node goes. Take one word of two and the `**` still
        // has a job to do — over the word that's left, with the space the delete
        // pushed against the opening delimiter moved out in front of it, or the
        // run would be no run at all (`** words**` is literal asterisks — see
        // the mark-edge rule on `splice`).
        let src = "a **two words** c\n";
        let mut d = wysiwyg_doc("wys_word_del_partial", src);
        d.caret = src.find(" words").unwrap();
        d.delete_word_back();
        assert_eq!(d.source, "a  **words** c\n");
    }

    #[test]
    fn source_view_word_motion_still_walks_the_markup() {
        // The other half of the decision: in the source view the `**` are
        // characters like any other — they're on the screen, so word motion has
        // to stop at them and a word-delete has to leave them behind. Only
        // WYSIWYG hides them, so only WYSIWYG steps over them.
        let g = |n, m, f: fn(&mut Doc)| golden(n, m, f);
        assert_eq!(
            g("src_word_motion", "a |**bold** c\n", |d| d
                .move_word_right(false)),
            "a **bold|** c\n"
        );
        // The same caret as the WYSIWYG reproduction, and the opposite outcome:
        // here "a ** c\n" is right, because `bold**` is what's to the left of it.
        assert_eq!(
            g("src_word_del", "a **bold**| c\n", |d| d.delete_word_back()),
            "a **| c\n"
        );
    }

    #[test]
    fn every_wysiwyg_motion_lands_on_a_caret_stop() {
        // The single invariant both bugs violated: the caret draws and edits at
        // the same place only when it's on a stop. `debug_assert_on_a_stop`
        // makes the same claim in-place; this pins it from the outside, over a
        // document with every kind of thing the map has to be careful about.
        // At two widths: the wide one every other test builds at, where no
        // fixture folds, and one narrow enough that they all do. A soft wrap is
        // where an offset stops being on exactly one row, and testing only the
        // width that never wraps is how the caret came to be pinned at the first
        // one Down reached.
        let src = "# Title\n\na **bold** e\u{0301}mo👨‍👩‍👧ji `x` c\n\n\
                   - item one\n\n| A | B |\n|---|---|\n| x | y |\n";
        // A table of named operations, which is what it looks like.
        #[allow(clippy::type_complexity)]
        let motions: [(&str, fn(&mut Doc)); 8] = [
            ("right", |d| d.move_right(false)),
            ("left", |d| d.move_left(false)),
            ("word_right", |d| d.move_word_right(false)),
            ("word_left", |d| d.move_word_left(false)),
            ("down", |d| d.move_down(false)),
            ("up", |d| d.move_up(false)),
            ("home", |d| d.move_home(false)),
            ("end", |d| d.move_end(false)),
        ];
        for width in [80, 12] {
            let mut d = wysiwyg_doc("stop_invariant", src);
            d.build_visual(width);
            let stops: Vec<usize> = (0..=src.len()).filter(|&o| d.vmap.is_stop(o)).collect();
            assert!(stops.len() > 20, "fixture should have plenty of stops");
            for start in stops {
                for (name, motion) in &motions {
                    d.caret = start;
                    d.anchor = None;
                    motion(&mut d);
                    assert!(
                        d.vmap.is_stop(d.caret),
                        "{name} from {start} at width {width} landed at {} — not a caret stop",
                        d.caret
                    );
                }
            }
        }
    }

    #[test]
    fn no_wysiwyg_motion_is_a_dead_end() {
        // Down held to the bottom of a document reaches the bottom, and Up held
        // to the top reaches the top — from anywhere, at a width that wraps. The
        // invariant above says a motion lands somewhere legal; this one says it
        // gets somewhere at all, which is what a caret pinned at a wrap boundary
        // was quietly failing to do while every assertion around it held.
        let src = "# Title\n\none two three four five six seven eight nine ten\n\n\
                   - item one two three four five\n\nlast\n";
        for width in [80, 12] {
            let mut d = wysiwyg_doc("no_dead_end", src);
            d.build_visual(width);
            let stops: Vec<usize> = (0..=src.len()).filter(|&o| d.vmap.is_stop(o)).collect();
            let (first, last) = (stops[0], stops[stops.len() - 1]);
            for &start in &stops {
                for (name, motion, want) in [
                    (
                        "down",
                        (|d: &mut Doc| d.move_down(false)) as fn(&mut Doc),
                        last,
                    ),
                    ("up", |d: &mut Doc| d.move_up(false), first),
                ] {
                    d.caret = start;
                    d.anchor = None;
                    d.goal_col = None;
                    // Every row, plus the presses the edges take, plus slack.
                    for _ in 0..d.vmap.num_rows() + 4 {
                        motion(&mut d);
                    }
                    assert_eq!(
                        d.caret, want,
                        "{name} held from {start} at width {width} never arrived"
                    );
                }
            }
        }
    }
    // ── display columns ──────────────────────────────────────────────────────
    // A `col` is a terminal cell, not a character. The two are the same number
    // for the ASCII the fixtures above are written in, which is how they came
    // apart in the first place: `你` is one character drawn in two cells, so a
    // column counted in characters names a cell the text isn't in — one earlier
    // for every wide character to its left.

    #[test]
    fn a_wide_character_is_two_columns_wide() {
        // The reproduction: `你` is one char and two cells, so the caret just
        // past it drew at column 1 — inside the character it had already left.
        for (view, tag) in VIEWS {
            let mut d = doc_in(view, &format!("wide_col_{tag}"), "你好\n");
            d.caret = "你".len();
            assert_eq!(d.caret_pos(), (0, 2), "{tag}: caret drew inside 你");
            d.caret = "你好".len();
            assert_eq!(d.caret_pos(), (0, 4), "{tag}");
        }
    }

    #[test]
    fn a_cluster_is_as_wide_as_it_is_drawn_not_as_its_codepoints_measure() {
        // `👨‍👩‍👧` is five codepoints — two-cell, joiner, two-cell, joiner,
        // two-cell — measuring six cells one at a time, but the character they
        // spell is drawn in two. Width belongs to the cluster, not the glyph,
        // and the frontends measure it the same way.
        let family = "👨‍👩‍👧";
        for (view, tag) in VIEWS {
            let src = format!("a{family}b\n");
            let mut d = doc_in(view, &format!("wide_cluster_{tag}"), &src);
            d.caret = 1 + family.len();
            assert_eq!(
                d.caret_pos(),
                (0, 3),
                "{tag}: 'a' is one cell, the family two"
            );
        }
    }

    #[test]
    fn both_cells_of_a_wide_character_mean_the_character() {
        // Clicking the far half of `好` is still clicking `好`: half a character
        // is not a place the caret can be, so it comes to rest at the
        // character's start — the column it would have been drawn at anyway.
        for (view, tag) in VIEWS {
            let mut d = doc_in(view, &format!("wide_click_{tag}"), "你好\n");
            for col in [2, 3] {
                d.caret = 0;
                d.click(0, col, false);
                assert_eq!(d.caret, "你".len(), "{tag}: click at col {col}");
                assert_eq!(d.caret_pos(), (0, 2), "{tag}: click at col {col}");
            }
            // Past the last cell is the line's end, as it is for ASCII.
            d.click(0, 9, false);
            assert_eq!(d.caret, "你好".len(), "{tag}: click past the end");
        }
    }

    #[test]
    fn every_offset_survives_the_trip_out_to_a_column_and_back() {
        // The mapping is only a mapping if it inverts: the cell the caret is
        // drawn in has to be the cell that brings it back to the same offset.
        // Over a fixture where a character may be one cell or two, and one
        // codepoint or five.
        use unicode_segmentation::UnicodeSegmentation;

        let src = "ab 你好 c\n\n👨‍👩‍👧 e\u{0301}x 漢字\n\nplain ascii\n";

        let mut d = doc_in(View::Source, "roundtrip_source", src);
        // Every offset the source view's caret can occupy: it steps by grapheme
        // cluster, so those are its boundaries.
        for (off, _) in src
            .grapheme_indices(true)
            .chain(std::iter::once((src.len(), "")))
        {
            d.caret = off;
            let (row, col) = d.caret_pos();
            d.click(row, col, false);
            assert_eq!(d.caret, off, "source: {off} → ({row}, {col}) → {}", d.caret);
        }

        // And in WYSIWYG, where the offsets the caret can occupy are the map's
        // stops rather than every boundary.
        let mut d = doc_in(View::Wysiwyg, "roundtrip_wysiwyg", src);
        let stops: Vec<usize> = (0..=src.len()).filter(|&o| d.vmap.is_stop(o)).collect();
        assert!(stops.len() > 20, "fixture should have plenty of stops");
        for off in stops {
            d.caret = off;
            let (row, col) = d.caret_pos();
            d.click(row, col, false);
            assert_eq!(
                d.caret, off,
                "wysiwyg: {off} → ({row}, {col}) → {}",
                d.caret
            );
        }
    }

    #[test]
    fn vertical_motion_aims_at_a_column_the_reader_can_see() {
        // Down from under `世` lands under the glyph in that cell, not two
        // characters further along the line. The goal is a column, so a line of
        // wide characters and a line of ASCII line up the way they're drawn.
        //
        // The gap differs by view: a bare newline inside a paragraph is a soft
        // break, which WYSIWYG draws as a space on a single row. The views share
        // a grid only where the source's lines are the renderer's rows too.
        for (view, tag) in VIEWS {
            let gap = if view == View::Source { "\n" } else { "\n\n" };
            let src = format!("你好世{gap}abcdef\n");
            let mut d = doc_in(view, &format!("goal_wide_{tag}"), &src);
            d.caret = "你好".len();
            assert_eq!(d.caret_pos().1, 4, "{tag}: `世` is drawn at column 4");
            d.move_down(false);
            assert_eq!(d.caret_pos().1, 4, "{tag}: goal column lost");
            assert!(
                d.source[d.caret..].starts_with('e'),
                "{tag}: landed on the wrong glyph"
            );
        }
    }

    #[test]
    fn a_goal_column_landing_inside_a_wide_character_lands_on_it() {
        // Down from column 3 onto `你好`, whose characters start at columns 0
        // and 2: column 3 is the *second* cell of `好`. There is nowhere to be
        // between the cells of one character, so the caret rests on it — and on
        // its start, which is the only offset there that is a caret stop.
        for (view, tag) in VIEWS {
            let gap = if view == View::Source { "\n" } else { "\n\n" };
            let src = format!("abcdef{gap}你好\n");
            let mut d = doc_in(view, &format!("goal_inside_{tag}"), &src);
            let line = src.find('你').unwrap();
            d.caret = 3;
            d.move_down(false);
            assert_eq!(d.caret, line + "你".len(), "{tag}: landed off `好`'s start");
            assert_eq!(d.caret_pos().1, 2, "{tag}: drew between `好`'s cells");
        }
    }

    #[test]
    fn a_caret_in_a_table_cell_of_wide_text_draws_where_the_text_is() {
        // The column the cell's text is laid out in is measured in cells, so the
        // caret walking that text has to be too — the two agreeing is the whole
        // point of the grid staying square.
        let mut d = wysiwyg_doc("table_wide", "| A | B |\n|---|---|\n| 你好 | y |\n");
        let at = d.source.find("你").unwrap();
        d.caret = at;
        let (row, col) = d.caret_pos();
        // `│ ` opens the row, so the cell's text starts at column 2; `好` is two
        // cells further along.
        assert_eq!(col, 2, "the cell's first character");
        d.move_right(false);
        assert_eq!(
            d.caret_pos(),
            (row, 4),
            "`好` is drawn past `你`'s two cells"
        );
        assert_eq!(d.caret, at + "你".len());
    }

    // ── active inline marks ───────────────────────────────────────────────────

    /// The marks at a `|`-marked fixture's caret, in `InlineMarks::iter` order.
    fn marks(view: View, name: &str, marked: &str) -> Vec<InlineKind> {
        let (src, caret) = parse_caret(marked);
        let mut d = doc_in(view, name, &src);
        d.caret = caret;
        d.active_inline_marks().iter().collect()
    }

    /// The marks over the selection `[start, end)`.
    fn marks_over(view: View, name: &str, src: &str, start: usize, end: usize) -> Vec<InlineKind> {
        let mut d = doc_in(view, name, src);
        d.anchor = Some(start);
        d.caret = end;
        d.active_inline_marks().iter().collect()
    }

    #[test]
    fn a_caret_in_a_mark_reports_it() {
        for (view, tag) in VIEWS {
            let m = |marked| marks(view, &format!("marks_in_{tag}"), marked);
            assert_eq!(m("a **bo|ld** b"), [InlineKind::Strong], "{tag}");
            assert_eq!(m("a *it|alic* b"), [InlineKind::Emph], "{tag}");
            assert_eq!(m("a `co|de` b"), [InlineKind::Verbatim], "{tag}");
            // Plain text under no mark lights nothing — the toolbar's resting state.
            assert_eq!(m("a| **bold** b"), [], "{tag}");
            assert!(m("plain t|ext").is_empty(), "{tag}");
        }
    }

    #[test]
    fn nested_marks_all_report() {
        // Bold *and* italic: a toolbar lights both buttons, so the set has both —
        // the ancestor chain is a chain, and every mark on it is in force.
        for (view, tag) in VIEWS {
            assert_eq!(
                marks(
                    view,
                    &format!("marks_nested_{tag}"),
                    "**bold and *bo|th*** end"
                ),
                [InlineKind::Strong, InlineKind::Emph],
                "{tag}"
            );
        }
    }

    #[test]
    fn the_caret_at_a_marks_edge_reports_it_where_typing_would_extend_it() {
        // The offsets a WYSIWYG caret actually reaches at a bold run's edges are
        // the first byte of its text and the byte after its last — both inside
        // the mark's span, both places typing lands inside the bold. The offset
        // past the closing delimiter is the next text, and reports nothing.
        let src = "a **bold** b";
        let inner_start = src.find("bold").unwrap(); // 4
        let inner_end = inner_start + "bold".len(); // 8, on the closing `**`
        for (view, tag) in VIEWS {
            let mut d = doc_in(view, &format!("marks_edge_{tag}"), src);
            for off in [2, 3, inner_start, inner_end, 9] {
                d.caret = off;
                assert!(
                    d.active_inline_marks().contains(InlineKind::Strong),
                    "{tag}: offset {off} is inside the strong span"
                );
            }
            for off in [0, 1, 10, 11, 12] {
                d.caret = off;
                assert!(
                    !d.active_inline_marks().contains(InlineKind::Strong),
                    "{tag}: offset {off} is outside the strong run"
                );
            }
        }
    }

    #[test]
    fn a_mark_ends_the_same_way_at_the_end_of_the_buffer_as_in_the_middle() {
        // Regression: twig resolves an offset that is one node's end and the
        // next one's start to the node that *starts* there, so `**bold**|\n`
        // isn't bold. With nothing following there's no tie to break and the
        // chain still ended at the mark, which made a trailing `\n` — not the
        // text — decide whether the caret after a bold word reported bold. It's
        // the offset past the mark either way, and typing there is plain either
        // way. A blank document typed into is exactly this shape.
        for (view, tag) in VIEWS {
            let m = |name: String, marked| marks(view, &name, marked);
            assert_eq!(
                m(format!("marks_eob_{tag}"), "**bold**|"),
                [],
                "{tag}: no trailing newline"
            );
            assert_eq!(
                m(format!("marks_eol_{tag}"), "**bold**|\n"),
                [],
                "{tag}: with one"
            );
            // And the last offset that *is* in the mark still is.
            assert_eq!(
                m(format!("marks_eob_in_{tag}"), "**bold*|*"),
                [InlineKind::Strong],
                "{tag}"
            );
        }
    }

    #[test]
    fn a_selection_reports_a_mark_only_when_it_covers_the_whole_thing() {
        let src = "a **bold** b";
        let (b, d_) = (src.find("bold").unwrap(), src.find("bold").unwrap() + 4);
        for (view, tag) in VIEWS {
            let m = |s, e| marks_over(view, &format!("marks_sel_{tag}"), src, s, e);
            // The whole bold word, and a slice of it.
            assert_eq!(m(b, d_), [InlineKind::Strong], "{tag}: the whole word");
            assert_eq!(m(b + 1, d_ - 1), [InlineKind::Strong], "{tag}: a slice");
            // Ending exactly at the closing delimiter's start is still all-bold:
            // an exclusive end sits *past* the last selected character, so the
            // question is asked of the character, not the boundary.
            assert_eq!(
                m(b, d_ + 2),
                [InlineKind::Strong],
                "{tag}: through the close"
            );
            // Half in, half out: Bold lit here would claim a press turns it off.
            assert_eq!(m(0, d_), [], "{tag}: leading plain text");
            assert_eq!(m(b, src.len()), [], "{tag}: trailing plain text");
        }
    }

    #[test]
    fn a_selection_across_two_runs_of_the_same_mark_reports_nothing() {
        // Both ends are bold, but the space between them isn't — two runs are two
        // nodes, which is exactly what the node id catches and a kind-only
        // comparison would not.
        let src = "**one** **two**";
        for (view, tag) in VIEWS {
            let m = marks_over(view, &format!("marks_runs_{tag}"), src, 2, 13);
            assert_eq!(m, [], "{tag}: `one** **two` is not all bold");
        }
    }

    #[test]
    fn marks_read_the_document_as_it_is_edited() {
        // The point of asking twig every frame instead of caching: the answer has
        // to follow the toggle that changed it.
        let mut d = wysiwyg_doc("marks_live", "one two\n");
        d.anchor = Some(0);
        d.caret = 3;
        assert!(d.active_inline_marks().is_empty(), "plain to start");
        d.toggle(InlineKind::Strong);
        assert_eq!(d.source, "**one** two\n");
        // `toggle` leaves the bolded text selected, so the button it lit stays lit.
        assert!(d.active_inline_marks().contains(InlineKind::Strong));
        d.toggle(InlineKind::Strong);
        assert!(d.active_inline_marks().is_empty(), "and off again");
    }

    #[test]
    fn a_link_is_not_an_inline_mark() {
        // `link`/`str` are inline nodes, but nothing on the inline toolbar
        // toggles them — a set with a "link mark" in it would have no button.
        for (view, tag) in VIEWS {
            assert_eq!(
                marks(view, &format!("marks_link_{tag}"), "a [te|xt](u) b"),
                [],
                "{tag}"
            );
        }
    }

    // ── blank documents ───────────────────────────────────────────────────────

    #[test]
    fn a_blank_document_is_untitled_empty_and_markdown() {
        let mut d = Doc::blank().unwrap();
        assert!(d.is_untitled());
        assert_eq!(d.path, PathBuf::new());
        assert_eq!(
            d.file_name(),
            "untitled",
            "the header has to show something"
        );
        assert_eq!(d.format_name(), "markdown");
        assert_eq!(d.source, "");
        assert!(!d.dirty, "nothing typed yet is nothing to lose");
        assert_eq!(d.disk_state(), DiskState::Untitled);
        // And it's a document you can be in: the default view renders it.
        d.build_visual(80);
        assert_eq!(d.caret, 0);
    }

    #[test]
    fn saving_an_untitled_document_asks_for_a_name_instead_of_writing() {
        let mut d = Doc::blank().unwrap();
        d.insert("hello");
        assert!(d.dirty);
        d.save();
        assert_eq!(d.status.as_deref(), Some("untitled — save as…"));
        assert!(d.dirty, "it must not come away believing it saved");
        assert!(d.is_untitled(), "and it still has no file");
    }

    #[test]
    fn a_blank_document_becomes_a_real_one_at_the_first_save_as() {
        let p = temp_path("blank_save_as");
        let mut d = Doc::blank().unwrap();
        // Plain text — a blank doc opens in Hidden mode, where a typed `#` would
        // be kept literal (`\#`); this test is about save-as, not escaping (which
        // has its own test), so it types nothing that escaping would touch.
        d.insert("hi");
        d.save_as(p.clone());
        assert_eq!(std::fs::read_to_string(&p).unwrap(), "hi");
        assert!(!d.is_untitled());
        assert!(!d.dirty);
        assert_eq!(d.file_name(), p.file_name().unwrap().to_string_lossy());
        assert_eq!(
            d.disk_state(),
            DiskState::Unchanged,
            "the watermark is stamped"
        );
        // And ⌘S is a plain save from here on.
        d.insert("!");
        d.save();
        assert_eq!(std::fs::read_to_string(&p).unwrap(), "hi!");
        let _ = std::fs::remove_file(&p);
    }

    // ── a file that isn't there yet ───────────────────────────────────────────

    /// A unique path in the temp dir with the given extension, guaranteed not to
    /// exist — what `leaf notes.md` is handed when the file has never been made.
    fn missing_path(name: &str, ext: &str) -> PathBuf {
        static SEQ: std::sync::atomic::AtomicUsize = std::sync::atomic::AtomicUsize::new(0);
        let seq = SEQ.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
        let mut p = std::env::temp_dir();
        p.push(format!("leaf_test_new_{name}_{seq}.{ext}"));
        let _ = std::fs::remove_file(&p);
        p
    }

    #[test]
    fn a_file_that_doesnt_exist_opens_as_an_empty_named_document() {
        let p = missing_path("named", "md");
        let mut d = Doc::open_or_create(p.clone()).unwrap();

        assert_eq!(d.source, "", "nothing was read, so there's nothing in it");
        assert!(!d.dirty, "an untouched new buffer has nothing to lose");
        assert!(
            !d.is_untitled(),
            "it has the name the user asked for — ^S must not detour to Save As"
        );
        assert_eq!(d.file_name(), p.file_name().unwrap().to_str().unwrap());
        assert!(d.path.is_absolute(), "the same absolute path `open` stores");
        assert!(!p.exists(), "and opening it wrote nothing");
        // And it's a document you can be in.
        d.build_visual(80);
        assert_eq!(d.caret, 0);
    }

    #[test]
    fn a_new_file_is_created_by_its_first_save() {
        let p = missing_path("first_save", "md");
        let mut d = Doc::open_or_create(p.clone()).unwrap();
        d.insert("hello\n");
        assert!(d.dirty);
        d.save();

        assert_eq!(
            std::fs::read_to_string(&p).unwrap(),
            "hello\n",
            "a plain ^S wrote it — no Save As, no name to invent"
        );
        assert!(!d.dirty);
        assert_eq!(d.disk_state(), DiskState::Unchanged);
        let _ = std::fs::remove_file(&p);
    }

    #[test]
    fn a_new_file_takes_its_format_from_the_extension() {
        // The one thing `blank` can't do: with no name it has to assume Markdown,
        // and typing djot into a Markdown parse is the wrong buffer.
        let dj = missing_path("format", "dj");
        assert_eq!(Doc::open_or_create(dj).unwrap().format_name(), "djot");
        let md = missing_path("format", "md");
        assert_eq!(Doc::open_or_create(md).unwrap().format_name(), "markdown");
    }

    #[test]
    fn a_new_file_reports_itself_missing_until_it_is_saved() {
        // Not `Untitled` — that's the answer for a document with no path, and it
        // would tell a frontend there is nothing a save could collide with. Here
        // there is a path, and the file simply isn't at it yet.
        let p = missing_path("disk_state", "md");
        let mut d = Doc::open_or_create(p.clone()).unwrap();
        assert_eq!(d.disk_state(), DiskState::Missing);

        // Somebody else creates it while the buffer is open: that's an overwrite
        // the frontend has to be able to prompt about, exactly as for an opened
        // file. Their bytes, not ours, so `Changed`.
        std::fs::write(&p, "theirs\n").unwrap();
        assert_eq!(d.disk_state(), DiskState::Changed);

        // Saving makes the file ours and re-stamps the watermark.
        d.insert("ours\n");
        d.save();
        assert_eq!(d.disk_state(), DiskState::Unchanged);
        assert_eq!(std::fs::read_to_string(&p).unwrap(), "ours\n");
        let _ = std::fs::remove_file(&p);
    }

    #[test]
    fn open_or_create_still_opens_a_file_that_is_there() {
        let d = doc_with("open_or_create_existing", "body\n");
        let reopened = Doc::open_or_create(d.path.clone()).unwrap();
        assert_eq!(reopened.source, "body\n");
        assert_eq!(reopened.disk_state(), DiskState::Unchanged);
    }

    #[test]
    fn a_missing_file_with_no_readable_extension_is_still_an_error() {
        // A mistyped flag or a stray argument must not become a buffer promising
        // to save somewhere — the same refusal `open` gives a real file.
        let mut p = std::env::temp_dir();
        p.push("leaf_test_new_bad_ext.wat");
        assert!(Doc::open_or_create(p).is_err());
        let mut none = std::env::temp_dir();
        none.push("leaf_test_new_no_ext");
        assert!(Doc::open_or_create(none).is_err());
    }

    #[test]
    fn a_new_file_in_a_directory_that_doesnt_exist_opens_but_wont_save() {
        // Opening reads nothing, so there is nothing to fail on yet; the write is
        // where it fails, and it says so rather than claiming a save.
        let p = std::env::temp_dir().join("leaf_test_no_such_dir_c41/doc.md");
        let mut d = Doc::open_or_create(p).unwrap();
        d.insert("x");
        d.save();
        assert!(
            d.status.as_deref().unwrap().starts_with("save failed:"),
            "got {:?}",
            d.status
        );
        assert!(d.dirty, "it must not come away believing it saved");
    }

    // ── save as ───────────────────────────────────────────────────────────────

    /// A unique path in the temp dir that no fixture wrote — a Save As target.
    fn temp_path(name: &str) -> PathBuf {
        static SEQ: std::sync::atomic::AtomicUsize = std::sync::atomic::AtomicUsize::new(0);
        let seq = SEQ.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
        let mut p = std::env::temp_dir();
        p.push(format!("leaf_test_target_{name}_{seq}.md"));
        let _ = std::fs::remove_file(&p);
        p
    }

    #[test]
    fn save_as_moves_the_document_and_leaves_the_old_file_alone() {
        let mut d = doc_with("save_as_move", "original\n");
        let old = d.path.clone();
        let new = temp_path("save_as_move");
        d.insert("edited: ");
        d.save_as(new.clone());

        assert_eq!(std::fs::read_to_string(&new).unwrap(), "edited: original\n");
        assert_eq!(
            std::fs::read_to_string(&old).unwrap(),
            "original\n",
            "Save As doesn't touch the file it came from"
        );
        assert_eq!(d.path, new, "the document moved");
        assert!(!d.dirty);
        assert_eq!(
            d.status.as_deref(),
            Some(&*format!("saved {}", d.file_name()))
        );

        // Every later save follows it, which is the whole difference from a copy.
        d.caret = 0;
        d.insert("re-");
        d.save();
        assert_eq!(
            std::fs::read_to_string(&new).unwrap(),
            "re-edited: original\n"
        );
        assert_eq!(std::fs::read_to_string(&old).unwrap(), "original\n");
        let _ = std::fs::remove_file(&new);
    }

    #[test]
    fn save_as_overwrites_an_existing_target() {
        // The picker already asked; asking again down here is the same question
        // twice, and the second one has no way to be answered.
        let new = temp_path("save_as_over");
        std::fs::write(&new, "theirs\n").unwrap();
        let mut d = doc_with("save_as_over", "ours\n");
        d.save_as(new.clone());
        assert_eq!(std::fs::read_to_string(&new).unwrap(), "ours\n");
        let _ = std::fs::remove_file(&new);
    }

    #[test]
    fn a_save_as_that_fails_leaves_the_document_where_it_was() {
        let mut d = doc_with("save_as_fail", "body\n");
        let old = d.path.clone();
        d.insert("x");
        // A directory that doesn't exist: the write can't land.
        let bad = std::env::temp_dir().join("leaf_test_no_such_dir_9f2/doc.md");
        d.save_as(bad);

        assert_eq!(
            d.path, old,
            "the document must not move to a file that isn't there"
        );
        assert!(d.dirty, "and must not believe it saved");
        assert!(
            d.status.as_deref().unwrap().starts_with("save failed:"),
            "the same failure a plain save reports, got {:?}",
            d.status
        );
        // The original is still the document's file, and still saveable.
        d.save();
        assert_eq!(std::fs::read_to_string(&old).unwrap(), "xbody\n");
        assert!(!d.dirty);
    }

    #[test]
    fn save_as_renames_without_reparsing_the_format() {
        // `.dj` on the name doesn't make the buffer djot: it was parsed as
        // Markdown and still is, and saying otherwise would be a conversion the
        // user never asked for (and an undo history thrown away to do it).
        let mut d = doc_with("save_as_format", "**b**\n");
        let mut new = temp_path("save_as_format");
        new.set_extension("dj");
        d.save_as(new.clone());
        assert_eq!(d.format_name(), "markdown");
        let _ = std::fs::remove_file(&new);
    }

    // ── external change / reload ──────────────────────────────────────────────

    #[test]
    fn an_untouched_file_reports_unchanged() {
        let mut d = doc_with("disk_clean", "body\n");
        assert_eq!(d.disk_state(), DiskState::Unchanged);
        // Editing the buffer is not editing the file.
        d.insert("x");
        assert_eq!(d.disk_state(), DiskState::Unchanged);
        assert!(d.dirty);
        // Saving re-stamps the watermark rather than reporting our own bytes back.
        d.save();
        assert_eq!(d.disk_state(), DiskState::Unchanged);
    }

    #[test]
    fn a_file_written_underneath_reports_changed() {
        let mut d = doc_with("disk_changed", "body\n");
        std::fs::write(&d.path, "someone else\n").unwrap();
        assert_eq!(d.disk_state(), DiskState::Changed);
        // Dirty *and* changed is the clobber: both halves are readable, and
        // leaf-core takes neither side.
        d.insert("x");
        assert!(d.dirty && d.disk_state() == DiskState::Changed);
        // Saving anyway is allowed — the frontend asked, or chose not to.
        d.save();
        assert_eq!(std::fs::read_to_string(&d.path).unwrap(), "xbody\n");
        assert_eq!(d.disk_state(), DiskState::Unchanged);
    }

    #[test]
    fn a_file_rewritten_with_the_same_bytes_is_unchanged() {
        // The hash is what makes this honest: the file was written (a fresh
        // mtime), and nothing about the document is stale.
        let d = doc_with("disk_same_bytes", "body\n");
        std::fs::write(&d.path, "body\n").unwrap();
        assert_eq!(d.disk_state(), DiskState::Unchanged);
    }

    #[test]
    fn a_deleted_file_reports_missing() {
        let mut d = doc_with("disk_missing", "body\n");
        std::fs::remove_file(&d.path).unwrap();
        assert_eq!(d.disk_state(), DiskState::Missing);
        // A save recreates it, and the document is whole again.
        d.save();
        assert_eq!(d.disk_state(), DiskState::Unchanged);
        assert_eq!(std::fs::read_to_string(&d.path).unwrap(), "body\n");
    }

    #[test]
    fn reload_replaces_the_document_with_the_file() {
        for (view, tag) in VIEWS {
            let mut d = doc_in(view, &format!("reload_{tag}"), "one\n\ntwo\n");
            d.insert("edited ");
            assert!(d.dirty);
            std::fs::write(&d.path, "one\n\ntwo\n\nthree\n").unwrap();
            d.reload();

            assert_eq!(d.source, "one\n\ntwo\n\nthree\n", "{tag}");
            assert!(!d.dirty, "{tag}: the file is what we have");
            assert_eq!(d.disk_state(), DiskState::Unchanged, "{tag}");
            assert_eq!(
                d.status.as_deref(),
                Some(&*format!("reloaded {}", d.file_name()))
            );
            // The reloaded tree is live, not the old parse.
            d.caret = d.source.find("three").unwrap();
            assert_eq!(d.breadcrumb(), "doc › para › str", "{tag}");
        }
    }

    #[test]
    fn reload_clamps_the_caret_and_drops_the_selection() {
        let mut d = doc_with("reload_caret", "a long first line\n");
        d.caret = 12;
        d.anchor = Some(4);
        std::fs::write(&d.path, "short\n").unwrap();
        d.reload();
        assert_eq!(d.caret, d.source.len(), "clamped into the shorter file");
        assert_eq!(
            d.anchor, None,
            "a selection over bytes that changed is a lie"
        );
        assert!(d.selection().is_none());

        // A caret the file still has room for stays put.
        let mut d = doc_with("reload_caret_keep", "one\n\ntwo\n");
        d.caret = 2;
        std::fs::write(&d.path, "one\n\ntwo\n\nthree\n").unwrap();
        d.reload();
        assert_eq!(d.caret, 2);
    }

    /// A silent reload is something that happened *to* a reader — a formatter,
    /// a `git checkout` — so it has to be undoable like anything else that
    /// changes the document, and undoable as one step rather than as however
    /// many the file happens to differ by.
    #[test]
    fn reload_is_one_undo_step_and_keeps_the_history_under_it() {
        let mut d = doc_with("reload_undo", "body\n");
        d.insert("x");
        assert_eq!(d.source, "xbody\n");
        std::fs::write(&d.path, "replaced\n").unwrap();
        d.reload();
        assert_eq!(d.source, "replaced\n");
        assert!(!d.dirty, "a reload lands clean");

        // One ^Z takes the whole swap off, and hands back the unsaved work it
        // replaced — which is unsaved again, because the file no longer says it.
        d.undo();
        assert_eq!(d.source, "xbody\n", "the reload comes off in one step");
        assert!(d.dirty, "and what it comes back to is unsaved");
        // …and the history under it is still there.
        d.undo();
        assert_eq!(
            d.source, "body\n",
            "the typing before the reload undoes too"
        );
        // Redo walks back up through the reload.
        d.redo();
        d.redo();
        assert_eq!(d.source, "replaced\n");
    }

    /// A file rewritten with the bytes it already had is not an edit, so it
    /// must not leave an undo step behind for something nobody did.
    #[test]
    fn reloading_identical_bytes_pushes_no_undo_step() {
        let mut d = doc_with("reload_same", "body\n");
        d.insert("x");
        std::fs::write(&d.path, "xbody\n").unwrap();
        d.reload();
        assert_eq!(d.source, "xbody\n");
        assert!(!d.dirty, "the file now says what the buffer does");
        d.undo();
        assert_eq!(
            d.source, "body\n",
            "one step back is the typing, not a no-op"
        );
    }

    #[test]
    fn a_reload_that_cant_read_leaves_the_document_alone() {
        let mut d = doc_with("reload_gone", "body\n");
        d.insert("x");
        std::fs::remove_file(&d.path).unwrap();
        d.reload();
        assert_eq!(d.source, "xbody\n", "the unsaved work is still here");
        assert!(d.dirty);
        assert!(
            d.status.as_deref().unwrap().starts_with("reload failed:"),
            "{:?}",
            d.status
        );

        // And an untitled document has nothing to reload from.
        let mut d = Doc::blank().unwrap();
        d.insert("typed");
        d.reload();
        assert_eq!(d.source, "typed");
        assert_eq!(d.status.as_deref(), Some("no file to reload"));
    }

    #[test]
    fn a_read_only_document_refuses_every_door() {
        let mut d = doc_with("readonly", "one two three\n");
        d.insert("x");
        assert!(d.dirty, "writable first, so the undo step exists");
        d.set_read_only(true);
        let before = d.source.clone();
        d.insert("y");
        d.backspace();
        d.undo();
        d.redo();
        assert_eq!(d.source, before, "no door moved a byte");
        d.set_read_only(false);
        d.undo();
        assert_ne!(d.source, before, "off again, the same doors work");
    }

    /// The doors that go to twig's own verbs rather than through the splice.
    /// Typed text in the rendered view under the default markup mode is the
    /// everyday one — it is what a keystroke in leaf-web or the Apple views
    /// becomes — and it walked straight past the gate.
    #[test]
    fn a_read_only_document_refuses_the_doors_around_the_splice() {
        let mut d = wysiwyg_doc(
            "readonly-doors",
            "one two three\n\n| a | b |\n|---|---|\n| c | d |\n",
        );
        d.set_markup_mode(MarkupMode::None);
        d.set_read_only(true);
        let before = d.source.clone();
        d.place_caret(3, false);
        d.insert("y");
        d.insert_link("https://example.com");
        d.insert_image("a.png", "alt");
        d.insert_thematic_break();
        d.insert_footnote();
        d.place_caret(0, false);
        d.place_caret(3, true);
        d.toggle(InlineKind::Strong);
        d.toggle_heading(2);
        d.set_block(BlockKind::Paragraph);
        d.toggle_list(false);
        d.toggle_blockquote();
        d.toggle_task_item();
        d.newline();
        d.indent();
        d.set_code_language("rust");
        let in_cell = d.source.find("| c").unwrap() + 2;
        d.place_caret(in_cell, false);
        assert!(d.caret_in_table(), "the caret is in the grid");
        assert!(!d.cell_line_break(), "the cell break reports the refusal");
        assert_eq!(d.source, before, "no door moved a byte");
        assert!(!d.dirty, "nothing to save");
        d.set_read_only(false);
        d.place_caret(3, false);
        d.insert("y");
        assert_ne!(d.source, before, "off again, the same doors work");
    }

    #[test]
    fn a_selection_quote_carries_its_context_on_char_boundaries() {
        let mut d = doc_with("quote", "before 你好 exact 世界 after\n");
        let start = d.source.find("exact").unwrap();
        d.place_caret(start, false);
        d.place_caret(start + "exact".len(), true);
        let q = d.selection_quote(3).unwrap();
        assert_eq!(q.exact, "exact");
        assert_eq!(
            q.prefix, "你好 ",
            "chars, not bytes — the multibyte pair counts as two"
        );
        assert_eq!(q.suffix, " 世界");
        assert_eq!(&d.source[q.start..q.end], "exact");
        // At the edges the context clips rather than erring.
        d.place_caret(0, false);
        d.place_caret(6, true);
        let q = d.selection_quote(40).unwrap();
        assert_eq!(q.prefix, "");
        assert_eq!(q.exact, "before");
        // No selection is no quote.
        d.place_caret(0, false);
        assert!(d.selection_quote(3).is_none());
    }

    #[test]
    fn highlights_are_kept_sorted_and_answer_point_queries() {
        let mut d = doc_with("hl", "one two three\n");
        d.set_highlights(vec![
            Highlight {
                start: 8,
                end: 13,
                id: "b".into(),
                color: None,
                marker: None,
            },
            Highlight {
                start: 0,
                end: 3,
                id: "a".into(),
                color: Some("#ffe066".into()),
                marker: None,
            },
            Highlight {
                start: 5,
                end: 5,
                id: "empty".into(),
                color: None,
                marker: None,
            },
        ]);
        assert_eq!(
            d.highlights()
                .iter()
                .map(|h| h.id.as_str())
                .collect::<Vec<_>>(),
            ["a", "b"],
            "sorted by start, the empty range dropped"
        );
        assert_eq!(d.highlight_at(1).map(|h| h.id.as_str()), Some("a"));
        assert_eq!(d.highlight_at(3), None, "end is exclusive");
        assert_eq!(d.highlight_at(8).map(|h| h.id.as_str()), Some("b"));
        d.set_highlights(Vec::new());
        assert!(d.highlights().is_empty(), "a replace is a replace");
    }

    /// `Highlight::covering` and the cursor over it are what both painters ask
    /// per glyph, so they have to answer the same as the scan they replaced —
    /// including in the gaps, which is where most glyphs are.
    #[test]
    fn covering_answers_from_a_sorted_list_without_scanning_all_of_it() {
        let hl = |start: usize, end: usize, id: &str| Highlight {
            start,
            end,
            id: id.into(),
            color: None,
            marker: None,
        };
        // Disjoint, as search hits are: in a range, in a gap, and past the end.
        let hits: Vec<Highlight> = (0..20).map(|i| hl(i * 10, i * 10 + 3, "hit")).collect();
        assert_eq!(Highlight::covering(&hits, 0).map(|h| h.start), Some(0));
        assert_eq!(Highlight::covering(&hits, 102).map(|h| h.start), Some(100));
        assert_eq!(
            Highlight::covering(&hits, 105),
            None,
            "a gap covers nothing"
        );
        assert_eq!(Highlight::covering(&hits, 103), None, "end is exclusive");
        assert_eq!(Highlight::covering(&hits, 9_999), None);
        assert_eq!(Highlight::covering(&[], 0), None);

        // Nested: first by start, so a hit inside an annotation still resolves
        // to the annotation — and the range that stops short doesn't mask it.
        let nested = vec![hl(0, 20, "outer"), hl(5, 10, "inner")];
        assert_eq!(
            Highlight::covering(&nested, 7).map(|h| h.id.as_str()),
            Some("outer")
        );
        assert_eq!(
            Highlight::covering(&nested, 15).map(|h| h.id.as_str()),
            Some("outer")
        );
    }

    /// The cursor is an optimisation, so the only thing worth asserting is that
    /// it is not also a change of answer — at every offset, over a list with a
    /// nest in it, walked forwards and then backwards.
    #[test]
    fn the_highlight_cursor_answers_exactly_what_a_fresh_scan_would() {
        let hl = |start: usize, end: usize, id: &str| Highlight {
            start,
            end,
            id: id.into(),
            color: None,
            marker: None,
        };
        let mut list = vec![
            hl(0, 20, "outer"),
            hl(5, 10, "inner"),
            hl(30, 33, "hit"),
            hl(40, 43, "hit"),
        ];
        list.sort_by_key(|h| (h.start, h.end));

        let mut cursor = HighlightCursor::new(&list);
        for offset in 0..50 {
            assert_eq!(
                cursor.at(offset).map(|h| h.id.as_str()),
                Highlight::covering(&list, offset).map(|h| h.id.as_str()),
                "cursor disagrees at {offset}"
            );
        }
        // Backwards: the cursor re-seats rather than answering from where it
        // had got to, so a painter that revisits a row is still told the truth.
        for offset in (0..50).rev() {
            assert_eq!(
                cursor.at(offset).map(|h| h.id.as_str()),
                Highlight::covering(&list, offset).map(|h| h.id.as_str()),
                "cursor disagrees walking back at {offset}"
            );
        }
    }

    // ── the presentation vocabulary ─────────────────────────────────────────

    /// A document in `format`, for the gesture tests that want more than the
    /// Markdown `doc_with` writes.
    fn fmt_doc(body: &str, format: Format) -> Doc {
        Doc::from_source(body.to_string(), format).unwrap()
    }

    /// Alignment is a block property, so the gesture is `set_block_attrs` on
    /// the caret's block whatever is selected — and each format spells it its
    /// own way: djot's `{…}` line above the block, a `<div>` around it in
    /// Markdown (the format has nowhere else to put it), the tag in HTML.
    #[test]
    fn set_alignment_spells_the_class_the_format_s_own_way() {
        let mut dj = fmt_doc("hello\n", Format::Djot);
        dj.caret = 1;
        dj.set_alignment(Some(Align::Center));
        assert_eq!(dj.source, "{.center}\nhello\n");
        assert!(dj.dirty);
        assert_eq!(dj.status, None);

        let mut md = fmt_doc("hello\n", Format::Markdown);
        md.caret = 1;
        md.set_alignment(Some(Align::Right));
        assert_eq!(md.source, "<div class=\"right\">\n\nhello\n\n</div>\n");

        let mut html = fmt_doc("<p>hello</p>\n", Format::Html);
        html.caret = html.source.find("hello").unwrap();
        html.set_alignment(Some(Align::Justify));
        assert_eq!(html.source, "<p class=\"justify\">hello</p>\n");
    }

    /// Each gesture edits **one key and keeps the rest** — twig's contract is
    /// replace-not-merge, so leaf reads the node's attributes, edits its own
    /// key out of them, and passes the list back whole. A document from
    /// elsewhere passes through the editor unharmed.
    #[test]
    fn a_presentation_gesture_keeps_every_attribute_it_did_not_write() {
        let mut d = fmt_doc(
            "{.lead .center #intro data-line-height=\"1.5\"}\nhello\n",
            Format::Djot,
        );
        d.caret = d.source.find("hello").unwrap();
        d.set_alignment(Some(Align::Right));
        // `center` goes, `lead` stays, and neither the id nor the spacing is
        // touched.
        // The serializer picks the order; what matters is which keys survive.
        assert!(d.source.contains(".lead"), "{:?}", d.source);
        assert!(d.source.contains(".right"), "{:?}", d.source);
        assert!(!d.source.contains(".center"), "{:?}", d.source);
        assert!(d.source.contains("#intro"), "{:?}", d.source);
        assert!(
            d.source.contains("data-line-height=\"1.5\""),
            "{:?}",
            d.source
        );
        assert_eq!(d.alignment_at_caret(), Some(Align::Right));
        assert_eq!(
            d.line_spacing_at_caret(),
            Some(LineHeight::Step(LineSpacing::OneHalf))
        );

        // And the other way round: the spacing gesture leaves the classes be.
        d.set_line_spacing(Some(LineHeight::Step(LineSpacing::Double)));
        assert!(d.source.contains(".lead"), "{:?}", d.source);
        assert!(d.source.contains(".right"), "{:?}", d.source);
        assert_eq!(
            d.line_spacing_at_caret(),
            Some(LineHeight::Step(LineSpacing::Double))
        );
    }

    /// Clearing is the same gesture with `None`: the key goes, the tokens leaf
    /// owns go out of `class`, and a block left with nothing at all is spelled
    /// bare again — in Markdown by unwrapping the div twig wrapped it in.
    #[test]
    fn none_clears_a_key_and_an_empty_set_unwraps_the_block() {
        let mut dj = fmt_doc("{.lead .center}\nhello\n", Format::Djot);
        dj.caret = dj.source.find("hello").unwrap();
        dj.set_alignment(None);
        assert_eq!(dj.source, "{.lead}\nhello\n", "the foreign class stays");
        assert_eq!(dj.alignment_at_caret(), None);

        let mut bare = fmt_doc("{.center}\nhello\n", Format::Djot);
        bare.caret = bare.source.find("hello").unwrap();
        bare.set_alignment(None);
        assert_eq!(
            bare.source, "hello\n",
            "the last key takes the line with it"
        );

        let mut md = fmt_doc("hello\n", Format::Markdown);
        md.caret = 1;
        md.set_alignment(Some(Align::Center));
        assert_eq!(md.source, "<div class=\"center\">\n\nhello\n\n</div>\n");
        md.caret = md.source.find("hello").unwrap();
        md.set_line_spacing(Some(LineHeight::Step(LineSpacing::OneFifteen)));
        assert_eq!(
            md.source, "<div class=\"center\" data-line-height=\"1.15\">\n\nhello\n\n</div>\n",
            "the second key rewrites the div rather than nesting a second"
        );
        md.caret = md.source.find("hello").unwrap();
        md.set_alignment(None);
        md.caret = md.source.find("hello").unwrap();
        md.set_line_spacing(None);
        assert_eq!(md.source, "hello\n", "an empty set unwraps the div");
    }

    /// Size, face and colour are the run's over a selection and the block's
    /// with none — so "make this paragraph larger" is a click with the caret in
    /// it rather than a select-all first.
    #[test]
    fn a_run_gesture_wraps_a_selection_and_sets_the_block_without_one() {
        // With a selection: a span, in each format's own spelling.
        let mut dj = fmt_doc("a big b\n", Format::Djot);
        dj.anchor = Some(2);
        dj.caret = 5;
        dj.set_font_size(Some(FontSize::Step(SizeStep::Large)));
        assert_eq!(dj.source, "a [big]{data-size=\"large\"} b\n");
        assert_eq!(
            dj.font_size_at_caret(),
            Some(FontSize::Step(SizeStep::Large))
        );

        let mut md = fmt_doc("a big b\n", Format::Markdown);
        md.anchor = Some(2);
        md.caret = 5;
        md.set_text_color(Some(TextColor::Named(MarkColor::Blue)));
        assert_eq!(md.source, "a <span data-color=\"blue\">big</span> b\n");
        assert_eq!(
            md.text_color_at_caret(),
            Some(TextColor::Named(MarkColor::Blue))
        );

        // Without one: the caret's block, through the block gesture.
        let mut block = fmt_doc("a big b\n", Format::Djot);
        block.caret = 3;
        block.set_font_family(Some(FontFace::Generic(FontFamily::Monospace)));
        assert_eq!(block.source, "{data-font=\"monospace\"}\na big b\n");
        assert_eq!(
            block.font_family_at_caret(),
            Some(FontFace::Generic(FontFamily::Monospace))
        );
    }

    /// The *Other…* row of each of the four menus: a value goes into the
    /// document in its canonical spelling and comes back out of the query as
    /// the same value. One round trip per property, because the four go out
    /// through different doors — two block gestures, and the run three through
    /// the span that `wrap_range_attrs` mints.
    #[test]
    fn an_exact_value_round_trips_through_the_gesture_and_the_query() {
        // Size: the run three, over a selection.
        let mut d = fmt_doc("a big b\n", Format::Djot);
        d.anchor = Some(2);
        d.caret = 5;
        d.set_font_size(FontSize::points(14.0));
        assert_eq!(d.source, "a [big]{data-size=\"14pt\"} b\n");
        assert_eq!(d.font_size_at_caret(), FontSize::points(14.0));

        // Colour, onto the same span — the gesture keeps the size it finds.
        d.set_text_color(Some(TextColor::Rgb {
            r: 0xc0,
            g: 0x30,
            b: 0x30,
        }));
        assert_eq!(
            d.source,
            "a [big]{data-size=\"14pt\" data-color=\"#c03030\"} b\n"
        );
        assert_eq!(
            d.text_color_at_caret(),
            Some(TextColor::Rgb {
                r: 0xc0,
                g: 0x30,
                b: 0x30
            })
        );

        // Face: a family name, as given.
        d.set_font_family(Some(FontFace::Named("Garamond".into())));
        assert!(
            d.source.contains("data-font=\"Garamond\""),
            "{:?}",
            d.source
        );
        assert_eq!(
            d.font_family_at_caret(),
            Some(FontFace::Named("Garamond".into()))
        );

        // Line spacing: a block gesture, and an exact ratio.
        let mut block = fmt_doc("hello\n", Format::Djot);
        block.caret = 1;
        block.set_line_spacing(LineHeight::ratio(1.3));
        assert_eq!(block.source, "{data-line-height=\"1.3\"}\nhello\n");
        assert_eq!(block.line_spacing_at_caret(), LineHeight::ratio(1.3));

        // And a value spelled long is written back short, so the same press
        // twice writes the same bytes: `14.0pt` in, `14pt` out.
        let mut long = fmt_doc("{data-size=\"14.0pt\"}\nhello\n", Format::Djot);
        long.caret = long.source.find("hello").unwrap();
        assert_eq!(long.font_size_at_caret(), FontSize::points(14.0));
        let in_force = long.font_size_at_caret();
        long.set_font_size(in_force);
        assert_eq!(long.source, "{data-size=\"14pt\"}\nhello\n");
    }

    /// A value the grammar does not cover is what it was before the vocabulary
    /// opened: carried untouched by the document, answered `None` by the query
    /// so the menu ticks *Default*, and rewritten only by a gesture on its own
    /// key. leaf is not going to grow a CSS parser to guess at `1.3em`.
    #[test]
    fn a_value_outside_the_grammar_is_carried_and_the_menu_ticks_the_default() {
        let src =
            "{data-size=\"huge\" data-color=\"rgb(1,2,3)\" data-line-height=\"1.3em\"}\nhello\n";
        let mut d = fmt_doc(src, Format::Djot);
        d.caret = d.source.find("hello").unwrap();
        assert_eq!(d.font_size_at_caret(), None);
        assert_eq!(d.text_color_at_caret(), None);
        assert_eq!(d.line_spacing_at_caret(), None);

        // The keys are still there, untouched, after a gesture on a *different*
        // key — "edit one key and keep the rest" holds for a value it cannot
        // read as readily as for one it can.
        d.set_alignment(Some(Align::Center));
        assert!(d.source.contains("data-size=\"huge\""), "{:?}", d.source);
        assert!(
            d.source.contains("data-color=\"rgb(1,2,3)\""),
            "{:?}",
            d.source
        );
        assert!(
            d.source.contains("data-line-height=\"1.3em\""),
            "{:?}",
            d.source
        );
        // And the gesture on its *own* key replaces it, which is the one way a
        // carried value ever changes.
        d.caret = d.source.find("hello").unwrap();
        d.set_font_size(FontSize::points(12.0));
        assert!(d.source.contains("data-size=\"12pt\""), "{:?}", d.source);
        assert!(!d.source.contains("huge"), "{:?}", d.source);
    }

    /// The nearest node wins whichever *form* either node wrote: a value inside
    /// a name, a name inside a value. The fold has one rule and does not learn
    /// a second one for exact values.
    #[test]
    fn the_nearest_node_wins_whether_it_named_a_size_or_measured_one() {
        // A value inside a name: the block says `small`, the span says `14pt`.
        let mut d = fmt_doc(
            "{data-size=\"small\"}\nx [y]{data-size=\"14pt\"} z\n",
            Format::Djot,
        );
        d.caret = d.source.find('y').unwrap();
        assert_eq!(d.font_size_at_caret(), FontSize::points(14.0));
        d.caret = d.source.find('x').unwrap();
        assert_eq!(
            d.font_size_at_caret(),
            Some(FontSize::Step(SizeStep::Small))
        );

        // And a name inside a value, which is the same rule read the other way.
        let mut e = fmt_doc(
            "{data-size=\"14pt\" data-color=\"#c03030\"}\nx [y]{data-size=\"small\"} z\n",
            Format::Djot,
        );
        e.caret = e.source.find('y').unwrap();
        assert_eq!(
            e.font_size_at_caret(),
            Some(FontSize::Step(SizeStep::Small))
        );
        assert_eq!(
            e.text_color_at_caret(),
            Some(TextColor::Rgb {
                r: 0xc0,
                g: 0x30,
                b: 0x30
            }),
            "the block's colour still reaches the span"
        );
        e.caret = e.source.find('x').unwrap();
        assert_eq!(e.font_size_at_caret(), FontSize::points(14.0));
    }

    /// twig re-styles the span a range already lies in rather than nesting a
    /// second, and an empty set unwraps it — so a second press of the menu
    /// fixes the size instead of building `[[big]{.a}]{.b}`, and the entry that
    /// means "the theme's own" takes the span away.
    #[test]
    fn a_second_run_gesture_re_styles_the_span_and_none_unwraps_it() {
        let mut d = fmt_doc("a big b\n", Format::Djot);
        d.anchor = Some(2);
        d.caret = 5;
        d.set_font_size(Some(FontSize::Step(SizeStep::Large)));
        assert_eq!(d.source, "a [big]{data-size=\"large\"} b\n");

        // The selection `wrap_range_attrs` left behind covers the whole span;
        // colouring it now keeps the size, because the gesture reads the span's
        // attributes before it edits its own key.
        d.set_text_color(Some(TextColor::Named(MarkColor::Red)));
        assert_eq!(
            d.source, "a [big]{data-size=\"large\" data-color=\"red\"} b\n",
            "one span, both keys"
        );
        assert_eq!(
            d.font_size_at_caret(),
            Some(FontSize::Step(SizeStep::Large))
        );
        assert_eq!(
            d.text_color_at_caret(),
            Some(TextColor::Named(MarkColor::Red))
        );

        d.set_text_color(None);
        assert_eq!(d.source, "a [big]{data-size=\"large\"} b\n");
        d.set_font_size(None);
        assert_eq!(d.source, "a big b\n", "the last key unwraps the span");
        assert_eq!(d.font_size_at_caret(), None);
    }

    /// The queries read the nearest node that names the property: the span the
    /// caret is in, then its block, then the `div`s around it.
    #[test]
    fn a_presentation_query_reads_the_nearest_node_that_names_it() {
        let mut d = fmt_doc(
            "{.center data-size=\"small\" data-font=\"serif\"}\nx [y]{data-size=\"xx-large\"} z\n",
            Format::Djot,
        );
        // In the span: its own size, the block's face and alignment.
        d.caret = d.source.find('y').unwrap();
        assert_eq!(
            d.font_size_at_caret(),
            Some(FontSize::Step(SizeStep::XxLarge))
        );
        assert_eq!(
            d.font_family_at_caret(),
            Some(FontFace::Generic(FontFamily::Serif))
        );
        assert_eq!(d.alignment_at_caret(), Some(Align::Center));
        assert_eq!(d.line_spacing_at_caret(), None);
        assert_eq!(d.text_color_at_caret(), None);

        // Outside it: the block's size.
        d.caret = d.source.find('x').unwrap();
        assert_eq!(
            d.font_size_at_caret(),
            Some(FontSize::Step(SizeStep::Small))
        );

        // And through a Markdown div, which is where a Markdown block's
        // attributes live.
        let mut md = fmt_doc(
            "<div class=\"center\" data-size=\"large\">\n\nhello\n\n</div>\n",
            Format::Markdown,
        );
        md.caret = md.source.find("hello").unwrap();
        assert_eq!(md.alignment_at_caret(), Some(Align::Center));
        assert_eq!(
            md.font_size_at_caret(),
            Some(FontSize::Step(SizeStep::Large))
        );

        // A document that names none of it answers `None` everywhere, which is
        // "the theme's own" and what every toolbar draws unlit.
        let mut plain = doc_with("plain_presentation", "hello\n");
        plain.caret = 1;
        assert_eq!(plain.alignment_at_caret(), None);
        assert_eq!(plain.line_spacing_at_caret(), None);
        assert_eq!(plain.font_size_at_caret(), None);
        assert_eq!(plain.font_family_at_caret(), None);
        assert_eq!(plain.text_color_at_caret(), None);
    }

    /// A djot fenced div is anonymous the way an attributed span is, and is a
    /// block all the same — the *form* is the whole of what tells them apart.
    /// Read as a span it poisoned both halves: the run gesture copied the div's
    /// entire attribute set onto the span it minted, duplicating the `id`, and
    /// the run and block queries answered off a node the walker draws nothing
    /// for.
    #[test]
    fn a_djot_fenced_div_is_not_an_attributed_span() {
        let src = "{.center data-size=\"small\" #box}\n:::\nhello world\n:::\n";
        let mut d = fmt_doc(src, Format::Djot);
        let at = d.source.find("world").unwrap();
        d.anchor = Some(at);
        d.caret = at + "world".len();
        d.set_text_color(Some(TextColor::Named(MarkColor::Red)));
        assert_eq!(
            d.source,
            "{.center data-size=\"small\" #box}\n:::\nhello [world]{data-color=\"red\"}\n:::\n",
            "the span carries its own key and nothing of the div's"
        );

        // And the queries stop at the block: a djot div is not a `<div>`, the
        // walker lends its keys to nothing inside it, and a query that said
        // otherwise would tick a menu entry no glyph on screen obeys.
        assert_eq!(
            d.text_color_at_caret(),
            Some(TextColor::Named(MarkColor::Red))
        );
        assert_eq!(d.font_size_at_caret(), None);
        assert_eq!(d.alignment_at_caret(), None);
    }

    /// Clearing a property the block does not name and a `div` around it does
    /// would write nothing and change nothing — twig's `set_block_attrs`
    /// reaches one node, and the div is not it. The gesture says so instead of
    /// leaving the author pressing an entry that never ticks.
    #[test]
    fn clearing_a_property_an_enclosing_div_names_says_so_and_writes_nothing() {
        // Markdown, two paragraphs in one div: not the sole-child shape twig
        // writes, so `block_attrs_at_caret` reads the paragraph and the
        // paragraph names none of it.
        let src = "<div class=\"center\" data-line-height=\"1.5\" data-size=\"large\">\n\nhello\n\nworld\n\n</div>\n";
        let mut md = fmt_doc(src, Format::Markdown);
        md.caret = md.source.find("hello").unwrap();
        assert_eq!(md.alignment_at_caret(), Some(Align::Center));

        md.set_alignment(None);
        assert_eq!(md.source, src, "nothing written");
        assert!(!md.dirty);
        assert_eq!(
            md.status.as_deref(),
            Some("alignment: set on the div around the block")
        );
        assert_eq!(md.alignment_at_caret(), Some(Align::Center));

        // The same for a `data-` key, at both levels — the block pair and the
        // run three, the run three at a bare caret being the block gesture.
        md.set_line_spacing(None);
        assert_eq!(md.source, src);
        assert_eq!(
            md.status.as_deref(),
            Some("line spacing: set on the div around the block")
        );
        md.set_font_size(None);
        assert_eq!(md.source, src);
        assert_eq!(
            md.status.as_deref(),
            Some("size: set on the div around the block")
        );

        // HTML has no sole-child fold at all: a block's attributes go on the
        // block, so the div around one is always out of reach.
        let html_src = "<div class=\"center\"><p>hi</p></div>\n";
        let mut html = fmt_doc(html_src, Format::Html);
        html.caret = html.source.find("hi").unwrap();
        assert_eq!(html.alignment_at_caret(), Some(Align::Center));
        html.set_alignment(None);
        assert_eq!(html.source, html_src);
        assert!(!html.dirty);
        assert_eq!(
            html.status.as_deref(),
            Some("alignment: set on the div around the block")
        );

        // And it is a refusal, not a rule against clearing: a block that names
        // the property itself still loses it, div or no div.
        let mut own = fmt_doc(
            "<div class=\"center\"><p class=\"right\">hi</p></div>\n",
            Format::Html,
        );
        own.caret = own.source.find("hi").unwrap();
        own.set_alignment(None);
        assert_eq!(own.source, "<div class=\"center\"><p>hi</p></div>\n");
        assert_eq!(own.status, None);
    }

    /// An edited key is rewritten **where it stands**. The proposal's worked
    /// example is the test: a paragraph that came in as `id="intro"
    /// class="lead center" data-line-height="1.5"` and is right-aligned goes
    /// out as the same list with one token changed. Removing the key and
    /// pushing it back shuffled a document's attributes on every press.
    #[test]
    fn an_edited_key_keeps_its_place_among_the_attributes() {
        let mut html = fmt_doc(
            "<p id=\"intro\" class=\"lead center\" data-line-height=\"1.5\">hello</p>\n",
            Format::Html,
        );
        html.caret = html.source.find("hello").unwrap();
        html.set_alignment(Some(Align::Right));
        assert_eq!(
            html.source,
            "<p id=\"intro\" class=\"lead right\" data-line-height=\"1.5\">hello</p>\n"
        );

        // A `data-` key the same way, and a key the block did not have still
        // goes on the end.
        html.caret = html.source.find("hello").unwrap();
        html.set_line_spacing(Some(LineHeight::Step(LineSpacing::Double)));
        assert_eq!(
            html.source,
            "<p id=\"intro\" class=\"lead right\" data-line-height=\"2\">hello</p>\n"
        );
        html.caret = html.source.find("hello").unwrap();
        html.set_font_size(Some(FontSize::Step(SizeStep::Large)));
        assert_eq!(
            html.source,
            "<p id=\"intro\" class=\"lead right\" data-line-height=\"2\" data-size=\"large\">hello</p>\n"
        );

        // Djot writes the same list in its own spelling, and the order is the
        // author's there too.
        let mut dj = fmt_doc(
            "{#intro .lead .center data-line-height=\"1.5\"}\nhello\n",
            Format::Djot,
        );
        dj.caret = dj.source.find("hello").unwrap();
        dj.set_alignment(Some(Align::Right));
        assert_eq!(
            dj.source,
            "{#intro .lead .right data-line-height=\"1.5\"}\nhello\n"
        );
    }

    /// A page break is a block, so twig alone lands one after the caret's whole
    /// block; the paragraph is parted at the caret first, exactly as
    /// `insert_thematic_break` parts it, and each format spells the directive
    /// its own way.
    #[test]
    fn insert_page_break_parts_the_paragraph_and_spells_the_directive() {
        let mut md = doc_with("page_break_md", "hello world\n");
        md.caret = 5;
        md.insert_page_break();
        assert_eq!(md.source, "hello\n\n::page-break\n\nworld\n");
        assert!(md.dirty);
        assert_eq!(md.status, None);

        let mut dj = fmt_doc("hello world\n", Format::Djot);
        dj.caret = 5;
        dj.insert_page_break();
        assert_eq!(dj.source, "hello\n\n::: page-break\n:::\n\nworld\n");

        // At a block's end there is no second half to mint, so the break simply
        // follows the block — the rule the rule button already has.
        let mut end = doc_with("page_break_end", "hello\n");
        end.caret = 5;
        end.insert_page_break();
        assert_eq!(end.source, "hello\n\n::page-break\n\n");
        assert_eq!(end.caret, end.source.len(), "on the line under the break");

        // And it reaches the map as the placeholder row a frontend paginates on.
        end.view = View::Wysiwyg;
        end.build_visual(80);
        assert_eq!(
            end.vmap
                .rows
                .iter()
                .find_map(|r| r.leaf_directive.as_ref())
                .map(|m| m.name.as_str()),
            Some(PAGE_BREAK)
        );

        // HTML and AsciiDoc spell it their own way, and draw it the same.
        for (fmt, src, spelled) in [
            (Format::Html, "<p>hello</p>\n", "<page-break></page-break>"),
            (Format::Asciidoc, "hello\n", "<<<"),
        ] {
            let mut d = fmt_doc(src, fmt);
            d.caret = d.source.find("hello").unwrap() + 5;
            d.insert_page_break();
            assert!(d.source.contains(spelled), "{fmt:?}: {:?}", d.source);
            d.view = View::Wysiwyg;
            d.build_visual(80);
            let marks = d
                .vmap
                .rows
                .iter()
                .filter_map(|r| r.leaf_directive.as_ref())
                .map(|m| m.name.as_str())
                .collect::<Vec<_>>();
            assert_eq!(marks, [PAGE_BREAK], "{fmt:?}");
        }
    }

    /// The vocabulary's capabilities, per format. The two block properties are
    /// `SetBlockAttrs` and the three run ones `WrapRangeAttrs`, which is why
    /// AsciiDoc can align a paragraph and not size a run: its `[#id.role]#text#`
    /// keeps an id and a role and has no slot for a `data-` key.
    #[test]
    fn the_presentation_capabilities_are_ragged_per_format() {
        for fmt in [Format::Markdown, Format::Djot, Format::Html] {
            let c = Capabilities::of(fmt);
            assert!(c.alignment, "{fmt:?} alignment");
            assert!(c.line_spacing, "{fmt:?} line spacing");
            assert!(c.font_size, "{fmt:?} size");
            assert!(c.font_family, "{fmt:?} face");
            assert!(c.text_color, "{fmt:?} colour");
        }
        // Markdown spells both only under the extensions leaf parses with — a
        // `<div>` and a `<span>` read back as containers under `html_elements`,
        // and `::page-break` as a directive under `directives`. Ask twig's own
        // defaults and the answer is no, which is why `Capabilities` is built
        // with `supports_with`.
        assert!(!Format::Markdown.supports(Gesture::SetBlockAttrs));
        assert!(!Format::Markdown.supports(Gesture::WrapRangeAttrs));
        assert!(!Format::Markdown.supports(Gesture::InsertDirective));

        let adoc = Capabilities::of(Format::Asciidoc);
        assert!(adoc.alignment && adoc.line_spacing, "AsciiDoc's `[…]` line");
        assert!(
            !adoc.font_size && !adoc.font_family && !adoc.text_color,
            "AsciiDoc has no inline spelling that keeps a data- key"
        );

        // XML spells none of it, and neither page break — nor has it blocks a
        // caret could name for a move.
        let xml = Capabilities::of(Format::Xml);
        assert!(!xml.alignment && !xml.font_size && !xml.page_break && !xml.move_block);
        for f in [Format::Markdown, Format::Djot, Format::Html] {
            assert!(Capabilities::of(f).move_block, "{f:?} moves blocks");
        }
        for f in [
            Format::Markdown,
            Format::Djot,
            Format::Html,
            Format::Asciidoc,
        ] {
            assert!(Capabilities::of(f).page_break, "{f:?} breaks pages");
        }
    }

    /// A format that cannot spell a property refuses in its own words and
    /// writes nothing — the guard every other gesture has.
    #[test]
    fn a_presentation_gesture_a_format_cannot_spell_is_refused_with_a_reason() {
        let src = "<doc><p>hello</p></doc>\n";
        #[allow(clippy::type_complexity)]
        let ops: [(&str, &dyn Fn(&mut Doc)); 6] = [
            ("alignment", &|d: &mut Doc| {
                d.set_alignment(Some(Align::Center))
            }),
            ("line spacing", &|d: &mut Doc| {
                d.set_line_spacing(Some(LineHeight::Step(LineSpacing::Double)))
            }),
            ("size", &|d: &mut Doc| {
                d.set_font_size(Some(FontSize::Step(SizeStep::Large)))
            }),
            ("face", &|d: &mut Doc| {
                d.set_font_family(Some(FontFace::Generic(FontFamily::Serif)))
            }),
            ("colour", &|d: &mut Doc| {
                d.set_text_color(Some(TextColor::Named(MarkColor::Red)))
            }),
            ("page break", &|d: &mut Doc| d.insert_page_break()),
        ];
        for (name, op) in ops {
            let mut d = fmt_doc(src, Format::Xml);
            let at = d.source.find("hello").unwrap();
            d.caret = at;
            d.anchor = Some(at + 5);
            op(&mut d);
            assert_eq!(d.source, src, "{name} edited an XML document");
            assert!(!d.dirty, "{name} marked the document dirty");
            let status = d.status.as_deref().unwrap_or("");
            assert!(
                status.contains("xml"),
                "{name}: the refusal should name the format, got {status:?}"
            );
        }

        // AsciiDoc is the ragged one: the block gesture works where the run
        // gesture does not, and a *selection* is what tells the two apart.
        let mut adoc = fmt_doc("hello world\n", Format::Asciidoc);
        adoc.anchor = Some(0);
        adoc.caret = 5;
        adoc.set_font_size(Some(FontSize::Step(SizeStep::Large)));
        assert_eq!(adoc.source, "hello world\n", "no inline spelling");
        assert!(adoc.status.is_some());
    }

    /// A read-only document takes none of it, and a caret on a blank line has
    /// no block to carry an attribute — both say so rather than writing.
    #[test]
    fn a_presentation_gesture_respects_read_only_and_a_blank_line() {
        let mut ro = fmt_doc("hello\n", Format::Djot);
        ro.read_only = true;
        ro.caret = 1;
        ro.set_alignment(Some(Align::Center));
        assert_eq!(ro.source, "hello\n");

        let mut blank = fmt_doc("a\n\n\nb\n", Format::Djot);
        blank.caret = 2; // the empty line between the two paragraphs
        blank.set_alignment(Some(Align::Center));
        assert_eq!(blank.source, "a\n\n\nb\n");
        assert!(
            blank.status.as_deref().unwrap_or("").contains("no block"),
            "got {:?}",
            blank.status
        );
    }

    /// A block attribute gesture keeps the caret on the **text** it was on, not
    /// on the byte offset it had. Markdown has nowhere to put a paragraph's
    /// attributes but a `<div>` around it, and twig splices the div and the
    /// block it wraps as one region — so a caret that kept its offset landed in
    /// the markup, and every press after the first answered "no block at the
    /// caret" with the toolbar's queries reading nothing.
    #[test]
    fn a_markdown_block_gesture_keeps_the_caret_on_its_text() {
        let word = |d: &Doc| d.caret - d.source.find("brown").unwrap();
        let mut md = fmt_doc("the quick brown fox\n", Format::Markdown);
        md.caret = md.source.find("brown").unwrap() + 2; // "br|own"

        // Wrapping: the div and two blank lines open above the block.
        md.set_alignment(Some(Align::Center));
        assert_eq!(
            md.source,
            "<div class=\"center\">\n\nthe quick brown fox\n\n</div>\n"
        );
        assert_eq!(word(&md), 2, "the caret left its word: {}", md.caret);
        assert_eq!(md.alignment_at_caret(), Some(Align::Center));

        // Re-styling: the attribute line changes length under the same caret,
        // and the second press reaches the same block rather than nothing.
        md.set_alignment(Some(Align::Right));
        assert_eq!(
            md.source, "<div class=\"right\">\n\nthe quick brown fox\n\n</div>\n",
            "a second press re-styles the div"
        );
        assert_eq!(md.status, None);
        assert_eq!(word(&md), 2);

        // A second key on the same div — the line grows, the caret rides it.
        md.set_line_spacing(Some(LineHeight::Step(LineSpacing::Double)));
        assert_eq!(
            md.source,
            "<div class=\"right\" data-line-height=\"2\">\n\nthe quick brown fox\n\n</div>\n"
        );
        assert_eq!(word(&md), 2);
        assert_eq!(
            md.line_spacing_at_caret(),
            Some(LineHeight::Step(LineSpacing::Double))
        );

        // Unwrapping: the line shrinks, and then the div goes altogether.
        md.set_alignment(None);
        assert_eq!(
            md.source,
            "<div data-line-height=\"2\">\n\nthe quick brown fox\n\n</div>\n"
        );
        assert_eq!(word(&md), 2);
        md.set_line_spacing(None);
        assert_eq!(md.source, "the quick brown fox\n", "the last key unwraps");
        assert_eq!(word(&md), 2, "the caret came back down with the block");
        assert_eq!(md.alignment_at_caret(), None);
        assert_eq!(md.status, None);
    }

    /// The same rule in djot, where the spelling is a `{…}` line *above* the
    /// block rather than a wrapper around it: inserting it pushes the block
    /// down, re-styling it changes the line's length, and clearing the last key
    /// takes the line away again. The caret rides all three.
    #[test]
    fn a_djot_attribute_line_keeps_the_caret_on_its_text() {
        let word = |d: &Doc| d.caret - d.source.find("brown").unwrap();
        let mut dj = fmt_doc("the quick brown fox\n", Format::Djot);
        dj.caret = dj.source.find("brown").unwrap() + 2;

        dj.set_alignment(Some(Align::Center));
        assert_eq!(dj.source, "{.center}\nthe quick brown fox\n");
        assert_eq!(word(&dj), 2);
        assert_eq!(dj.alignment_at_caret(), Some(Align::Center));

        dj.set_line_spacing(Some(LineHeight::Step(LineSpacing::Double)));
        assert_eq!(
            dj.source, "{.center data-line-height=\"2\"}\nthe quick brown fox\n",
            "a second press edits the line the first wrote"
        );
        assert_eq!(word(&dj), 2);

        dj.set_alignment(None);
        assert_eq!(dj.source, "{data-line-height=\"2\"}\nthe quick brown fox\n");
        assert_eq!(word(&dj), 2);

        dj.set_line_spacing(None);
        assert_eq!(dj.source, "the quick brown fox\n");
        assert_eq!(word(&dj), 2);
        assert_eq!(dj.status, None);
    }

    /// The run gestures with no selection are the block gesture, so they keep
    /// the caret the same way — and a heading keeps it inside the heading's own
    /// text, past the `# ` its content span starts after. A selection rides
    /// along whole: a block gesture is not a run gesture, and what was selected
    /// before the press is still selected after it.
    #[test]
    fn a_block_gesture_carries_a_selection_and_a_heading_caret_too() {
        // No selection: the run gesture goes through the block door.
        let mut md = fmt_doc("the quick brown fox\n", Format::Markdown);
        md.caret = md.source.find("brown").unwrap() + 2;
        md.set_font_size(Some(FontSize::Step(SizeStep::Large)));
        assert_eq!(
            md.source,
            "<div data-size=\"large\">\n\nthe quick brown fox\n\n</div>\n"
        );
        assert_eq!(md.caret - md.source.find("brown").unwrap(), 2);
        assert_eq!(
            md.font_size_at_caret(),
            Some(FontSize::Step(SizeStep::Large))
        );
        md.set_font_size(Some(FontSize::Step(SizeStep::Small)));
        assert_eq!(
            md.font_size_at_caret(),
            Some(FontSize::Step(SizeStep::Small)),
            "the second press reached the same block"
        );

        // A selection: alignment is the block's whatever is selected, and the
        // words stay selected.
        let mut sel = fmt_doc("the quick brown fox\n", Format::Markdown);
        let at = sel.source.find("brown").unwrap();
        sel.anchor = Some(at);
        sel.caret = at + 5;
        sel.set_alignment(Some(Align::Center));
        let now = sel.source.find("brown").unwrap();
        assert_eq!(sel.selection(), Some((now, now + 5)), "the words moved out");

        // A heading: the content span starts past the `# `.
        let mut h = fmt_doc("# hi there\n\nbody\n", Format::Markdown);
        h.caret = h.source.find("there").unwrap() + 1;
        h.set_alignment(Some(Align::Right));
        assert_eq!(
            h.source,
            "<div class=\"right\">\n\n# hi there\n\n</div>\n\nbody\n"
        );
        assert_eq!(h.caret, h.source.find("there").unwrap() + 1);
        assert_eq!(h.alignment_at_caret(), Some(Align::Right));
    }

    /// A djot document open in the rich view, with its map built as
    /// [`wysiwyg_doc`] builds a Markdown one's.
    fn wysiwyg_djot(body: &str) -> Doc {
        let mut d = fmt_doc(body, Format::Djot);
        d.view = View::Wysiwyg;
        d.build_visual(80);
        d
    }

    /// Backspace at the start of a block whose presentation is spelled as
    /// hidden markup before it strips that presentation, the way Backspace at
    /// a heading's start strips its `#`. The ordinary delete fused djot's
    /// `{.center}` line onto the text and took the blank line a Markdown div
    /// needs between its tag and its paragraph.
    #[test]
    fn backspace_at_the_start_of_a_centred_paragraph_strips_its_attributes() {
        let mut md = wysiwyg_doc(
            "wys_attr_bksp",
            "above\n\n<div class=\"center\">\n\nhello\n\n</div>\n\nbelow\n",
        );
        md.caret = md.source.find("hello").unwrap();
        md.backspace();
        assert_eq!(
            md.source, "above\n\nhello\n\nbelow\n",
            "the div is unwrapped"
        );
        assert_eq!(md.caret, 7, "the caret stays at the start of its text");
        md.backspace();
        assert_eq!(
            md.source, "above\nhello\n\nbelow\n",
            "the next press joins the paragraphs, as it always did"
        );

        let mut dj = wysiwyg_djot("above\n\n{.center}\nhello\n\nbelow\n");
        dj.caret = dj.source.find("hello").unwrap();
        dj.backspace();
        assert_eq!(
            dj.source, "above\n\nhello\n\nbelow\n",
            "the attribute line goes"
        );
        assert_eq!(dj.caret, 7);

        // A heading's own marker is the nearer hidden markup, and goes first;
        // the attributes are the next press's.
        let mut dj = wysiwyg_djot("{.center}\n# Title\n");
        dj.caret = dj.source.find("Title").unwrap();
        dj.backspace();
        assert_eq!(dj.source, "{.center}\nTitle\n", "the `#` first");
        dj.build_visual(80);
        dj.backspace();
        assert_eq!(dj.source, "Title\n", "then the attributes");
        assert_eq!(dj.caret, 0);
    }

    /// A div around several blocks has no sole child for twig to unwrap, so
    /// at its first block the caret steps back to the stop before rather than
    /// taking the div apart; a later block has an ordinary paragraph above it
    /// and joins as any paragraph does.
    #[test]
    fn backspace_at_the_first_of_a_div_s_blocks_steps_back_and_a_later_one_joins() {
        let src = "above\n\n<div class=\"center\">\n\nhello\n\nworld\n\n</div>\n";
        let mut d = wysiwyg_doc("wys_div_first", src);
        d.caret = d.source.find("hello").unwrap();
        d.backspace();
        assert_eq!(d.source, src, "nothing is deleted");
        assert_eq!(d.caret, 5, "the caret steps back to the end of `above`");

        let mut d = wysiwyg_doc("wys_div_later", src);
        d.caret = d.source.find("world").unwrap();
        d.backspace();
        assert_eq!(
            d.source, "above\n\n<div class=\"center\">\n\nhello\nworld\n\n</div>\n",
            "a later block joins the one above it"
        );
    }

    /// Backspace at the start of the paragraph after a Markdown div joins it
    /// into the div's last paragraph — the join any two paragraphs make, with
    /// the hidden `</div>` carried past the joined text. The ordinary delete
    /// took the newline under the tag, which drew nothing different, and the
    /// next press took the `>` and left the div unclosed.
    #[test]
    fn backspace_after_a_div_joins_the_paragraph_into_it() {
        let src = "above\n\n<div class=\"center\">\n\nhello\n\n</div>\n\nbelow\n";
        let mut d = wysiwyg_doc("wys_div_join", src);
        d.caret = d.source.find("below").unwrap();
        d.backspace();
        assert_eq!(
            d.source,
            "above\n\n<div class=\"center\">\n\nhello\nbelow\n\n</div>\n"
        );
        assert_eq!(
            d.caret,
            d.source.find("below").unwrap(),
            "the caret stays at the start of the joined text"
        );
        d.build_visual(80);
        assert_eq!(
            d.alignment_at_caret(),
            Some(Align::Center),
            "and is centred now"
        );
        d.undo();
        assert_eq!(d.source, src, "one undo step");

        // A list closes the div: the paragraph joins the last item's text,
        // under the item's continuation indent, inside the div.
        let src = "<div class=\"center\">\n\n- item\n\n</div>\n\nbelow\n";
        let mut d = wysiwyg_doc("wys_div_list", src);
        d.caret = d.source.find("below").unwrap();
        d.backspace();
        assert_eq!(
            d.source, "<div class=\"center\">\n\n- item\n  below\n\n</div>\n",
            "the paragraph joins the item"
        );
        assert_eq!(d.caret, d.source.find("below").unwrap());

        // And where twig has nothing to join into — a code block above — the
        // caret steps back to the stop before, and nothing is deleted.
        let src = "```\ncode\n```\n\nbelow\n";
        let mut d = wysiwyg_doc("wys_code_then_para", src);
        d.caret = d.source.find("below").unwrap();
        d.backspace();
        assert_eq!(d.source, src, "nothing is deleted");
        assert!(
            d.caret < d.source.find("below").unwrap(),
            "the caret stepped back"
        );
    }

    /// Backspace on a blank line collapses to the stop before it — but not
    /// across hidden markup, which that collapse deleted whole: a `</div>`,
    /// or a comment between two blocks. There the blank line goes alone, and
    /// the caret lands where the collapse would have put it.
    #[test]
    fn backspace_on_a_blank_line_after_hidden_markup_keeps_the_markup() {
        let mut d = wysiwyg_doc(
            "wys_div_blank",
            "<div class=\"center\">\n\nhello\n\n</div>\n\n\n\nbelow\n",
        );
        d.caret = d.source.find("below").unwrap() - 2; // the empty paragraph
        assert!(
            d.vmap.is_stop(d.caret),
            "the empty paragraph is a caret home"
        );
        d.backspace();
        assert_eq!(
            d.source, "<div class=\"center\">\n\nhello\n\n</div>\n\nbelow\n",
            "the blank line goes and the div stays closed"
        );
        assert_eq!(
            d.caret,
            d.source.find("hello").unwrap() + 5,
            "onto the end of `hello`"
        );

        let mut d = wysiwyg_doc("wys_comment_blank", "above\n\n<!-- note -->\n\n\n\nbelow\n");
        d.caret = d.source.find("below").unwrap() - 2;
        assert!(d.vmap.is_stop(d.caret));
        d.backspace();
        assert_eq!(
            d.source, "above\n\n<!-- note -->\n\nbelow\n",
            "the comment stays"
        );
        assert_eq!(d.caret, 5);
    }

    /// Backspace at the end of an attributed span steps inside its hidden
    /// closing tag the way it steps inside a `**`, and takes the span with
    /// its last letter. Before, the byte-step took the `>` of `</span>`,
    /// which left the paragraph unparseable: it vanished from the rich view,
    /// and the Backspace after that joined the next block into the wreck.
    #[test]
    fn backspace_walks_into_a_sized_span_and_takes_the_emptied_span_with_its_space() {
        let src = "above\n\nThis <span data-size=\"x-large\">is</span> a test\n\nTest 2\n";
        let mut d = wysiwyg_doc("wys_span_bs", src);
        d.caret = d.source.find("a test").unwrap() + 6;
        for _ in 0..7 {
            d.backspace();
        }
        assert_eq!(
            d.source,
            "above\n\nThis <span data-size=\"x-large\">is</span>\n\nTest 2\n"
        );
        d.backspace();
        assert_eq!(
            d.source, "above\n\nThis <span data-size=\"x-large\">i</span>\n\nTest 2\n",
            "the first Backspace after the tag takes the letter, not the `>`"
        );
        d.backspace();
        assert_eq!(
            d.source, "above\n\nThis \n\nTest 2\n",
            "the last letter takes the span with it"
        );
        assert_eq!(d.caret, 12, "the caret is where the letter was");
        d.backspace();
        assert_eq!(d.source, "above\n\nThis\n\nTest 2\n");
        assert_eq!(d.caret, 11);
        d.build_visual(80);
        assert!(
            d.vmap
                .rows
                .iter()
                .any(|r| r.glyphs.iter().map(|g| g.ch).collect::<String>() == "This"),
            "the paragraph is still drawn"
        );
    }

    #[test]
    fn backspace_walks_into_a_djot_sized_span_too() {
        let mut d = wysiwyg_djot("This [is]{data-size=\"x-large\"}\n\nTest 2\n");
        d.caret = d.source.find("\n\nTest 2").unwrap();
        // The caret home at the paragraph's end is inside the span, before
        // its `]`: the map offers no stop after `]{…}`.
        d.build_visual(80);
        assert_eq!(d.vmap.stop_before(31), Some(8));
        d.caret = 8;
        d.backspace();
        assert_eq!(d.source, "This [i]{data-size=\"x-large\"}\n\nTest 2\n");
        d.backspace();
        assert_eq!(
            d.source, "This \n\nTest 2\n",
            "the attribute block outside the span goes with it"
        );
        d.backspace();
        assert_eq!(d.source, "This\n\nTest 2\n");
        assert_eq!(d.caret, 4);
    }

    /// A span that is empty as the file was written has no stop of its own;
    /// Backspace reaching it from behind takes it with the character before
    /// it, the character the key looked aimed at.
    #[test]
    fn backspace_over_an_already_empty_span_takes_it_with_the_character_before() {
        let src = "This <span data-size=\"x-large\"></span> a test\n";
        let mut d = wysiwyg_doc("wys_span_empty", src);
        d.caret = d.source.find(" a test").unwrap();
        d.backspace();
        assert_eq!(d.source, "This a test\n");
        assert_eq!(d.caret, 4);
    }

    /// The mirror: Delete in front of a span's opening tag takes its first
    /// letter, and the span with its last.
    #[test]
    fn delete_walks_into_a_sized_span_and_takes_the_span_with_its_last_letter() {
        let src = "This <span data-size=\"x-large\">is</span> a test\n";
        let mut d = wysiwyg_doc("wys_span_del", src);
        d.caret = 5;
        d.delete_forward();
        assert_eq!(
            d.source,
            "This <span data-size=\"x-large\">s</span> a test\n"
        );
        d.delete_forward();
        assert_eq!(d.source, "This  a test\n");
        assert_eq!(d.caret, 5);
        d.delete_forward();
        assert_eq!(d.source, "This a test\n");
        assert_eq!(d.caret, 5);
    }

    /// The block version of the span's emptying rule. Centre a one-letter
    /// paragraph — Markdown spells that as a `<div>` around it — and
    /// Backspace the letter: the div goes with it, leaving a plain blank line
    /// the caret is at home on, in the incremental map and the from-scratch
    /// one alike. Before, the letter went alone; the emptied div drew a
    /// caret home only the stale map had, and the next Backspace collapsed
    /// the line and left `<div class="center">\n\n</div>` standing invisibly
    /// in the file.
    #[test]
    fn backspace_that_empties_a_centred_paragraph_takes_its_div_with_the_letter() {
        let mut d = wysiwyg_doc("wys_div_empty", "Try the toolbar.\n\nT\n");
        d.caret = d.source.len() - 1;
        d.set_alignment(Some(Align::Center));
        assert_eq!(
            d.source,
            "Try the toolbar.\n\n<div class=\"center\">\n\nT\n\n</div>\n"
        );
        d.backspace();
        assert_eq!(d.source, "Try the toolbar.\n\n\n");
        assert_eq!(d.caret, 18, "on the blank line where the letter was");
        // The host rebuilds the map after every key; the spliced map and a
        // fresh one both give the line a caret home.
        d.build_visual_unwrapped();
        assert!(d.vmap.is_stop(18));
        wysiwyg::assert_maps_eq(&d.vmap, &reference_map(&d.source), "after the div goes");
        d.backspace();
        assert_eq!(
            d.source, "Try the toolbar.\n",
            "then the blank line collapses"
        );
        assert_eq!(d.caret, 16);

        // With a block after the div the blank line keeps a gap each side.
        let mut d = wysiwyg_doc(
            "wys_div_empty_mid",
            "Try the toolbar.\n\n<div class=\"center\">\n\nT\n\n</div>\n\nbelow\n",
        );
        d.caret = d.source.find("T\n").unwrap() + 1;
        d.backspace();
        assert_eq!(d.source, "Try the toolbar.\n\n\n\nbelow\n");
        assert_eq!(d.caret, 18);
        d.build_visual_unwrapped();
        assert!(d.vmap.is_stop(18));
        wysiwyg::assert_maps_eq(&d.vmap, &reference_map(&d.source), "after the div goes");
    }

    /// djot spells the same paragraph as a `{.center}` line above it, and
    /// twig has no node at all for that line once the paragraph is gone —
    /// so the line goes with the letter too.
    #[test]
    fn backspace_that_empties_a_centred_paragraph_takes_its_djot_attrs_line_too() {
        let mut d = fmt_doc("Try the toolbar.\n\nT\n", Format::Djot);
        d.build_visual(80);
        d.caret = d.source.len() - 1;
        d.set_alignment(Some(Align::Center));
        assert_eq!(d.source, "Try the toolbar.\n\n{.center}\nT\n");
        d.backspace();
        assert_eq!(d.source, "Try the toolbar.\n\n\n");
        assert_eq!(d.caret, 18);
        d.build_visual(80);
        assert!(d.vmap.is_stop(18));
    }

    /// The rule is for a block that would be no block: a div holding more
    /// keeps its tags, and an emptied heading is still a heading.
    #[test]
    fn emptying_a_paragraph_keeps_a_div_that_holds_more_and_a_heading_its_marker() {
        let mut d = wysiwyg_doc(
            "wys_div_more",
            "<div class=\"center\">\n\nText\n\nT\n\n</div>\n",
        );
        d.caret = d.source.find("T\n").unwrap() + 1;
        d.backspace();
        assert_eq!(d.source, "<div class=\"center\">\n\nText\n\n\n\n</div>\n");
        assert_eq!(d.caret, 28, "the blank line inside the div, as after Enter");

        let mut d = wysiwyg_doc(
            "wys_div_heading",
            "<div class=\"center\">\n\n# T\n\n</div>\n",
        );
        d.caret = d.source.find("T\n").unwrap() + 1;
        d.backspace();
        assert_eq!(d.source, "<div class=\"center\">\n\n# \n\n</div>\n");
        assert_eq!(d.caret, 24);
        d.build_visual(80);
        assert!(
            d.vmap.is_stop(24),
            "the empty heading is still a caret home"
        );
    }

    /// The mirror: Delete in front of the letter takes the div with it.
    #[test]
    fn delete_that_empties_a_centred_paragraph_takes_its_div_with_the_letter() {
        let mut d = wysiwyg_doc(
            "wys_div_empty_del",
            "Try the toolbar.\n\n<div class=\"center\">\n\nT\n\n</div>\n",
        );
        d.caret = d.source.find("T\n").unwrap();
        d.delete_forward();
        assert_eq!(d.source, "Try the toolbar.\n\n\n");
        assert_eq!(d.caret, 18);
        d.build_visual(80);
        assert!(d.vmap.is_stop(18));

        let mut d = fmt_doc("Try the toolbar.\n\n{.center}\nT\n", Format::Djot);
        d.build_visual(80);
        d.caret = d.source.find("T\n").unwrap();
        d.delete_forward();
        assert_eq!(d.source, "Try the toolbar.\n\n\n");
        assert_eq!(d.caret, 18);
    }

    /// Backspace at a block's start is twig's join, spelled per format — so
    /// the cases the one-newline delete got wrong come out right: a
    /// paragraph joins onto a heading's line, HTML's `</p><p>` goes as one,
    /// and a quote's prefix is written on the joined line.
    #[test]
    fn backspace_at_a_block_start_joins_it_the_format_s_way() {
        let mut d = wysiwyg_doc("wys_join_heading", "# Title\n\nbelow\n");
        d.caret = d.source.find("below").unwrap();
        d.backspace();
        assert_eq!(d.source, "# Title below\n", "onto the heading's line");
        assert_eq!(d.caret, d.source.find("below").unwrap());
        d.undo();
        assert_eq!(d.source, "# Title\n\nbelow\n", "one undo step");

        let mut d = wysiwyg_doc("wys_join_quote", "> a\n\nb\n");
        d.caret = d.source.find('b').unwrap();
        d.backspace();
        assert_eq!(d.source, "> a\n> b\n", "into the quote, with its prefix");
        assert_eq!(d.caret, d.source.find('b').unwrap());

        let mut h = fmt_doc("<p>above</p>\n<p class=\"x\">below</p>\n", Format::Html);
        h.view = View::Wysiwyg;
        h.build_visual(80);
        h.caret = h.source.find("below").unwrap();
        h.backspace();
        assert_eq!(
            h.source, "<p>above\nbelow</p>\n",
            "one paragraph, the tag gone whole"
        );
        assert_eq!(h.caret, h.source.find("below").unwrap());
    }

    /// Delete at the end of a block's content is the same join aimed at the
    /// block after it, and the caret stays where the joined text now begins.
    #[test]
    fn delete_at_a_block_end_joins_the_next_block_into_it() {
        let src = "above\n\n<div class=\"center\">\n\nhello\n\n</div>\n\nbelow\n";
        let mut d = wysiwyg_doc("wys_del_join", src);
        d.caret = d.source.find("hello").unwrap() + 5;
        d.delete_forward();
        assert_eq!(
            d.source, "above\n\n<div class=\"center\">\n\nhello\nbelow\n\n</div>\n",
            "below joins hello inside the div"
        );
        assert_eq!(
            d.caret,
            d.source.find("hello").unwrap() + 5,
            "the caret stays"
        );

        let mut d = wysiwyg_doc("wys_del_join_head", "above\n\n# Title\n");
        d.caret = 5;
        d.delete_forward();
        assert_eq!(
            d.source, "above\nTitle\n",
            "the heading's marker goes with the join"
        );
        assert_eq!(d.caret, 5);

        // A code block after the paragraph: nothing to join, the caret steps
        // forward onto the next stop and nothing is deleted.
        let src = "above\n\n```\ncode\n```\n";
        let mut d = wysiwyg_doc("wys_del_code", src);
        d.caret = 5;
        d.delete_forward();
        assert_eq!(d.source, src);
        assert!(d.caret > 5, "the caret stepped forward");
    }

    // ── Text statistics ──────────────────────────────────────────────────────

    /// The counts of `body`, from a document open in the WYSIWYG view — the
    /// shape every case below starts from.
    fn counts_of(name: &str, body: &str) -> TextCounts {
        doc_in(View::Wysiwyg, name, body).counts()
    }

    #[test]
    fn counts_tally_plain_prose() {
        let c = counts_of(
            "counts_prose",
            "The quick brown fox jumps over the lazy dog.\n",
        );
        assert_eq!(
            c,
            TextCounts {
                words: 9,
                characters: 44,
                characters_without_spaces: 36,
                paragraphs: 1,
            }
        );
    }

    #[test]
    fn counts_read_the_text_and_not_the_markup() {
        // The `**` are four bytes of source and no part of the word.
        assert_eq!(
            counts_of("counts_marks", "a **bold** word\n"),
            TextCounts {
                words: 3,
                characters: 11,
                characters_without_spaces: 9,
                paragraphs: 1,
            }
        );
        // A link is its label; the destination is plumbing, however long.
        assert_eq!(
            counts_of(
                "counts_link",
                "see [the label](https://example.com/a/b/c) here\n"
            ),
            TextCounts {
                words: 4,
                characters: 18,
                characters_without_spaces: 15,
                paragraphs: 1,
            }
        );
    }

    #[test]
    fn counts_spend_nothing_on_a_picture() {
        // A block image renders as a `🖼 alt` placeholder — a picture, not a
        // sentence, and not a paragraph either.
        assert_eq!(
            counts_of("counts_image", "![a long caption](pic.png)\n"),
            TextCounts::default()
        );
        // And it adds nothing to the prose around it.
        assert_eq!(
            counts_of("counts_image_prose", "text\n\n![a long caption](pic.png)\n"),
            TextCounts {
                words: 1,
                characters: 4,
                characters_without_spaces: 4,
                paragraphs: 1,
            }
        );
    }

    #[test]
    fn counts_spend_nothing_on_drawn_furniture() {
        // A thematic break is drawn, not written, and an empty paragraph has
        // nothing in it — neither is a paragraph of the document.
        assert_eq!(
            counts_of("counts_rule", "one\n\n---\n\ntwo\n"),
            TextCounts {
                words: 2,
                characters: 6,
                characters_without_spaces: 6,
                paragraphs: 2,
            }
        );
    }

    #[test]
    fn counts_measure_characters_as_a_reader_does() {
        // Four Han characters (each its own word under UAX#29), one ZWJ emoji
        // family that is a single grapheme cluster, and two letters.
        let c = counts_of(
            "counts_graphemes",
            "你好世界 👩\u{200d}👩\u{200d}👧\u{200d}👦 ok\n",
        );
        assert_eq!(
            c,
            TextCounts {
                words: 5,
                characters: 9,
                characters_without_spaces: 7,
                paragraphs: 1,
            }
        );
    }

    #[test]
    fn counts_take_a_code_block_as_one_paragraph() {
        let c = counts_of("counts_code", "```rust\nlet x = 1;\n\nlet y = 2;\n```\n");
        assert_eq!(
            c,
            TextCounts {
                words: 6,
                characters: 20,
                characters_without_spaces: 14,
                paragraphs: 1,
            }
        );
    }

    #[test]
    fn counts_take_a_table_as_one_paragraph() {
        // The box-drawn borders and the column padding are the renderer's, not
        // the author's; the cells are what was written.
        let c = counts_of("counts_table", "| a b | c |\n| - | - |\n| d | e |\n");
        assert_eq!(
            c,
            TextCounts {
                words: 5,
                characters: 6,
                characters_without_spaces: 5,
                paragraphs: 1,
            }
        );
    }

    #[test]
    fn counts_give_every_item_and_every_quoted_paragraph_its_own_paragraph() {
        let c = counts_of(
            "counts_blocks",
            "- one\n- two\n- three\n\n> first quoted\n>\n> second quoted\n",
        );
        assert_eq!(
            c,
            TextCounts {
                words: 7,
                characters: 36,
                characters_without_spaces: 34,
                paragraphs: 5,
            }
        );
    }

    #[test]
    fn counts_leave_the_frontmatter_out() {
        // The WYSIWYG view doesn't render it and a writer didn't write it.
        let c = counts_of(
            "counts_frontmatter",
            "---\ntitle: Hidden\n---\n\nvisible words here\n",
        );
        assert_eq!(
            c,
            TextCounts {
                words: 3,
                characters: 18,
                characters_without_spaces: 16,
                paragraphs: 1,
            }
        );
    }

    #[test]
    fn counts_of_an_empty_document_are_all_zero() {
        assert_eq!(counts_of("counts_empty", ""), TextCounts::default());
    }

    /// UAX#29 puts a boundary at the hyphen, so a hyphenated compound is two
    /// words. Recorded rather than corrected: it is what the algorithm says,
    /// and what every other UAX#29 counter reports.
    #[test]
    fn counts_split_a_hyphenated_compound_in_two() {
        let c = counts_of("counts_hyphen", "well-known example\n");
        assert_eq!(c.words, 3);
        assert_eq!(c.characters, 18);
        // Punctuation on its own is no word, and an apostrophe doesn't split one.
        assert_eq!(counts_of("counts_punct", "don't ... stop\n").words, 2);
    }

    #[test]
    fn selection_counts_measure_the_selection_and_nothing_without_one() {
        let src = "alpha beta\n\ngamma delta\n";
        let mut d = doc_in(View::Wysiwyg, "counts_sel", src);
        assert_eq!(d.selection_counts(), None, "no selection, no counts");

        // From the `b` of `beta` to the end of `gamma`: two blocks clipped.
        d.select_range(6, 17);
        assert_eq!(
            d.selection_counts(),
            Some(TextCounts {
                words: 2,
                characters: 9,
                characters_without_spaces: 9,
                paragraphs: 2,
            })
        );
    }

    /// The count is of the document, not of the window it is shown in — so
    /// ⌘E must not move it, and neither must a resize or an edit made with no
    /// map built at all.
    #[test]
    fn counts_agree_across_the_views() {
        let src =
            "# Head\n\nA **bold** word, a [label](http://x), and more.\n\n- item one\n- item two\n";
        let mut d = doc_in(View::Wysiwyg, "counts_views", src);
        let wysiwyg = d.counts();
        assert!(wysiwyg.words > 0 && wysiwyg.paragraphs == 4);

        d.toggle_view();
        assert_eq!(d.view, View::Source);
        d.build_source();
        assert_eq!(d.counts(), wysiwyg, "the source view counts the same text");

        // A narrower measure is a narrower window, not a shorter document.
        d.toggle_view();
        d.build_visual(24);
        assert_eq!(d.counts(), wysiwyg, "wrapping is not an input");

        // And an edit made in the source view, with the visual map left stale,
        // still counts the document as it now stands.
        d.toggle_view();
        d.caret = d.source.len();
        d.insert("\n\ntail words\n");
        let after = d.counts();
        assert_eq!(after.paragraphs, wysiwyg.paragraphs + 1);
        assert_eq!(after.words, wysiwyg.words + 2);
    }

    // ── math ─────────────────────────────────────────────────────────────────

    #[test]
    fn a_formula_reveals_on_the_caret_line_in_the_hidden_modes() {
        // The rule the proposal states: a formula's content is its TeX, not
        // its picture, so it reveals on the caret's line in *every* mode —
        // and nothing else on that line does outside `Full`.
        let mut d = doc_in(
            View::Wysiwyg,
            "math_reveal",
            "*one* $x+y$ here\n\ntwo there\n",
        );
        d.set_inline_pictures(true);
        assert_eq!(d.markup_mode(), MarkupMode::None);

        // Away from the formula's line: the atom, and no reveal at all.
        caret_at(&mut d, "two");
        assert!(
            drawn_rows(&d).iter().any(|r| r == "one ∑ here"),
            "{:?}",
            drawn_rows(&d)
        );
        assert_eq!(d.vmap.math.len(), 1);
        assert_eq!(d.reveal_line(), None, "a line with no math keys nothing");

        // On it: the formula is its source, the emphasis is still resolved.
        caret_at(&mut d, "here");
        assert!(
            drawn_rows(&d).iter().any(|r| r == "one $x+y$ here"),
            "{:?}",
            drawn_rows(&d)
        );
        assert!(d.vmap.math.is_empty());
        assert_eq!(d.reveal_line(), Some(Reveal::math(0..16)));

        // Off again, and the picture is back.
        caret_at(&mut d, "two");
        assert!(drawn_rows(&d).iter().any(|r| r == "one ∑ here"));

        // The same in Shortcuts; and Full reveals the emphasis too.
        d.set_markup_mode(MarkupMode::Shortcuts);
        caret_at(&mut d, "here");
        assert!(drawn_rows(&d).iter().any(|r| r == "one $x+y$ here"));
        d.set_markup_mode(MarkupMode::Full);
        caret_at(&mut d, "here");
        assert!(drawn_rows(&d).iter().any(|r| r == "*one* $x+y$ here"));
    }

    #[test]
    fn an_unrevealed_map_shows_the_carets_line_as_a_page_does() {
        let mut d = doc_in(
            View::Wysiwyg,
            "math_unrevealed",
            "*one* $x+y$ here\n\ntwo there\n",
        );
        d.set_inline_pictures(true);
        d.set_markup_mode(MarkupMode::Full);
        caret_at(&mut d, "here");
        assert!(drawn_rows(&d).iter().any(|r| r == "*one* $x+y$ here"));
        let (screen, caret) = (d.visual_key(), d.caret);

        // On paper: the delimiters hidden and the formula a picture, though
        // the caret has not moved off the line.
        d.set_unrevealed(true);
        d.build_visual(80);
        assert!(
            drawn_rows(&d).iter().any(|r| r == "one ∑ here"),
            "{:?}",
            drawn_rows(&d)
        );
        assert_eq!(d.vmap.math.len(), 1);
        assert_eq!(d.caret, caret);

        // Off again: the screen's map is back as it was, and the next build
        // reuses it rather than building it over.
        d.set_unrevealed(false);
        assert_eq!(d.visual_key(), screen);
        d.build_visual(80);
        assert_eq!(d.visual_key(), screen);
        assert!(drawn_rows(&d).iter().any(|r| r == "*one* $x+y$ here"));
        assert_eq!(d.caret, caret);
    }

    #[test]
    fn a_formula_closed_by_typing_reveals_at_once() {
        // The reveal line is decided from the last build's layout, which
        // across an edit is stale: the keystroke that closes a `$…$` asks a
        // layout that knew no math. `build_map` asks again once the new
        // layout is in, so the formula does not snap to its picture under
        // the caret.
        let mut d = doc_in(View::Wysiwyg, "math_typed", "say \n");
        d.set_inline_pictures(true);
        d.set_markup_mode(MarkupMode::Shortcuts);
        d.caret = 4;
        for ch in ["$", "x", "$"] {
            d.insert(ch);
            d.build_visual(80);
        }
        assert_eq!(d.source, "say $x$\n");
        assert_eq!(
            drawn_rows(&d)[0],
            "say $x$",
            "source, not a picture, under the caret"
        );
        assert!(d.vmap.math.is_empty());
        // Leaving the line folds it — there is only one line, so add one.
        d.newline();
        d.insert("more");
        d.build_visual(80);
        assert_eq!(drawn_rows(&d)[0], "say ∑");
        assert_eq!(d.vmap.math.len(), 1);
        // And deleting the formula while revealed drops the reveal with it.
        d.caret = 7;
        d.build_visual(80);
        assert_eq!(drawn_rows(&d)[0], "say $x$");
        for _ in 0..3 {
            d.backspace();
        }
        d.build_visual(80);
        assert_eq!(d.source, "say \n\nmore\n");
        assert_eq!(d.reveal_line(), None);
    }

    #[test]
    fn a_display_block_is_edited_where_it_stands() {
        let mut d = doc_in(
            View::Wysiwyg,
            "math_block",
            "intro\n\n$$\n\\int_0^1 x\n$$\n\nend\n",
        );
        caret_at(&mut d, "end");
        assert_eq!(
            drawn_rows(&d),
            vec!["intro", "", "∑ \\int_0^1 x", "", "end"]
        );
        // Up from `end` lands on the placeholder, whose glyphs all carry the
        // block's start — which is on its `$$` line, so the block reveals.
        d.move_up(false);
        d.build_visual(80);
        assert_eq!(d.caret, 7);
        assert_eq!(
            drawn_rows(&d),
            vec!["intro", "", "$$", "\\int_0^1 x", "$$", "", "end"]
        );
        // Down walks the source lines, still revealed; typing edits the TeX.
        d.move_down(false);
        d.build_visual(80);
        assert_eq!(d.caret, 10);
        d.move_end(false);
        d.insert("^2");
        d.build_visual(80);
        assert_eq!(d.source, "intro\n\n$$\n\\int_0^1 x^2\n$$\n\nend\n");
        assert_eq!(drawn_rows(&d)[3], "\\int_0^1 x^2");
        // Out below, and it folds to the placeholder with the new TeX.
        d.move_down(false);
        d.move_down(false);
        d.move_down(false);
        d.build_visual(80);
        assert_eq!(drawn_rows(&d)[2], "∑ \\int_0^1 x^2");
        assert_eq!(d.vmap.math[0].tex, "\n\\int_0^1 x^2\n");
    }

    #[test]
    fn set_math_rows_reserves_filler_rows_by_tex() {
        let mut d = doc_in(View::Wysiwyg, "math_rows", "$$\nx\n$$\n\nend\n");
        caret_at(&mut d, "end");
        assert_eq!(d.vmap.math[0].rows_span, 0..1);
        d.set_math_rows(HashMap::from([(d.vmap.math[0].tex.clone(), 3)]));
        d.build_visual(80);
        assert_eq!(d.vmap.math[0].rows_span, 0..3);
        assert_eq!(drawn_rows(&d)[..3], ["∑ x", "", ""]);
        // Cheap when nothing changed.
        let key = d.visual_key();
        d.set_math_rows(HashMap::from([(d.vmap.math[0].tex.clone(), 3)]));
        d.build_visual(80);
        assert_eq!(d.visual_key(), key);
    }

    #[test]
    fn a_dollar_typed_in_shortcuts_authors_math() {
        let mut d = doc_in(View::Wysiwyg, "math_dollar_sc", "\n");
        d.set_markup_mode(MarkupMode::Shortcuts);
        d.caret = 0;
        d.insert("$x$");
        assert_eq!(d.source, "$x$\n");
        d.caret = 1;
        assert_eq!(d.breadcrumb(), "doc › para › inline_math");

        // `None` keeps typed syntax literal, and under the math extension a
        // `$` is syntax: twig escapes it, and the paragraph stays text.
        let mut d = doc_in(View::Wysiwyg, "math_dollar_none", "\n");
        assert_eq!(d.markup_mode(), MarkupMode::None);
        d.caret = 0;
        d.insert("$x$");
        assert_eq!(d.source, "\\$x\\$\n");
        d.caret = 2;
        assert_eq!(d.breadcrumb(), "doc › para › str");
    }

    #[test]
    fn counts_see_a_formula_as_a_picture_however_it_is_written() {
        let c = counts_of(
            "counts_math",
            "the sum $\\sum_i x_i$ and\n\n$$\ny = mx + c\n$$\n",
        );
        // `the sum … and` is three words; neither formula counts, and the
        // display block is not a paragraph of text.
        assert_eq!(c.words, 3);
        assert_eq!(c.paragraphs, 1);
    }
}