markdown/

render.rs

//! [`Doc`] → gpui elements.
//!
//! Numbers drive layout (sizes, line heights, paddings — the constants here);
//! colors are paint, read from [`Theme`]. Blocks are a flat list, so nesting is
//! left padding rather than nested containers, and the gap between two blocks
//! is decided by the pair: list items sit tight, everything else breathes.
//!
//! Ported from zeronsh/comet (MIT) and rebuilt against the flat block model.

use crate::AppExt as _;
use std::{
    cell::RefCell,
    collections::HashMap,
    hash::{DefaultHasher, Hash, Hasher},
    ops::Range,
    path::Path,
    rc::Rc,
};
use ui::AppExt as _;
use ui::widgets::Layout as _;

use gpui::{
    AnyElement, App, BorderStyle, Bounds, CursorStyle, ElementId, FontStyle, FontWeight, Hsla,
    ImageSource, InteractiveText, MouseButton, ObjectFit, Pixels, Point, SharedString,
    StrikethroughStyle, StyledImage as _, StyledText, TextLayout, TextRun, UnderlineStyle, Window,
    canvas, div, font, img, point, prelude::*, px, quad, size,
};
use theme::{TextStyle, Theme, Typeset};

use crate::{
    block,
    doc::{Align, Block, BlockKind, Doc, Form, Mark, Part, QuoteKind, Text},
    preview,
    select::{Cursor, Selection},
    typography::Typography,
};

/// Space between two ordinary blocks, and the tighter space inside a list.
mod code;
mod column;
mod layouts;
mod media;
mod table;
mod text;

pub use code::*;
use column::*;
pub use layouts::*;
use media::*;
use table::*;
pub use text::*;

const BLOCK_GAP: f32 = 12.0;
const LIST_GAP: f32 = 4.0;
/// One indent level. Wide enough to clear a marker and read as a level.
const INDENT_WIDTH: f32 = 22.0;
/// The marker column of a list row.
const MARKER_WIDTH: f32 = 18.0;
const MARKER_GAP: f32 = 8.0;
/// What a fence holds its code in, inside its border.
const CODE_PADDING_X: f32 = 12.0;
const CODE_PADDING_Y: f32 = 10.0;
/// What a fence with no info string calls itself, in its header and in a
/// picker — one spelling, so the label and the menu row cannot disagree.
pub const PLAIN_LANGUAGE: &str = "Plain";
/// Width of the caret. Wider than a hairline, because it has to read at a
/// glance against the text it sits in.
const CARET_WIDTH: f32 = 1.5;
/// Inline code's wash is a rounded quad painted under the glyphs: a run's
/// `background_color` can only ever be a square box.
const INLINE_CODE_RADIUS: f32 = 4.5;
const INLINE_CODE_PAD_X: f32 = 2.0;
const INLINE_CODE_INSET_Y: f32 = 2.0;
/// A mention's hover card.
const MENTION_CARD_WIDTH: f32 = 320.0;
/// Bookmark metrics. Notion's card: 180px of image beside the text, and a
/// height that fits a title, two lines of blurb and a footer. A cover moves
/// that image above the text and gives it the card's full width.
const CARD_HEIGHT: f32 = 116.0;
const CARD_IMAGE_WIDTH: f32 = 180.0;
const CARD_COVER_HEIGHT: f32 = 200.0;
const CARD_PADDING: f32 = 14.0;
const CARD_BORDER: f32 = 1.0;
const CARD_ICON: f32 = 16.0;
const CARD_COVER: f32 = 44.0;
/// Image metrics.
const IMAGE_EMPTY_HEIGHT: f32 = 52.0;
const CAPTION_GAP: f32 = 4.0;
/// What an image with no URL yet says, and what its caption says while empty.
const IMAGE_EMPTY: &str = "Add an image";
const CAPTION_HINT: &str = "Write a caption";
/// Table metrics. The design is frameless: hairlines between rows are the only
/// chrome — no outer box, no header fill, no rounding.
const TABLE_CELL_PADDING: f32 = 12.0;
/// Reserved lane for editor table controls, inside the block bounds.
pub const TABLE_CONTROL_SIZE: f32 = 16.0;
const TABLE_DIVIDER: f32 = 1.0;
/// Floor for a column's max-content share, so a short column ("1k") beside a
/// prose column keeps a readable width.
const TABLE_MIN_COLUMN_CONTENT: f32 = 48.0;
/// Narrowest a column wraps down to before the table scrolls instead.
const TABLE_MIN_COLUMN_WIDTH: f32 = 96.0;

/// What an image's one authored string is doing on the page.
///
/// SwiftUI keeps three things apart — `accessibilityLabel` for a reader that
/// cannot see, `.help` for the pointer, and a caption you compose out of a
/// `Text` under the picture. Markdown has one slot for all three, so this says
/// which of them it is playing here rather than in the document, where it is
/// the same string either way.
///
/// A named choice rather than a `bool`, so a surface that wants a third answer
/// gets a variant instead of a second flag.
#[derive(Clone, Copy, PartialEq, Eq, Debug, Default)]
pub enum Caption {
    /// Under the picture, where a caret can sit in it. The editor's shape.
    #[default]
    Shown,
    /// Kept by the document and painted nowhere — a picture on its own.
    Hidden,
}

/// Whether a fence paints the button that copies its text.
///
/// A named choice rather than a `bool`, the way [`Caption`] is.
#[derive(Clone, Copy, PartialEq, Eq, Debug, Default)]
pub enum CopyButton {
    /// Floating at the top right of the band, on the pointer and off it.
    #[default]
    Shown,
    /// Painted nowhere. A document with this and no [`Editing::toggle`] holds
    /// no listener at all.
    Hidden,
}

/// A range the caller wants washed, and which wash it gets.
///
/// None of what a comment or a highlight *says* is here: the caller keeps it
/// and hands over the range, the way it hands over a [`crate::Preview`]. A
/// closed set rather than a color, so the environment keeps deciding the paint.
#[derive(Clone, Copy, PartialEq, Eq, Debug, Default)]
#[non_exhaustive]
pub enum Annotation {
    /// A thread still waiting on someone.
    #[default]
    Open,
    /// Answered, and kept for the record.
    Resolved,
    /// The one whose thread the reader has in front of them.
    Active,
    /// A reader's highlight, in the wash [`crate::AppExt::set_highlight_paint`]
    /// gives its colour.
    Highlight(crate::HighlightColor),
    /// A find hit.
    Match,
    /// The find hit the reader is on.
    Current,
}

impl Annotation {
    fn wash(self, theme: &Theme, highlight: crate::HighlightPaint, find: crate::FindPaint) -> Hsla {
        match self {
            Self::Open => theme.warning.opacity(0.20),
            Self::Resolved => theme.warning.opacity(0.08),
            Self::Active => theme.warning.opacity(0.38),
            Self::Highlight(color) => highlight(color, theme),
            Self::Match => find(theme).0,
            Self::Current => find(theme).1,
        }
    }
}

/// Handed the block whose checkbox was clicked — see [`Toggle::Handled`].
///
/// Shared rather than borrowed: the press listener it is cloned into outlives
/// the frame that built it.
pub type OnToggle = Rc<dyn Fn(usize, &mut Window, &mut App)>;

/// Handed the image block whose picture was clicked — see [`Editing::image`].
pub type OnImage = Rc<dyn Fn(usize, &mut Window, &mut App)>;

/// Builds a picture's hover control from its block index and original URL.
pub type ImageOverlay = Rc<dyn Fn(usize, &str, &mut Window, &mut App) -> Option<AnyElement>>;

/// Handed a fence's block index and its new code — see [`FenceHost::rewrite`].
pub type OnRewrite = Rc<dyn Fn(usize, String, &mut Window, &mut App)>;

/// Handed the block a painted fence is leaving — see [`FenceHost::leave`].
pub type OnLeave = Rc<dyn Fn(usize, &mut Window, &mut App)>;

/// Handed a resizable painted block's index and where its grip was pressed —
/// see [`FenceHost::resize`].
pub type OnResize = Rc<dyn Fn(usize, Pixels, &mut Window, &mut App)>;

/// What an editor lends the blocks an installed [`crate::BlockRenderer`]
/// paints — see [`Editing::fence`].
#[derive(Clone)]
pub struct FenceHost {
    /// Replaces the code of the fence at a block index, as an edit of the
    /// document.
    pub rewrite: OnRewrite,
    /// Hands the keyboard back to the document from inside the painted block
    /// at a block index. Dispatched by [`LeaveBlock`].
    pub leave: OnLeave,
    /// Starts a height drag off the grip along a resizable painted block's
    /// foot, from the press's window `y`. `None` paints no grip.
    pub resize: Option<OnResize>,
}

/// The key context around every painted fence. An editor's bindings stop at
/// it, so keys typed into a control the block paints stay that control's.
pub const PAINTED_CONTEXT: &str = "MarkdownPaintedBlock";

gpui::actions!(
    markdown,
    [
        /// Return focus from inside a painted fence to the document around it.
        LeaveBlock
    ]
);

/// The picture corner holding an app's hover control.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub enum ImageOverlayCorner {
    TopLeft,
    TopRight,
    BottomLeft,
    #[default]
    BottomRight,
}

/// Who answers a press on a task block's checkbox.
///
/// Either variant paints the box as a control — the pointer over it is a hand.
/// [`Editing::toggle`] left unset paints it as a mark, and the press goes
/// wherever it would on any other glyph.
#[derive(Clone)]
pub enum Toggle {
    /// The box takes the press, stops it, and calls this with the block it
    /// belongs to. For a caller holding the [`Doc`] it renders itself.
    Handled(OnToggle),
    /// The box takes no press. The caller hit-tests
    /// [`BlockLayouts::checkbox_bounds`] in its own handler, which is what an
    /// editor does: the press it swallows is the one that also takes focus and
    /// closes an open menu, and the toggle belongs in the undo history beside
    /// the rest of its edits.
    HitTested,
}

/// What an editor paints over a document.
///
/// One value rather than six parameters, and the reason it is public: the
/// caret, the selection, the comment washes and the layout sink all arrive
/// together or not at all, and a read-only [`render`] sets none of them.
#[derive(Clone)]
pub struct Editing<'a> {
    /// The caret and what it has selected. `None` paints neither — a document
    /// nobody is editing.
    pub selection: Option<Selection>,
    /// The blink's lit half. A caret painted on every frame reads as frozen,
    /// and the phase belongs to whoever owns the focus.
    pub caret_on: bool,
    /// Filled as the document paints, for a caller resolving clicks against it.
    ///
    /// Given, only the blocks near the part of the window the document shows
    /// are built, and it answers for those alone. The rest are placed at their
    /// last measured height, or a guess at one, and held in it across frames.
    pub layouts: Option<&'a BlockLayouts>,
    /// Ranges washed under the text, in the order given.
    pub annotations: &'a [(Selection, Annotation)],
    /// Shown on the caret's block while it holds nothing.
    pub placeholder: Option<SharedString>,
    pub caption: Caption,
    /// What to set the document in. `None` takes the installed
    /// [`Typography`] — a caller sizing one document apart from the rest
    /// passes [`Typography::scaled`].
    pub typography: Option<Typography>,
    /// Makes a task block's checkbox a control, and says who answers the
    /// press. `None` paints a mark.
    pub toggle: Option<Toggle>,
    /// Handles a picture click with its block index, keeping the arrow cursor. A
    /// press and release more than a couple of pixels apart is a drag and
    /// calls nothing. The press still reaches whatever is under the picture.
    pub image: Option<OnImage>,
    /// An app control inset at `image_overlay_corner`, visible on hover.
    /// Its presses do not reach the picture. `None` adds no listeners.
    pub image_overlay: Option<ImageOverlay>,
    /// Corner for the picture control; defaults to bottom-right.
    pub image_overlay_corner: ImageOverlayCorner,
    /// Lets a painted fence rewrite its own code and give the keyboard back.
    /// `None` paints a fence that can do neither.
    pub fence: Option<FenceHost>,
    /// A painted block held at a height while its resize handle is dragged:
    /// the block index and the height, in place of the one the document says.
    pub sizing: Option<(usize, u32)>,
    /// Reserves lanes around tables for editor controls.
    pub table_controls: bool,
    /// Whether a fence offers to copy itself.
    pub copy: CopyButton,
    /// The directory a relative image path is joined onto. `None` leaves it
    /// relative, which gpui reads against the process's working directory.
    pub base: Option<&'a Path>,
    /// Blocks built wherever they are, alongside the ones near the part of the
    /// window the document shows — source lines, for [`render_source`]. The
    /// caret's and the selection anchor's are built without being named. Only
    /// read with `layouts` given.
    pub keep: &'a [usize],
    /// The scroll container the document sits in. When blocks above the text
    /// showing come out taller or shorter than they were placed at, its offset
    /// moves by the difference. Only read with `layouts` given.
    pub scroll: Option<&'a gpui::ScrollHandle>,
}

impl Default for Editing<'_> {
    fn default() -> Self {
        Self {
            selection: None,
            // Lit, so that a caller setting a selection and nothing else gets a
            // caret rather than a mystery.
            caret_on: true,
            layouts: None,
            annotations: &[],
            placeholder: None,
            caption: Caption::default(),
            typography: None,
            toggle: None,
            image: None,
            image_overlay: None,
            image_overlay_corner: ImageOverlayCorner::BottomRight,
            fence: None,
            sizing: None,
            table_controls: false,
            copy: CopyButton::default(),
            base: None,
            keep: &[],
            scroll: None,
        }
    }
}

/// What the editor needs painted into one text: which text it is, where the
/// caret sits, and where to record the layout a click resolves against.
///
/// One bundle rather than four parameters threaded through every block arm —
/// a read-only render builds it with no caret and no sink, and pays nothing.
#[derive(Clone, Copy)]
struct Overlay<'a> {
    block: usize,
    part: Part,
    selection: Option<Selection>,
    caret_on: bool,
    caret_shape: ui::input::CaretShape,
    caret_height: ui::input::CaretHeight,
    caret_inactive: ui::input::InactiveCaret,
    /// A block caret is hollow, and cuts no glyph out, while this is false.
    window_active: bool,
    layouts: Option<&'a BlockLayouts>,
    /// Ranges washed under the text, in the order the caller gave them.
    annotations: &'a [(Selection, Annotation)],
    /// Shown on the caret's block while it holds nothing. The renderer is the
    /// only thing that knows where that text sits, so the string comes to it.
    placeholder: Option<&'a SharedString>,
    caption: Caption,
    /// Borrowed so [`Overlay`] stays `Copy` — the clone is made at the one
    /// press listener that needs an owned handle.
    toggle: Option<&'a Toggle>,
    image: Option<&'a OnImage>,
    image_overlay: Option<&'a ImageOverlay>,
    image_overlay_corner: ImageOverlayCorner,
    fence: Option<&'a FenceHost>,
    sizing: Option<(usize, u32)>,
    table_controls: bool,
    copy: CopyButton,
    base: Option<&'a Path>,
    highlight: crate::HighlightPaint,
    find: crate::FindPaint,
}

/// What a resizable painted block carries besides itself: where it landed,
/// for the height drag, and the grip along its foot that starts one, shown
/// while the block is hovered or being sized.
fn resizable(overlay: &Overlay, theme: &Theme) -> Vec<AnyElement> {
    let ix = overlay.block;
    let mut parts = Vec::new();
    if let Some(layouts) = overlay.layouts {
        let layouts = layouts.clone();
        parts.push(
            canvas(
                move |bounds, _, _| layouts.record_painted(ix, bounds),
                |_, _, _, _| (),
            )
            .absolute()
            .size_full()
            .into_any_element(),
        );
    }
    if let Some(resize) = overlay.fence.and_then(|host| host.resize.clone()) {
        let dragging = overlay.held_height().is_some();
        // Inside the block's foot rather than across its edge: the grip is
        // shown while the block is hovered, and the pointer has to stay over
        // the block to reach it. A strip only as wide as it needs, so the
        // controls a block keeps in its corners stay pressable.
        parts.push(
            div()
                .absolute()
                .left_0()
                .right_0()
                .bottom_0()
                .flex()
                .justify_center()
                .child(
                    theme
                        .grip_handle()
                        .id(ElementId::named_usize("painted-grip", ix))
                        .debug_selector(|| "painted-grip".into())
                        .w(px(PAINTED_GRIP_WIDTH))
                        .when(!dragging, |el| {
                            el.invisible()
                                .group_hover(painted_group(ix), |el| el.visible())
                        })
                        .on_mouse_down(MouseButton::Left, move |event, window, cx| {
                            cx.stop_propagation();
                            resize(ix, event.position.y, window, cx);
                        }),
                )
                .into_any_element(),
        );
    }
    parts
}

/// How wide the grip along a resizable painted block's foot can be pressed.
const PAINTED_GRIP_WIDTH: f32 = 120.0;

fn painted_group(ix: usize) -> SharedString {
    SharedString::from(format!("painted-{ix}"))
}

impl<'a> Overlay<'a> {
    fn at(self, part: Part) -> Self {
        Self { part, ..self }
    }

    /// The height this block's resize handle is holding it at, if it is.
    fn held_height(&self) -> Option<u32> {
        self.sizing
            .filter(|(ix, _)| *ix == self.block)
            .map(|(_, height)| height)
    }

    fn here(&self) -> Cursor {
        Cursor::new(self.block, self.part, 0)
    }

    /// The caret to paint: where it is, only on the blink's lit half, and
    /// never over a non-empty selection.
    ///
    /// Separate from [`Self::caret`] because the blink must not reach anything
    /// but the quad — a block whose paint depends on holding the caret would
    /// otherwise swap itself out twice a second.
    fn caret_painted(&self) -> Option<usize> {
        let collapsed = self.selection.is_some_and(|s| s.is_collapsed());
        let shown = self
            .caret_shape
            .shown(self.window_active, self.caret_inactive);
        (self.caret_on && collapsed && shown)
            .then(|| self.caret())
            .flatten()
    }

    fn caret_hollow(&self) -> bool {
        !self.window_active
    }

    /// The caret's byte offset, if the head is in *this* text.
    fn caret(&self) -> Option<usize> {
        self.selection
            .map(|selection| selection.head)
            .filter(|head| head.block == self.block && head.part == self.part)
            .map(|head| head.offset)
    }

    /// The selected slice of this text, clipped to it.
    fn selected(&self, len: usize) -> Option<Range<usize>> {
        self.clip(self.selection?, len)
    }

    /// The annotated slices of this text, already resolved to their paint —
    /// the wash goes into a `move` closure that the theme does not travel into.
    fn annotated(&self, len: usize, theme: &Theme) -> Vec<(Range<usize>, Hsla)> {
        self.annotations
            .iter()
            .filter_map(|(range, kind)| {
                Some((
                    self.clip(*range, len)?,
                    kind.wash(theme, self.highlight, self.find),
                ))
            })
            .collect()
    }

    /// A range clipped to this text, and `None` when it does not reach it.
    ///
    /// The comparison is on `(block, part)` alone: a range covers this text
    /// entirely when it starts before and ends after, and the offsets only
    /// matter at the two ends.
    fn clip(&self, selection: Selection, len: usize) -> Option<Range<usize>> {
        if selection.is_collapsed() {
            return None;
        }
        let (start, end) = selection.ordered();
        let here = self.here();
        let (first, last) = (
            Cursor::new(start.block, start.part, 0),
            Cursor::new(end.block, end.part, 0),
        );
        if here < first || here > last {
            return None;
        }
        let from = if here == first { start.offset } else { 0 };
        let to = if here == last { end.offset } else { len };
        (from < to).then_some(from..to.min(len))
    }

    /// Whether a block painting something a caret cannot enter — a rule, a
    /// picture — falls inside the selection, and so should show that it is
    /// going to be taken.
    fn covers_block(&self) -> bool {
        let Some(selection) = self.selection.filter(|s| !s.is_collapsed()) else {
            return false;
        };
        let (start, end) = selection.ordered();
        start.block < self.block && self.block < end.block
    }
}

/// What gpui loads for an image URL as written in a document.
///
/// Anything with `://` is fetched as it stands. Anything else is a file: an
/// absolute path as it stands, a relative one joined onto `base` when there is
/// one.
pub fn image_source(url: &str, base: Option<&Path>) -> ImageSource {
    if url.contains("://") {
        return SharedString::from(url.to_string()).into();
    }
    // gpui reads a file only from a `PathBuf` — handed a string it looks for
    // an asset built into the binary and paints nothing.
    match base {
        Some(base) => base.join(url).into(),
        None => std::path::PathBuf::from(url).into(),
    }
}

/// Parse and render in one step — the common case for read-only content.
pub fn markdown(source: &str, window: &mut Window, cx: &mut App) -> AnyElement {
    let doc = crate::parse_with(source, &cx.marks());
    render(&doc, Caption::default(), window, cx)
}

/// Render a document.
pub fn render(doc: &Doc, caption: Caption, window: &mut Window, cx: &mut App) -> AnyElement {
    render_with(
        doc,
        Editing {
            caption,
            ..Editing::default()
        },
        window,
        cx,
    )
}

/// Render a document with a caret and a selection in it.
///
/// Both are paint-time concerns and nothing else: they read their positions off
/// the shaped text's own layout handle, the same way the inline-code wash does,
/// so nothing about layout depends on where the caret sits. An editor supplies
/// the selection and owns the focus and the keys; painting a caret and a few
/// quads is not worth a second renderer.
pub fn render_with(doc: &Doc, editing: Editing, window: &mut Window, cx: &mut App) -> AnyElement {
    let Editing {
        selection,
        caret_on,
        layouts,
        annotations,
        placeholder,
        caption,
        typography,
        toggle,
        image,
        image_overlay,
        image_overlay_corner,
        fence,
        sizing,
        table_controls,
        copy,
        base,
        keep,
        scroll,
    } = editing;
    // Cloned once so the theme is readable while `cx` stays free for the
    // element state the copy button needs.
    let theme = Theme::of(cx).clone();
    let typography = typography.unwrap_or_else(|| cx.typography());
    let highlight = crate::marks::highlight_paint_of(cx);
    let find = crate::find::find_paint_of(cx);
    let gaps: Vec<Pixels> = doc
        .blocks
        .iter()
        .enumerate()
        .map(|(ix, block)| {
            px(match doc.blocks.get(ix.wrapping_sub(1)) {
                None => 0.0,
                Some(previous) if tight(previous, block) => LIST_GAP,
                Some(_) => BLOCK_GAP,
            })
        })
        .collect();

    let Some(layouts) = layouts else {
        let mut column = div().flex().flex_col();
        for (ix, block) in doc.blocks.iter().enumerate() {
            let overlay = Overlay {
                block: ix,
                part: Part::Body,
                selection,
                caret_on,
                caret_shape: cx.caret_shape(),
                caret_height: cx.caret_height(),
                caret_inactive: cx.inactive_caret(),
                window_active: window.is_window_active(),
                layouts: None,
                annotations,
                placeholder: placeholder.as_ref(),
                caption,
                toggle: toggle.as_ref(),
                image: image.as_ref(),
                image_overlay: image_overlay.as_ref(),
                image_overlay_corner,
                fence: fence.as_ref(),
                sizing,
                table_controls,
                copy,
                base,
                highlight,
                find,
            };
            column = column
                .child(block_box(block, overlay, &typography, &theme, window, cx).mt(gaps[ix]));
        }
        return column.into_any_element();
    };

    let keys: Vec<u64> = doc
        .blocks
        .iter()
        .map(|block| block_key(block, &typography, table_controls))
        .collect();
    layouts.prune(&keys);
    let guesses: Vec<Guess> = doc
        .blocks
        .iter()
        .map(|block| {
            let mut guess = guess(block, &typography);
            if table_controls && matches!(block.kind, BlockKind::Table { .. }) {
                guess.extra += px(2.0 * TABLE_CONTROL_SIZE);
            }
            guess
        })
        .collect();
    let mut kept = keep.to_vec();
    kept.extend(selection.map(|selection| selection.head.block));
    kept.extend(selection.map(|selection| selection.anchor.block));
    let owned = Owned {
        blocks: doc.blocks.clone(),
        selection,
        caret_on,
        layouts: layouts.clone(),
        annotations: annotations.to_vec(),
        placeholder,
        caption,
        toggle,
        image,
        image_overlay,
        image_overlay_corner,
        fence,
        sizing,
        table_controls,
        copy,
        base: base.map(Path::to_path_buf),
        highlight,
        find,
        typography,
        theme,
    };
    Column {
        layouts: layouts.clone(),
        keys: keys.into(),
        gaps: gaps.into(),
        guesses: guesses.into(),
        keep: kept,
        scroll: scroll.cloned(),
        item_of: Box::new(|at| at.block),
        build: Box::new(move |ix, window, cx| {
            let overlay = Overlay {
                block: ix,
                part: Part::Body,
                selection: owned.selection,
                caret_on: owned.caret_on,
                caret_shape: cx.caret_shape(),
                caret_height: cx.caret_height(),
                caret_inactive: cx.inactive_caret(),
                window_active: window.is_window_active(),
                layouts: Some(&owned.layouts),
                annotations: &owned.annotations,
                placeholder: owned.placeholder.as_ref(),
                caption: owned.caption,
                toggle: owned.toggle.as_ref(),
                image: owned.image.as_ref(),
                image_overlay: owned.image_overlay.as_ref(),
                image_overlay_corner: owned.image_overlay_corner,
                fence: owned.fence.as_ref(),
                sizing: owned.sizing,
                table_controls: owned.table_controls,
                copy: owned.copy,
                base: owned.base.as_deref(),
                highlight: owned.highlight,
                find: owned.find,
            };
            block_box(
                &owned.blocks[ix],
                overlay,
                &owned.typography,
                &owned.theme,
                window,
                cx,
            )
            .into_any_element()
        }),
    }
    .into_any_element()
}

/// What [`Column`] builds a block from, owned so it can build one at prepaint.
struct Owned {
    blocks: Vec<Block>,
    selection: Option<Selection>,
    caret_on: bool,
    layouts: BlockLayouts,
    annotations: Vec<(Selection, Annotation)>,
    placeholder: Option<SharedString>,
    caption: Caption,
    toggle: Option<Toggle>,
    image: Option<OnImage>,
    image_overlay: Option<ImageOverlay>,
    image_overlay_corner: ImageOverlayCorner,
    fence: Option<FenceHost>,
    sizing: Option<(usize, u32)>,
    table_controls: bool,
    copy: CopyButton,
    base: Option<std::path::PathBuf>,
    highlight: crate::HighlightPaint,
    find: crate::FindPaint,
    typography: Typography,
    theme: Theme,
}

/// A block's box: its indent outside, and inside it the recorder and the
/// block itself.
fn block_box(
    block: &Block,
    overlay: Overlay,
    typography: &Typography,
    theme: &Theme,
    window: &mut Window,
    cx: &mut App,
) -> gpui::Div {
    let ix = overlay.block;
    // The block's own box, recorded for a gutter handle and a drop target.
    // A rule and an image hold no text, so a layout would not find them.
    let frame = overlay.layouts.map(|layouts| {
        let layouts = layouts.clone();
        canvas(
            move |bounds, _, _| layouts.record_block(ix, bounds),
            |_, _, _, _| (),
        )
        .absolute()
        .size_full()
    });
    // The indent sits on the outside and the recorder on the inside, so what
    // is recorded is the box the block's text actually occupies. Recorded
    // outside the padding, every level answered with the same left edge, and a
    // gutter handle placed from it stayed at the margin while the block it
    // belongs to moved right.
    div()
        .w_full()
        .pl(px(block.indent as f32 * INDENT_WIDTH))
        .child(
            div()
                .w_full()
                .relative()
                .children(frame)
                // What a caret cannot enter still has to show it is inside the
                // selection, or a rule between two paragraphs looks untouched
                // right up until it disappears.
                .when(overlay.covers_block() && block.opaque(), |el| {
                    el.rounded(px(4.0)).bg(theme.selection)
                })
                .child(block_element(block, overlay, typography, theme, window, cx)),
        )
}

/// What a block's height is cached under: its content and the type it is set
/// in, so an edit elsewhere that shifts its index keeps the height.
fn block_key(block: &Block, typography: &Typography, table_controls: bool) -> u64 {
    let mut hasher = DefaultHasher::new();
    table_controls.hash(&mut hasher);
    block.hash(&mut hasher);
    typography.body.size().to_bits().hash(&mut hasher);
    typography.body.line_height().to_bits().hash(&mut hasher);
    hasher.finish()
}

/// What a block is placed at before it has ever been built.
fn guess(block: &Block, typography: &Typography) -> Guess {
    let body = px(typography.body.line_height());
    let prose = |chars: usize, line: Pixels| Guess {
        chars,
        line,
        rows: 0,
        extra: px(0.0),
        indent: px(0.0),
    };
    let guess = match &block.kind {
        BlockKind::Paragraph(text)
        | BlockKind::Bullet(text)
        | BlockKind::Ordered { text, .. }
        | BlockKind::Task { text, .. }
        | BlockKind::Quote { text, .. } => prose(text.text.len(), body),
        BlockKind::Heading { level, text } => prose(
            text.text.len(),
            px(typography.heading(*level).line_height()),
        ),
        BlockKind::Code { code, .. } => Guess {
            rows: code.text.lines().count().max(1),
            line: px(typography.code.line_height()),
            extra: px(2.0 * CODE_PADDING_Y) + body,
            ..prose(0, body)
        },
        BlockKind::Table { rows, .. } => Guess {
            rows: rows.len() + 1,
            extra: px(2.0 * TABLE_CELL_PADDING + TABLE_DIVIDER) * (rows.len() + 1) as f32,
            ..prose(0, body)
        },
        BlockKind::Image { alt, .. } => Guess {
            extra: px(240.0),
            ..prose(alt.text.len(), px(typography.caption.line_height()))
        },
        BlockKind::Bookmark { .. } => Guess {
            extra: body * 3.0,
            ..prose(0, body)
        },
        BlockKind::Rule => Guess {
            extra: body,
            ..prose(0, px(0.0))
        },
    };
    Guess {
        indent: px(block.indent as f32 * INDENT_WIDTH),
        ..guess
    }
}

/// Whether two adjacent blocks belong to the same list and should sit close.
fn tight(previous: &Block, next: &Block) -> bool {
    let marker = |block: &Block| {
        matches!(
            block.kind,
            BlockKind::Bullet(_) | BlockKind::Ordered { .. } | BlockKind::Task { .. }
        )
    };
    marker(previous) && (marker(next) || next.indent > previous.indent)
}

fn block_element(
    block: &Block,
    overlay: Overlay,
    typography: &Typography,
    theme: &Theme,
    window: &mut Window,
    cx: &mut App,
) -> AnyElement {
    let body = overlay.at(Part::Body);
    match &block.kind {
        BlockKind::Paragraph(text) => text_element(
            text,
            typography.body.size(),
            typography.body.line_height(),
            FontWeight::NORMAL,
            body,
            theme,
            cx,
        ),
        BlockKind::Heading { level, text } => {
            let heading = typography.heading(*level);
            text_element(
                text,
                heading.size(),
                heading.line_height(),
                heading.weight,
                body,
                theme,
                cx,
            )
        }
        BlockKind::Bullet(text) => {
            marker_row(disc(typography, theme), text, body, typography, theme, cx)
        }
        BlockKind::Ordered { number, text } => marker_row(
            div()
                .flex_none()
                .w(px(MARKER_WIDTH))
                .text_size(px(typography.body.size()))
                .line_height(px(typography.body.line_height()))
                .text_color(theme.text_muted)
                .child(SharedString::from(format!("{number}.")))
                .into_any_element(),
            text,
            body,
            typography,
            theme,
            cx,
        ),
        BlockKind::Task { checked, text } => marker_row(
            checkbox(*checked, overlay, typography, theme),
            text,
            body,
            typography,
            theme,
            cx,
        ),
        BlockKind::Quote { kind, text } => div()
            .border_l_2()
            .border_color(kind.map_or(theme.border_strong, |kind| alert_color(kind, theme)))
            .pl(px(12.0))
            .pr(px(10.0))
            .py(px(2.0))
            .text_color(theme.text_muted)
            .children(kind.map(|kind| {
                div()
                    .pb(px(2.0))
                    .text_size(px(typography.body.size()))
                    .line_height(px(typography.body.line_height()))
                    .font_weight(FontWeight::SEMIBOLD)
                    .text_color(alert_color(kind, theme))
                    .child(kind.label())
            }))
            .child(text_element(
                text,
                typography.body.size(),
                typography.body.line_height(),
                FontWeight::NORMAL,
                body,
                theme,
                cx,
            ))
            .into_any_element(),
        BlockKind::Code {
            language,
            code,
            height,
        } => {
            let overlay = overlay.at(Part::Code);
            let height = overlay.held_height().or(*height);
            // The caret in the fence gives the source back. A painted block is
            // still an editable one, and typing into it otherwise edits what
            // the reader cannot see.
            let painted = overlay
                .caret()
                .is_none()
                .then(|| {
                    let fence = block::Fence {
                        language: language.as_deref()?,
                        code: &code.text,
                        height,
                        rewrite: overlay.fence.map(|host| {
                            let (rewrite, ix) = (host.rewrite.clone(), overlay.block);
                            Rc::new(move |code: String, window: &mut Window, cx: &mut App| {
                                rewrite(ix, code, window, cx)
                            }) as block::Rewrite
                        }),
                    };
                    block::render(&fence, window, cx)
                })
                .flatten();
            match painted {
                // Painted, there is no text under the selection to carry it —
                // the wash an opaque block gets at the container comes here.
                Some(element) => div()
                    .key_context(PAINTED_CONTEXT)
                    .relative()
                    .when_some(overlay.fence, |el, host| {
                        let (leave, ix) = (host.leave.clone(), overlay.block);
                        el.on_action(move |_: &LeaveBlock, window, cx| leave(ix, window, cx))
                    })
                    .when(overlay.covers_block(), |el| {
                        el.rounded(px(4.0)).bg(theme.selection)
                    })
                    .group(painted_group(overlay.block))
                    .child(element)
                    .children(resizable(&overlay, theme))
                    .into_any_element(),
                None => code_block(
                    language.as_deref(),
                    &code.text,
                    overlay,
                    typography,
                    theme,
                    window,
                    cx,
                ),
            }
        }
        BlockKind::Image { url, alt, width } => {
            image(url, alt, *width, overlay, typography, theme, window, cx)
        }
        BlockKind::Bookmark { url, form } => match crate::preview::card(
            url,
            match (*form, overlay.held_height()) {
                (Form::Embed(_), Some(held)) => Form::Embed(Some(held)),
                (form, _) => form,
            },
            window,
            cx,
        ) {
            // The app's own, painted where a fence's would be and kept apart
            // from the editor's keys the same way.
            Some(element) => div()
                .key_context(PAINTED_CONTEXT)
                .relative()
                .when_some(overlay.fence, |el, host| {
                    let (leave, ix) = (host.leave.clone(), overlay.block);
                    el.on_action(move |_: &LeaveBlock, window, cx| leave(ix, window, cx))
                })
                .when(overlay.covers_block(), |el| {
                    el.rounded(px(4.0)).bg(theme.selection)
                })
                .group(painted_group(overlay.block))
                .child(element)
                .children(match form {
                    Form::Embed(_) => resizable(&overlay, theme),
                    _ => Vec::new(),
                })
                .into_any_element(),
            None => bookmark(overlay.block, url, *form, typography, theme, cx),
        },
        BlockKind::Table {
            align,
            header,
            rows,
        } => table(align, header, rows, overlay, typography, theme, window, cx),
        BlockKind::Rule => div()
            .h(px(1.0))
            .w_full()
            .bg(theme.border)
            .into_any_element(),
    }
}

/// A real 5px disc rather than the "•" glyph, which reads too small at body size.
fn disc(typography: &Typography, theme: &Theme) -> AnyElement {
    div()
        .flex_none()
        .w(px(MARKER_WIDTH))
        .h(px(typography.body.line_height()))
        .flex()
        .items_center()
        .child(
            div()
                .ml(px(1.0))
                .w(px(5.0))
                .h(px(5.0))
                .rounded_full()
                .bg(theme.text_faint),
        )
        .into_any_element()
}

fn checkbox(checked: bool, overlay: Overlay, typography: &Typography, theme: &Theme) -> AnyElement {
    let ix = overlay.block;
    let mut box_ = div()
        .relative()
        .w(px(13.0))
        .h(px(13.0))
        .rounded(px(3.5))
        .border_1()
        .flex()
        .items_center()
        .justify_center();
    box_ = if checked {
        box_.bg(theme.solid)
            .border_color(theme.solid)
            .text_style(TextStyle::Caption)
            .text_color(theme.on_solid)
            .child("✓")
    } else {
        box_.border_color(theme.border_strong)
    };
    // The box's own bounds rather than the marker column's: a caller hit-tests
    // these to tell a toggle from a caret placed in the gutter beside it.
    box_ = box_.children(overlay.layouts.map(|layouts| {
        let layouts = layouts.clone();
        canvas(
            move |bounds, _, _| layouts.record_checkbox(ix, bounds),
            |_, _, _, _| (),
        )
        .absolute()
        .size_full()
    }));
    // The cursor answers to either variant: a box an editor hit-tests is as
    // pressable as one the renderer listens to, and only the pointer says so.
    if overlay.toggle.is_some() {
        box_ = box_.cursor_pointer();
    }
    if let Some(Toggle::Handled(toggle)) = overlay.toggle.cloned() {
        box_ = box_.on_mouse_down(MouseButton::Left, move |_, window, cx| {
            // Stopped, or the press goes on to whatever placed a caret
            // under it and the toggle reads as a click that moved the
            // caret as well.
            cx.stop_propagation();
            toggle(ix, window, cx);
        });
    }

    div()
        .flex_none()
        .w(px(MARKER_WIDTH))
        .h(px(typography.body.line_height()))
        .flex()
        .items_center()
        .child(box_)
        .into_any_element()
}

/// What an alert paints its rule and its label in.
fn alert_color(kind: QuoteKind, theme: &Theme) -> Hsla {
    match kind {
        QuoteKind::Note => theme.accent,
        QuoteKind::Tip => theme.success,
        QuoteKind::Important => theme.busy,
        QuoteKind::Warning => theme.warning,
        QuoteKind::Caution => theme.danger,
    }
}

fn marker_row(
    marker: AnyElement,
    text: &Text,
    overlay: Overlay,
    typography: &Typography,
    theme: &Theme,
    cx: &App,
) -> AnyElement {
    div()
        .flex()
        .flex_row()
        .gap(px(MARKER_GAP))
        .child(marker)
        .child(div().flex_1().min_w_0().child(text_element(
            text,
            typography.body.size(),
            typography.body.line_height(),
            FontWeight::NORMAL,
            overlay,
            theme,
            cx,
        )))
        .into_any_element()
}