cellix 0.5.2

use std::cell::{Cell, OnceCell, RefCell};
use std::collections::HashMap;
use std::io;
use std::path::{Path, PathBuf};
use std::rc::Rc;

use cellix_core::{Buffer, Motion, Selection, Selections};
use cellix_doc::changes::{self, Change, Kind, Syntax};
use cellix_doc::latex::Project;
use cellix_doc::notebook::{Marker, Notebook, marker};
use cellix_doc::prose::{Issue, Source};
use cellix_doc::table::Edit as TableEdit;
use cellix_input::{Command, Direction, Format};
use cellix_lang::{Registry, Span, Worker};
use cellix_render::{
    Bounds, Font, Grid, INTERFACE_SIZE, Icon, Measure, Paint, Rect, Rgb, Strip, units,
};
use cellix_review::LineChange;
use unicode_width::UnicodeWidthChar;

use crate::panels::Symbol;
use crate::rendered::{
    self, Callout, Dialect, Extras, Figure, Glyph, Intent, Library, Row, Target, Tone,
};
use crate::snippets::Snippet;
use crate::theme::Theme;

const TAB_WIDTH: usize = 4;
const GUTTER_MIN_DIGITS: usize = 4;
const NARROW_PANE: u16 = 60;
/// Left margin of a rendered page, which also carries comment marks.
/// Heading levels the toolbar steps through before going back to plain text.
const MAX_HEADING_CYCLE: usize = 3;
const INDENT: &str = "    ";
const MINIMAP_WIDTH: u16 = 13;
/// Lines of code a minimap fits in one row of the grid.
const MINIMAP_LINES_PER_ROW: usize = 6;
/// Columns of code a minimap shows, past which lines are cut off.
const MINIMAP_COLUMNS: usize = 96;
/// Narrower panes spend every column on the code, as beside an agent.
const MIN_MINIMAP_PANE: u16 = 90;
/// Files up to this size highlight as they're drawn. Past it, a background thread does it,
/// because a full reparse on each key starts to show.
const MAX_INLINE_HIGHLIGHT: usize = 32 * 1024;
/// How far a tall heading sits below its row: closer to its own text than to what's above.
const TALL_DROP: f32 = 0.9;
const PAGE_MARGIN: u16 = 2;
const WIDE_PAGE_MARGIN: u16 = 4;
/// How wide a page can be set.
const MAX_PAGE_WIDTH: u16 = 512;

/// Per-line marks for the gutter, as 1-based lines.
#[derive(Default)]
pub struct Marks {
    pub comments: Vec<usize>,
    pub errors: Vec<usize>,
}

/// A page's rows, its figures, and where its formulas and images are.
type LaidOut = (
    Rc<Vec<Row>>,
    Vec<Figure>,
    Vec<(usize, usize)>,
    Option<usize>,
);

/// A laid-out page, and what it was made for.
struct PageCache {
    revision: u64,
    width: usize,
    /// The formula showing as source.
    open: Option<(usize, usize)>,
    rows: Rc<Vec<Row>>,
    figures: Rc<Vec<Figure>>,
    /// Every formula and image, which the caret opens by moving into.
    spans: Rc<Vec<(usize, usize)>>,
    /// The picture of a formula being edited in a line.
    preview: Option<usize>,
}

/// One open document and where the view is within it.
/// A colour pulled towards the background, as text is at a minimap's size.
fn dimmed(color: Rgb, bg: Rgb) -> Rgb {
    let mix = |a: u8, b: u8| ((a as u16 * 3 + b as u16 * 2) / 5) as u8;
    Rgb(mix(color.0, bg.0), mix(color.1, bg.1), mix(color.2, bg.2))
}

/// Bars per row, with the revision, height and width they fit.
type MinimapRows = ((u64, u16, u16), Vec<Vec<Strip>>);

pub struct Editor {
    pub buffer: Buffer,
    pub selections: Selections,
    /// Char ranges to mark, while find is open.
    pub found: Vec<(usize, usize)>,
    /// The last commit's copy of this file, to mark changed lines against.
    pub committed: Option<String>,
    /// Changed lines by number, with the revision they were worked out at.
    changes: RefCell<(u64, HashMap<usize, LineChange>)>,
    /// Bars for each row of the minimap, with what they were built for.
    minimap: RefCell<Option<MinimapRows>>,
    pub top: usize,
    pub left: usize,
    spans: RefCell<Option<(u64, Vec<Span>)>>,
    /// How many rows the pane showed last, which is what bounds scrolling.
    view_rows: Cell<u16>,
    /// Started for the first large file, and only then.
    worker: OnceCell<Worker>,
    /// The revision the worker was last asked about.
    asked: Cell<u64>,
    /// What the cached spans were read from, to carry them across an edit.
    spans_bytes: Cell<usize>,
    /// Show Markdown as an editable page rather than its source, as Xcode does.
    pub rendered: bool,
    /// The file changed on disk under unsaved edits, so saving would erase that change.
    pub conflict: bool,
    /// Whether the minimap may show, given the room.
    pub minimap_on: bool,
    /// The laid-out page, for one revision and width.
    page: RefCell<Option<PageCache>>,
    /// Formulas and images already drawn, so typing doesn't redo them.
    library: RefCell<Library>,
    /// Whether the sheet is dark, which diagrams are drawn for.
    dark: Cell<bool>,
    /// How wide the text was drawn, for what's drawn beside it.
    view_width: Cell<u16>,
    /// Whether typing here is kept as suggested changes.
    pub tracking: bool,
    /// While an agent's edits stand as suggestions nobody has touched: the text from before
    /// its first edit, and the buffer as Cellix last marked it. A later edit from the same
    /// turn is marked against that text again, so the edits pile up into one review.
    pub agent_review: Option<(String, String)>,
    /// The word count, how many files it took in, and what it was counted at: this file's
    /// revision, and a stamp of the other open files' unsaved edits.
    words: Cell<Option<Counted>>,
    /// The paper's other files as last read.
    paper_files: RefCell<cellix_doc::paper::Files>,
    /// The paper a LaTeX file belongs to, looked for again only once the file is saved.
    paper_root: RefCell<Option<PathBuf>>,
    /// The notebook this page was read from, which saving writes back into.
    pub notebook: Option<Notebook>,
    /// Spelling and grammar, for the revision they were checked at.
    pub issues: (u64, Vec<Issue>),
    /// The revision last sent to be checked.
    pub checked: u64,
    /// Where built pieces of LaTeX are kept.
    pub built: Option<PathBuf>,
    /// Pieces of LaTeX this page is waiting for, for the app to have built.
    wanted: RefCell<Vec<Snippet>>,
    /// Text comments are about, as char ranges, tinted on a page.
    pub quoted: RefCell<Vec<(usize, usize)>>,
    /// The paper this file belongs to, and the revision it was read at.
    project: RefCell<Option<(u64, Rc<Project>)>>,
    show_preamble: bool,
    measure: Rc<dyn Measure>,
    registry: Rc<Registry>,
    /// The revision last sent to a language server.
    pub synced: u64,
}

/// A word count, how many files it took in, and what it was counted at: the file's revision and a
/// stamp of the other open files' unsaved edits.
type Counted = ((u64, u64), usize, usize);

fn display_width(c: char, column: usize) -> usize {
    if c == '\t' {
        TAB_WIDTH - column % TAB_WIDTH
    } else {
        c.width().unwrap_or(0)
    }
}

/// The colour a callout's bar and title take.
fn callout_colour(kind: Callout, theme: &Theme) -> Rgb {
    match kind {
        Callout::Note => theme.accent,
        Callout::Tip => theme.inserted(),
        Callout::Important => Rgb(0x88, 0x57, 0xe0),
        Callout::Warning => Rgb(0xc0, 0x86, 0x10),
        Callout::Caution => theme.deleted(),
    }
}

/// How far below a paper's figure the line naming its image can be.
const FIGURE_LINES: usize = 6;

impl Editor {
    pub fn new(buffer: Buffer, measure: Rc<dyn Measure>, registry: Rc<Registry>) -> Self {
        // A file of definitions is code: it has no page to show.
        let definitions = |text: &str| {
            let lines = text.lines().filter(|l| !l.trim().is_empty());
            let marks = [
                "\\newcommand",
                "\\renewcommand",
                "\\def",
                "\\usepackage",
                "\\Declare",
            ];
            let (total, defining) = lines.fold((0, 0), |(total, defining), line| {
                let is = marks.iter().any(|m| line.trim_start().starts_with(m));
                (total + 1, defining + usize::from(is))
            });
            total > 0 && defining * 2 > total
        };
        let is_page =
            buffer.path().is_some_and(|p| Dialect::of(p).is_some()) && !definitions(&buffer.text());
        Self {
            buffer,
            selections: Selections::default(),
            found: Vec::new(),
            committed: None,
            changes: RefCell::new((u64::MAX, HashMap::new())),
            minimap: RefCell::new(None),
            top: 0,
            left: 0,
            spans: RefCell::new(None),
            view_rows: Cell::new(0),
            worker: OnceCell::new(),
            asked: Cell::new(u64::MAX),
            spans_bytes: Cell::new(0),
            rendered: is_page,
            conflict: false,
            minimap_on: true,
            page: RefCell::new(None),
            library: RefCell::default(),
            dark: Cell::new(false),
            view_width: Cell::new(0),
            tracking: false,
            agent_review: None,
            words: Cell::new(None),
            paper_files: RefCell::default(),
            paper_root: RefCell::new(None),
            notebook: None,
            issues: (0, Vec::new()),
            checked: 0,
            built: None,
            wanted: RefCell::default(),
            quoted: RefCell::default(),
            project: RefCell::new(None),
            show_preamble: false,
            measure,
            registry,
            synced: 0,
        }
    }

    pub fn title(&self) -> String {
        self.buffer
            .path()
            .and_then(Path::file_name)
            .map_or_else(|| "untitled".into(), |n| n.to_string_lossy().into_owned())
    }

    /// Whether a pane has room for a minimap, which only pixels can draw.
    fn shows_minimap(&self, area: Rect) -> bool {
        // A document that fits has nothing for a minimap to show.
        let fits = self.rendered && {
            let cache = self.page.borrow();
            cache
                .as_ref()
                .is_some_and(|p| p.rows.len() <= area.h as usize)
        };
        self.minimap_on && !self.measure.in_cells() && area.w >= MIN_MINIMAP_PANE && !fits
    }

    /// Where text goes in `area`: all of it, less the minimap or the scrollbar.
    fn text_area(&self, area: Rect) -> Rect {
        let side = if self.shows_minimap(area) {
            MINIMAP_WIDTH
        } else {
            1
        };
        Rect::new(area.x, area.y, area.w.saturating_sub(side), area.h)
    }

    /// Forgets the changed lines, as when the file's committed copy changes.
    pub fn refresh_changes(&self) {
        self.changes.borrow_mut().0 = u64::MAX;
    }

    fn line_changes(&self) -> std::cell::Ref<'_, (u64, HashMap<usize, LineChange>)> {
        let revision = self.buffer.revision();
        if self.changes.borrow().0 != revision {
            let found = self.committed.as_ref().map_or_else(HashMap::new, |old| {
                cellix_review::line_changes(old, &self.buffer.text())
                    .into_iter()
                    .collect()
            });
            *self.changes.borrow_mut() = (revision, found);
        }
        self.changes.borrow()
    }

    /// The bar beside a changed line, in the margin of `x`.
    fn draw_change(&self, grid: &mut Grid, x: f32, y: u16, change: LineChange, theme: &Theme) {
        let color = match change {
            LineChange::Added => theme.git_added,
            LineChange::Modified => theme.git_modified,
            LineChange::DeletedAbove => theme.error,
        };
        let bounds = match change {
            LineChange::DeletedAbove => Bounds::cells(x, y as f32 - 0.06, 0.55, 0.12),
            _ => Bounds::cells(x, y as f32, 0.28, 1.0),
        };
        grid.panel(bounds, 0, Some(color), None);
        if self.measure.in_cells() {
            let style = cellix_render::Style {
                fg: color,
                ..theme.gutter
            };
            grid.put_str(x as u16, y, "▎", style, x as u16 + 1);
        }
    }

    /// The words of a line of code, as (column, length, colour).
    fn code_words(&self, line: usize, spans: &[Span], theme: &Theme) -> Vec<(usize, usize, Rgb)> {
        let start = self.buffer.rope().line_to_byte(line);
        let text = self.line_text(line);
        let mut words = Vec::new();
        let (mut column, mut byte) = (0, 0);
        for word in text.split_inclusive(char::is_whitespace) {
            let size = word.trim_end().chars().count();
            if size > 0 {
                let at = start + byte;
                let found = spans.partition_point(|s| s.end <= at);
                let span = spans.get(found).filter(|s| s.start <= at);
                let color = span.map_or(theme.text.fg, |s| theme.token(s.token));
                words.push((column, size, color));
            }
            column += word.chars().count();
            byte += word.len();
        }
        words
    }

    /// The words of a page row, in the same shape.
    fn page_words(row: &Row, theme: &Theme) -> Vec<(usize, usize, Rgb)> {
        let color = |tone| match tone {
            Tone::Heading => theme.heading,
            Tone::Code | Tone::Block(_) => theme.string,
            Tone::Link | Tone::Check(_) => theme.accent,
            _ => theme.text.fg,
        };
        let mut words = Vec::new();
        let mut run: Option<(usize, usize, Rgb)> = None;
        for g in &row.glyphs {
            if g.ch.is_whitespace() {
                words.extend(run.take());
            } else {
                let column = g.x as usize;
                let word = run.get_or_insert((column, 0, color(g.tone)));
                word.1 = (column + 1).saturating_sub(word.0);
            }
        }
        words.extend(run);
        words
    }

    /// Bars for each row of a minimap `rows` tall, in the colours of the code. The whole
    /// file always fits, so the map's fill shows how long the file is.
    fn minimap_rows(&self, rows: u16, text: Rect, theme: &Theme) -> Vec<Vec<Strip>> {
        let page = self.rendered.then(|| self.page(text));
        let lines = page
            .as_ref()
            .map_or_else(|| self.buffer.rope().len_lines(), |p| p.len());
        let capacity = rows as usize * MINIMAP_LINES_PER_ROW;
        let shown = lines.min(capacity);
        let mut out = vec![Vec::new(); rows as usize];
        let spans = self.spans.borrow();
        let no_spans = Vec::new();
        let spans = spans.as_ref().map_or(&no_spans, |(_, spans)| spans);
        for slot in 0..shown {
            let line = slot * lines / shown.max(1);
            let words = match &page {
                Some(page) => Self::page_words(&page[line], theme),
                None => self.code_words(line, spans, theme),
            };
            let row = slot / MINIMAP_LINES_PER_ROW;
            let y = ((slot % MINIMAP_LINES_PER_ROW) * 256 / MINIMAP_LINES_PER_ROW) as u8;
            for (column, size, color) in words {
                if column < MINIMAP_COLUMNS {
                    let x = (column * 256 / MINIMAP_COLUMNS) as u8;
                    let w = (size.min(MINIMAP_COLUMNS - column) * 256 / MINIMAP_COLUMNS) as u8;
                    let color = dimmed(color, theme.text.bg);
                    out[row].push(Strip { x, y, w, color });
                }
            }
        }
        out
    }

    /// Pictures in the map, small, where they stand in the document.
    fn draw_minimap_pictures(
        &self,
        grid: &mut Grid,
        track: Rect,
        (margin, width): (f32, f32),
        theme: &Theme,
    ) {
        if !self.rendered {
            return;
        }
        let area = self.text_area(Rect::new(
            track.x.saturating_sub(self.view_width.get()),
            track.y,
            self.view_width.get() + track.w,
            track.h,
        ));
        let page = self.page(area);
        let figures = self.figures();
        let per_row = MINIMAP_LINES_PER_ROW as f32;
        let capacity = track.h as f32 * per_row;
        let squeeze = (capacity / page.len().max(1) as f32).min(1.0);
        let column = width / MINIMAP_COLUMNS as f32;
        let clip = Bounds::cells(
            track.x as f32,
            track.y as f32,
            track.w as f32,
            track.h as f32,
        );
        for (index, row) in page.iter().enumerate() {
            let Some((figure, x)) = row.figure else {
                continue;
            };
            let Some(figure) = figures.get(figure) else {
                continue;
            };
            let top = track.y as f32 + index as f32 * squeeze / per_row;
            let bounds = Bounds::cells(
                track.x as f32 + margin + x * column,
                top,
                figure.width * column,
                figure.height * squeeze / per_row,
            );
            grid.paint.push(Paint::Image {
                bounds,
                clip,
                bitmap: figure.bitmap.clone(),
                color: dimmed(theme.text.fg, theme.text.bg),
            });
        }
    }

    fn draw_minimap(&self, grid: &mut Grid, full: Rect, theme: &Theme) {
        let text = self.text_area(full);
        let track = Rect::new(text.right(), full.y, full.w - text.w, full.h);
        grid.fill(track, theme.text);
        let total = if self.rendered {
            self.page(text).len()
        } else {
            self.buffer.rope().len_lines()
        }
        .max(1);
        let revision = self.buffer.revision();
        let built = (revision, track.h, text.w);
        let stale = !self
            .minimap
            .borrow()
            .as_ref()
            .is_some_and(|(key, _)| *key == built);
        if stale {
            let rows = self.minimap_rows(track.h, text, theme);
            *self.minimap.borrow_mut() = Some((built, rows));
        }
        let height = track.h as f32;
        let filled = (total as f32 / MINIMAP_LINES_PER_ROW as f32).min(height);
        let span = (text.h as f32 * filled / total as f32).clamp(1.0, height);
        // Past the fold the marker's top stops short, so the end of the file meets the end
        // of the map rather than sliding off it.
        let range = total.saturating_sub(text.h as usize).max(1) as f32;
        let travel = (filled - span).max(0.0);
        let top = track.y as f32 + (self.top as f32 / range).min(1.0) * travel;
        let margin = 0.6;
        let width = track.w as f32 - margin * 1.5;
        let view = Bounds::cells(track.x as f32 + margin - 0.3, top, width + 0.6, span);
        grid.panel(view, 12, Some(theme.minimap_view), Some(theme.popup_border));
        self.draw_minimap_pictures(grid, track, (margin, width), theme);
        if let Some((_, rows)) = self.minimap.borrow().as_ref() {
            for (row, strips) in rows.iter().enumerate().filter(|(_, s)| !s.is_empty()) {
                let bounds = Bounds::cells(
                    track.x as f32 + margin,
                    track.y as f32 + row as f32,
                    width,
                    1.0,
                );
                grid.paint.push(cellix_render::Paint::Strips {
                    bounds,
                    strips: strips.clone(),
                });
            }
        }
        // Changes down the edge, so they're findable in a long file.
        let lines = self.buffer.rope().len_lines().max(1);
        let fill_lines = lines as f32;
        for (line, change) in &self.line_changes().1 {
            let y = track.y as f32 + *line as f32 * filled / fill_lines;
            let color = match change {
                LineChange::Added => theme.git_added,
                LineChange::Modified => theme.git_modified,
                LineChange::DeletedAbove => theme.error,
            };
            let height = (filled / fill_lines).max(0.1);
            let mark = Bounds::cells(track.right() as f32 - 0.45, y, 0.3, height);
            grid.panel(mark, 0, Some(color), None);
        }
    }

    /// Scrolls to a point on the minimap or scrollbar.
    fn map_filled(&self, area: Rect, lines: usize) -> usize {
        if self.shows_minimap(area) {
            (lines.div_ceil(MINIMAP_LINES_PER_ROW)).clamp(1, area.h as usize)
        } else {
            area.h.max(1) as usize
        }
    }

    /// Pages use the whole pane, with room in the margin for marks.
    fn page_margin(area: Rect) -> u16 {
        if area.w < NARROW_PANE {
            PAGE_MARGIN
        } else {
            WIDE_PAGE_MARGIN
        }
    }

    fn page_width(area: Rect) -> usize {
        area.w
            .saturating_sub(Self::page_margin(area) * 2 + 1)
            .clamp(1, MAX_PAGE_WIDTH) as usize
    }

    /// The paper this file is part of, read again once each time the file is saved.
    fn project(&self, text: &str) -> Rc<Project> {
        let saved = self.buffer.is_dirty();
        let mut cache = self.project.borrow_mut();
        if let Some((revision, project)) = cache.as_ref()
            && (saved || *revision == self.buffer.revision())
        {
            return project.clone();
        }
        let project = match self.buffer.path() {
            Some(path) if !saved => {
                let root = cellix_doc::latex::root_of(path, text);
                Project::read(&root)
            }
            _ => Project::from_text(text),
        };
        let project = Rc::new(project);
        *cache = Some((self.buffer.revision(), project.clone()));
        project
    }

    /// Words in the document, and in the selection when there is one. A paper split over
    /// `\input` files counts whole, from its root, and the last says it did; any of its files
    /// open in `others` counts as edited there, saved or not. Counted once a revision.
    pub fn word_count(&self, others: &[&Editor]) -> (usize, Option<usize>, bool) {
        let latex = self.dialect() == Dialect::Latex;
        // Other open papers with unsaved edits, which count as they stand.
        let unsaved: Vec<&Editor> = others
            .iter()
            .copied()
            .filter(|e| latex && e.buffer.is_dirty() && e.dialect() == Dialect::Latex)
            .filter(|e| e.buffer.path().is_some() && !std::ptr::eq(*e, self))
            .collect();
        let stamp = {
            use std::hash::{Hash, Hasher};
            let mut hasher = std::collections::hash_map::DefaultHasher::new();
            for editor in &unsaved {
                (editor.buffer.path(), editor.buffer.revision()).hash(&mut hasher);
            }
            hasher.finish()
        };
        let revision = (self.buffer.revision(), stamp);
        let (all, files) = match self.words.get() {
            Some((counted, words, files)) if counted == revision => (words, files),
            _ => {
                let text = self.buffer.text();
                let (words, files) = match self.buffer.path().filter(|_| latex) {
                    Some(path) => {
                        let root = self.paper_root(path, &text);
                        let texts: Vec<(&Path, String)> = unsaved
                            .iter()
                            .filter_map(|e| Some((e.buffer.path()?, e.buffer.text())))
                            .collect();
                        let mut open = vec![(path, text.as_str())];
                        open.extend(texts.iter().map(|(p, t)| (*p, t.as_str())));
                        let mut files = self.paper_files.borrow_mut();
                        cellix_doc::words::paper(&root, path, &open, &mut files)
                    }
                    None => (cellix_doc::words::count(&text, latex), 1),
                };
                self.words.set(Some((revision, words, files)));
                (words, files)
            }
        };
        let selected = self
            .selected_text()
            .filter(|t| !t.trim().is_empty())
            .map(|t| cellix_doc::words::count(&t, false));
        (all, selected, files > 1)
    }

    /// Counts again on the next look, as when another file of the paper was saved.
    pub fn recount(&self) {
        self.words.set(None);
    }

    /// The root of the paper this file is in. Finding it can read the folder, so it's found
    /// again only while the file is as saved, not on every keystroke.
    fn paper_root(&self, path: &Path, text: &str) -> PathBuf {
        let mut known = self.paper_root.borrow_mut();
        match known.as_ref() {
            Some(root) if self.buffer.is_dirty() => root.clone(),
            _ => known.insert(cellix_doc::latex::root_of(path, text)).clone(),
        }
    }

    /// How this document writes a suggestion.
    pub fn change_syntax(&self) -> cellix_doc::changes::Syntax {
        if self.dialect() == Dialect::Latex {
            cellix_doc::changes::Syntax::Latex
        } else {
            cellix_doc::changes::Syntax::Critic
        }
    }

    /// Replaces the whole text as one edit, keeping the caret where it was as far as it can.
    pub fn set_all_text(&mut self, text: &str) {
        let head = self.selections.primary().head;
        self.selections.set_single(Selection {
            anchor: 0,
            head: self.buffer.rope().len_chars(),
            goal: None,
        });
        self.buffer.replace(&mut self.selections, text);
        let end = self.buffer.rope().len_chars();
        self.selections.set_single(Selection::cursor(head.min(end)));
    }

    /// The paper this file belongs to, with its labels and citations.
    pub fn latex_project(&self) -> Rc<Project> {
        self.project(&self.buffer.text())
    }

    /// Where a clickable row leads, if there's one at this place on screen.
    pub fn row_target(&self, area: Rect, y: u16) -> Option<Target> {
        let area = self.text_area(area);
        let rows = self.page(area);
        let row = self.top + y.saturating_sub(area.y) as usize;
        rows.get(row)?.target.clone()
    }

    /// Shows or hides a paper's preamble.
    pub fn toggle_preamble(&mut self) {
        self.show_preamble = !self.show_preamble;
        *self.page.borrow_mut() = None;
    }

    pub fn dialect(&self) -> Dialect {
        let path = self.buffer.path();
        path.and_then(Dialect::of).unwrap_or_default()
    }

    /// Lays the page out, with `open` showing as source.
    fn lay_out(&self, text: &str, width: usize, open: Option<(usize, usize)>) -> LaidOut {
        let folder = self.buffer.path().and_then(Path::parent);
        let dialect = self.dialect();
        let extras = Extras {
            dialect,
            project: (dialect == Dialect::Latex).then(|| self.project(text)),
            file: self.buffer.path(),
            preamble: self.show_preamble,
            open,
            folder,
            library: Some(&self.library),
            dark: self.dark.get(),
            built: self.built.as_deref(),
            notebook: self.notebook.as_ref(),
            ..Extras::default()
        };
        let rows = rendered::layout_with(text, width, &*self.measure, &self.registry, &extras);
        self.wanted.borrow_mut().extend(extras.wanted.take());
        (
            Rc::new(rows),
            extras.figures.into_inner(),
            extras.spans.into_inner(),
            extras.preview.into_inner(),
        )
    }

    /// The rendered page for this width, laid out again only after an edit, or when the
    /// caret moves into or out of a formula.
    fn page(&self, area: Rect) -> Rc<Vec<Row>> {
        let width = Self::page_width(area);
        let revision = self.buffer.revision();
        let head = self.selections.primary().head;
        let mut cache = self.page.borrow_mut();
        let stale = cache
            .as_ref()
            .is_none_or(|c| c.revision != revision || c.width != width);
        if stale {
            let text = self.buffer.text();
            let (rows, figures, spans, _) = self.lay_out(&text, width, None);
            *cache = Some(PageCache {
                revision,
                width,
                open: None,
                rows,
                figures: Rc::new(figures),
                spans: Rc::new(spans),
                preview: None,
            });
        }
        let page = cache.as_mut().expect("laid out above");
        let open = page
            .spans
            .iter()
            .copied()
            .find(|s| s.0 < head && head < s.1);
        if open != page.open {
            let text = self.buffer.text();
            let (rows, figures, _, preview) = self.lay_out(&text, width, open);
            page.open = open;
            page.rows = rows;
            page.figures = Rc::new(figures);
            page.preview = preview;
        }
        page.rows.clone()
    }

    fn figures(&self) -> Rc<Vec<Figure>> {
        let cache = self.page.borrow();
        cache
            .as_ref()
            .map_or_else(Rc::default, |page| page.figures.clone())
    }

    /// The picture a screen row is on, of any kind: its source line and its box on the
    /// screen, as left, right, top and bottom.
    pub fn figure_on_row(&self, area: Rect, y: u16) -> Option<(usize, [f32; 4])> {
        let area = self.text_area(area);
        let rows = self.page(area);
        let figures = self.figures();
        let left = (area.x + Self::page_margin(area)) as f32;
        let row = (y as f32) + 0.5;
        for (index, found) in rows.iter().enumerate() {
            let Some((figure, x)) = found.figure else {
                continue;
            };
            let figure = figures.get(figure)?;
            let top = area.y as f32 + index as f32 - self.top as f32 + 0.2;
            if row < top || row > top + figure.height {
                continue;
            }
            let shown = [left + x, left + x + figure.width, top, top + figure.height];
            return Some((found.line, shown));
        }
        None
    }

    /// The source line of the code block a screen row is in, and the screen row its first
    /// row is on.
    pub fn code_block_on_row(&self, area: Rect, y: u16) -> Option<(usize, u16)> {
        let area = self.text_area(area);
        let rows = self.page(area);
        let index = self.top + y.checked_sub(area.y)? as usize;
        let row = rows.get(index).filter(|r| r.shaded)?;
        let first = (0..=index).rev().take_while(|n| rows[*n].shaded).last()?;
        let top = first.max(self.top) - self.top;
        Some((row.line, area.y + top as u16))
    }

    /// The image a screen row is on. Formulas and diagrams have no size of their own to change.
    fn image_on_row(&self, area: Rect, y: u16) -> Option<(usize, [f32; 4])> {
        let (line, shown) = self.figure_on_row(area, y)?;
        let text = self.line_text(line);
        let names_one = cellix_doc::figure::image_line(&text).is_some()
            || self.dialect() == Dialect::Latex && {
                let lines = line..self.buffer.rope().len_lines();
                let mut lines = lines.take(FIGURE_LINES);
                lines.any(|n| self.line_text(n).contains("\\includegraphics"))
            };
        names_one.then_some((line, shown))
    }

    /// What a comment on a picture is about, in words an agent can act on.
    pub fn figure_quote(&self, line: usize) -> String {
        let text = self.line_text(line);
        let next = || self.line_text(line + 1).trim().to_string();
        if let Some(image) = cellix_doc::figure::image_line(&text) {
            return format!("Image {}", image.path);
        }
        let trimmed = text.trim();
        if let Some(language) = trimmed.strip_prefix("```") {
            return format!("{} diagram: {}", language.trim(), next());
        }
        if trimmed.starts_with("$$") || trimmed.starts_with("\\[") {
            return format!("Formula: {}", trimmed.trim_matches('$'));
        }
        let lines = line..self.buffer.rope().len_lines();
        let named = lines.take(FIGURE_LINES).find_map(|n| {
            let source = self.line_text(n);
            let open = source.find("\\includegraphics")?;
            let brace = open + source[open..].find('{')?;
            let close = brace + source[brace..].find('}')?;
            Some(source[brace + 1..close].to_string())
        });
        match named {
            Some(path) => format!("Image {path}"),
            None => format!("Figure: {trimmed}"),
        }
    }

    /// Shows a handle on the image under the pointer.
    pub fn draw_hover(&self, grid: &mut Grid, area: Rect, hover: (u16, u16), theme: &Theme) {
        if self.rendered && !self.measure.in_cells() {
            self.draw_image_handle(grid, area, hover, theme);
        }
    }

    /// The image whose right edge is under the pointer, ready to be dragged.
    pub fn image_edge(&self, area: Rect, x: f32, y: u16) -> Option<usize> {
        const REACH: f32 = 1.2;
        let (line, [_, right, ..]) = self.image_on_row(area, y)?;
        ((x - right).abs() <= REACH).then_some(line)
    }

    /// Sets an image's width from where its edge was dragged to. A picture stands in the
    /// middle of the page, so the edge is half its width from there.
    pub fn drag_image(&mut self, area: Rect, line: usize, x: f32) {
        const STEP: f32 = 5.0;
        let page = Self::page_width(self.text_area(area)) as f32;
        let middle = (area.x + Self::page_margin(self.text_area(area))) as f32 + page / 2.0;
        let share = (x - middle).max(0.0) * 2.0 / page.max(1.0) * 100.0;
        let percent = ((share / STEP).round() * STEP).clamp(STEP * 2.0, 100.0) as u32;
        let at = self.buffer.position(line, 0);
        self.selections.set_single(Selection::cursor(at));
        self.set_image_width(percent);
    }

    /// Draws the pictures in view, including those that started above it.
    fn draw_image_handle(&self, grid: &mut Grid, area: Rect, hover: (u16, u16), theme: &Theme) {
        let Some((_, [_, right, top, bottom])) = self.image_on_row(area, hover.1) else {
            return;
        };
        let middle = (top + bottom) / 2.0;
        let handle = Bounds::cells(right + 0.25, middle - 0.8, 0.3, 1.6);
        grid.panel(handle, 30, Some(theme.accent), None);
    }

    fn draw_figures(&self, grid: &mut Grid, area: Rect, rows: &[Row], theme: &Theme) {
        let figures = self.figures();
        let left = (area.x + Self::page_margin(area)) as f32;
        let end = (self.top + area.h as usize).min(rows.len());
        for (index, row) in rows.iter().enumerate().take(end) {
            let Some((figure, x)) = row.figure else {
                continue;
            };
            let Some(figure) = figures.get(figure) else {
                continue;
            };
            let top = area.y as f32 + index as f32 - self.top as f32;
            if top + figure.height < area.y as f32 {
                continue;
            }
            grid.paint.push(Paint::Image {
                bounds: Bounds::cells(left + x, top + 0.2, figure.width, figure.height),
                clip: Bounds::cells(area.x as f32, area.y as f32, area.w as f32, area.h as f32),
                bitmap: figure.bitmap.clone(),
                color: theme.text.fg,
            });
        }
    }

    /// The page as it stands, for tests to read.
    #[cfg(test)]
    pub fn page_rows(&self, area: Rect) -> Rc<Vec<Row>> {
        self.page(self.text_area(area))
    }

    /// What this page is waiting on LaTeX for.
    pub fn take_wanted(&self) -> Vec<Snippet> {
        self.wanted.take()
    }

    /// Sets the page again, now something it was waiting for has come.
    pub fn refresh_page(&mut self) {
        *self.page.borrow_mut() = None;
    }

    /// Text is measured differently from here on, so the page has to be set again.
    pub fn set_measure(&mut self, measure: Rc<dyn Measure>) {
        self.measure = measure;
        *self.page.borrow_mut() = None;
        *self.library.borrow_mut() = Library::default();
    }

    /// Moves a caret out of hidden source, so typing never lands inside a marker.
    pub fn prepare_edit(&mut self, area: Rect) {
        if !self.rendered {
            return;
        }
        let area = self.text_area(area);
        let primary = self.selections.primary();
        // Only for one caret: replacing it would end a multi-cursor edit after a letter.
        if primary.is_empty() && self.selections.len() == 1 {
            let at = rendered::snap(&self.page(area), primary.head);
            self.selections.set_single(Selection::cursor(at));
        }
    }

    /// Runs a move on the page, where rows and hidden markers change what it means.
    fn move_on_page(&mut self, area: Rect, direction: Direction, extend: bool) -> bool {
        let rows = self.page(area);
        // Start from a visible place, so a selection never takes in a hidden marker.
        let mut primary = self.selections.primary();
        if primary.is_empty() {
            let at = rendered::snap(&rows, primary.head);
            primary.anchor = at;
            primary.head = at;
        }
        let head = primary.head;
        let page = area.h.max(2) as isize - 1;
        let (_, column) = rendered::locate(&rows, head);
        // Selection keeps its goal as a whole number, so this one is in paint units.
        let goal = primary
            .goal
            .map_or(column, |g| g as f32 / units(1.0) as f32);
        let vertical = |delta| rendered::vertical(&rows, head, delta, goal);
        let (next, keep_goal) = match direction {
            Direction::Left => (rendered::step(&rows, head, false), false),
            Direction::Right => (rendered::step(&rows, head, true), false),
            Direction::Up => (vertical(-1), true),
            Direction::Down => (vertical(1), true),
            Direction::PageUp => (vertical(-page), true),
            Direction::PageDown => (vertical(page), true),
            Direction::LineStart | Direction::LineColumnZero => {
                (rendered::row_edge(&rows, head, false), false)
            }
            Direction::LineEnd => (rendered::row_edge(&rows, head, true), false),
            _ => return false,
        };
        self.selections.set_single(Selection {
            anchor: if extend { primary.anchor } else { next },
            head: next,
            goal: keep_goal.then_some(units(goal) as usize),
        });
        true
    }

    fn line_of_head(&self) -> (String, usize) {
        let rope = self.buffer.rope();
        let line = rope.char_to_line(self.selections.primary().head);
        (self.line_text(line), rope.line_to_char(line))
    }

    /// Enter on a page continues lists and quotes, and ends them on an empty item.
    fn enter_on_page(&mut self) {
        let (line, start) = self.line_of_head();
        let head = self.selections.primary().head;
        let at_end = head == start + line.chars().count();
        match rendered::enter_intent(&line) {
            Intent::Continue(marker) if at_end => {
                self.buffer
                    .replace(&mut self.selections, &format!("\n{marker}"));
            }
            Intent::EndList(hidden) => {
                self.buffer
                    .replace_range(&mut self.selections, start, start + hidden, "");
                self.selections.set_single(Selection::cursor(start));
            }
            _ => self.buffer.replace(&mut self.selections, "\n"),
        }
    }

    /// Backspace at the start of an item or heading removes its marker in one step.
    fn backspace_on_page(&mut self) {
        let (line, start) = self.line_of_head();
        let hidden = rendered::hidden_prefix(&line);
        let primary = self.selections.primary();
        if primary.is_empty() && hidden > 0 && primary.head == start + hidden {
            self.buffer
                .replace_range(&mut self.selections, start, start + hidden, "");
            self.selections.set_single(Selection::cursor(start));
        } else {
            self.buffer.delete_backward(&mut self.selections);
        }
    }

    /// Tab moves between table cells, and otherwise indents.
    /// The next empty `{}` of a formula being edited, for Tab.
    fn next_slot(&self, head: usize) -> Option<usize> {
        let cache = self.page.borrow();
        let (_, end) = cache.as_ref()?.open?;
        let rope = self.buffer.rope();
        let text: Vec<char> = rope
            .slice(head..end.min(rope.len_chars()))
            .chars()
            .collect();
        let found = text.windows(2).skip(1).position(|w| w == ['{', '}'])?;
        Some(head + found + 2)
    }

    fn tab_on_page(&mut self, area: Rect) {
        let rows = self.page(area);
        let head = self.selections.primary().head;
        if let Some(slot) = self.next_slot(head) {
            self.selections.set_single(Selection::cursor(slot));
            return;
        }
        if let Some(cell) = rendered::next_cell(&rows, head) {
            self.select_cell(area, cell);
            return;
        }
        // Past the last cell comes a new row, as in a sheet.
        if self.selection_in_table(area) {
            self.edit_table(TableEdit::RowBelow);
            let rows = self.page(area);
            let head = self.selections.primary().head;
            if let Some(cell) = rendered::cell_in_row(&rows, head, 0) {
                self.select_cell(area, cell);
            }
            return;
        }
        let (line, start) = self.line_of_head();
        if rendered::hidden_prefix(&line) > 0 {
            self.buffer
                .replace_range(&mut self.selections, start, start, "  ");
            self.selections.set_single(Selection::cursor(head + 2));
        } else {
            self.buffer.type_text(&mut self.selections, "    ");
        }
    }

    /// Whether the caret is in a table the page is showing.
    fn selection_in_table(&self, area: Rect) -> bool {
        let rows = self.page(area);
        let head = self.selections.primary().head;
        rendered::cell_column(&rows, head).is_some() && self.table_at_caret().is_some()
    }

    /// Lands on a cell with what it holds selected, so typing replaces it.
    fn select_cell(&mut self, area: Rect, cell: usize) {
        let rows = self.page(area);
        let (anchor, head) = rendered::cell_range(&rows, cell);
        self.selections.set_single(Selection {
            anchor,
            head,
            goal: None,
        });
    }

    /// Enter in a table: the same column one row down, with a new row past the last.
    fn next_row(&mut self, area: Rect) {
        let rows = self.page(area);
        let head = self.selections.primary().start();
        let column = rendered::cell_column(&rows, head).unwrap_or(0);
        if let Some(cell) = rendered::cell_below(&rows, head) {
            self.select_cell(area, cell);
            return;
        }
        self.edit_table(TableEdit::RowBelow);
        let rows = self.page(area);
        let head = self.selections.primary().head;
        if let Some(cell) = rendered::cell_in_row(&rows, head, column) {
            self.select_cell(area, cell);
        }
    }

    /// Toggles a task checkbox at a click. Returns true if there was one.
    pub fn click_checkbox(&mut self, area: Rect, x: f32, y: u16) -> bool {
        if !self.rendered {
            return false;
        }
        let area = self.text_area(area);
        let rows = self.page(area);
        let row = self.top + y.saturating_sub(area.y) as usize;
        let column = x - (area.x + Self::page_margin(area)) as f32;
        let Some((at, mark)) = rows.get(row).and_then(|r| r.checkbox) else {
            return false;
        };
        if column < at || column >= at + 1.3 {
            return false;
        }
        let checked = self.buffer.rope().char(mark) != ' ';
        let text = if checked { " " } else { "x" };
        self.buffer
            .replace_range(&mut self.selections, mark, mark + 1, text);
        true
    }

    /// True when a click at `x` lands on the scrollbar.
    pub fn on_scrollbar(&self, area: Rect, x: u16) -> bool {
        x >= self.text_area(area).right()
    }

    /// Scrolls so that row `y` of the scrollbar's track matches the document.
    pub fn scroll_to_row(&mut self, area: Rect, y: u16) {
        let lines = if self.rendered {
            self.page(self.text_area(area)).len()
        } else {
            self.buffer.rope().len_lines()
        };
        let row = y.saturating_sub(area.y) as usize;
        let filled = self.map_filled(area, lines);
        // The point clicked comes to the middle of the view.
        let middle = (row * lines / filled).saturating_sub(area.h as usize / 2);
        self.view_rows.set(area.h);
        self.top = middle.min(self.last_top());
    }

    /// Narrow panes keep only the digits they need: every column counts there.
    fn gutter_width(&self, pane_width: u16) -> u16 {
        let digits = self.buffer.rope().len_lines().to_string().len();
        let minimum = if pane_width < NARROW_PANE {
            2
        } else {
            GUTTER_MIN_DIGITS
        };
        (digits.max(minimum) + 2) as u16
    }

    pub fn line_text(&self, line: usize) -> String {
        let text = self.buffer.rope().line(line).to_string();
        text.trim_end_matches(['\n', '\r']).to_string()
    }

    /// Display column of a char position.
    fn column_of(&self, at: usize) -> usize {
        let rope = self.buffer.rope();
        let line = rope.char_to_line(at);
        let start = rope.line_to_char(line);
        let mut column = 0;
        for c in rope.slice(start..at).chars() {
            column += display_width(c, column);
        }
        column
    }

    /// Char position at a screen cell inside `area`, clamped to the text.
    pub fn position_at(&self, area: Rect, fine_x: f32, y: u16) -> usize {
        let x = fine_x as u16;
        let area = self.text_area(area);
        if self.rendered {
            let rows = self.page(area);
            let row = self.top + y.saturating_sub(area.y) as usize;
            let column = (fine_x - (area.x + Self::page_margin(area)) as f32).max(0.0);
            let row = row.min(rows.len().saturating_sub(1));
            // A tall heading's lower half is the spacer under it, and still the heading.
            let above = row
                .checked_sub(1)
                .and_then(|r| rows.get(r))
                .filter(|r| r.tall);
            // So is any part of a picture, which opens it wherever it's clicked.
            let picture = (!rows[row].stop)
                .then(|| {
                    rows[..=row]
                        .iter()
                        .rev()
                        .find(|r| r.stop || r.figure.is_some())
                })
                .flatten()
                .filter(|r| r.figure.is_some());
            let stop = above
                .or(picture)
                .or_else(|| rows[row..].iter().find(|r| r.stop));
            return stop.map_or(self.buffer.rope().len_chars(), |r| r.position_at(column));
        }
        let rope = self.buffer.rope();
        let line = (self.top + (y.saturating_sub(area.y)) as usize).min(rope.len_lines() - 1);
        let target = self.left + x.saturating_sub(area.x + self.gutter_width(area.w)) as usize;
        let start = rope.line_to_char(line);
        let mut column = 0;
        for (i, c) in self.line_text(line).chars().enumerate() {
            let width = display_width(c, column);
            if column + width > target {
                return start + i;
            }
            column += width;
        }
        start + self.line_text(line).chars().count()
    }

    pub fn top(&self) -> usize {
        self.top
    }

    /// The lines or page rows there are to scroll through.
    fn scroll_length(&self) -> usize {
        // A cached page is close enough to bound the scroll; draw corrects any slack.
        let rows = self.page.borrow().as_ref().map(|page| page.rows.len());
        rows.filter(|_| self.rendered)
            .unwrap_or_else(|| self.buffer.rope().len_lines())
    }

    /// The furthest down the view goes: the end of the file at the bottom of the pane.
    fn last_top(&self) -> usize {
        self.scroll_length()
            .saturating_sub(self.view_rows.get().max(1) as usize)
    }

    pub fn scroll_by(&mut self, delta: isize) {
        let last = self.last_top() as isize;
        self.top = (self.top as isize + delta).clamp(0, last) as usize;
    }

    /// What moves when the wheel does: the text, without the minimap beside it or the left
    /// column, where the sheet's rounded corners are.
    pub fn scroll_region(&self, area: Rect) -> Rect {
        let text = self.text_area(area);
        Rect::new(text.x + 1, text.y, text.w.saturating_sub(1), text.h)
    }

    /// Scrolls just enough to show the primary cursor.
    pub fn reveal(&mut self, area: Rect) {
        let area = self.text_area(area);
        if self.rendered {
            let (row, _) = rendered::locate(&self.page(area), self.selections.primary().head);
            let height = area.h.max(1) as usize;
            if row < self.top {
                self.top = row;
            } else if row >= self.top + height {
                self.top = row + 1 - height;
            }
            self.left = 0;
            return;
        }
        let head = self.selections.primary().head;
        let line = self.buffer.rope().char_to_line(head);
        let rows = area.h.max(1) as usize;
        if line < self.top {
            self.top = line;
        } else if line >= self.top + rows {
            self.top = line + 1 - rows;
        }
        let column = self.column_of(head);
        let cols = area.w.saturating_sub(self.gutter_width(area.w)).max(1) as usize;
        if column < self.left {
            self.left = column;
        } else if column >= self.left + cols {
            self.left = column + 1 - cols;
        }
    }

    pub fn run(&mut self, command: Command, area: Rect) {
        let page = area.h.max(2) as usize - 1;
        if self.rendered {
            let text_area = self.text_area(area);
            self.prepare_for_command(command, text_area);
            let handled = match command {
                Command::Move(direction, extend) => self.move_on_page(text_area, direction, extend),
                Command::DeleteBackward if self.tracking => {
                    self.tracked_delete(false);
                    true
                }
                Command::DeleteForward if self.tracking => {
                    self.tracked_delete(true);
                    true
                }
                Command::Newline if self.selection_in_table(text_area) => {
                    self.next_row(text_area);
                    true
                }
                Command::Outdent if self.selection_in_table(text_area) => {
                    let rows = self.page(text_area);
                    let start = self.selections.primary().start();
                    if let Some(cell) = rendered::previous_cell(&rows, start) {
                        self.select_cell(text_area, cell);
                    }
                    true
                }
                Command::Newline => {
                    self.enter_on_page();
                    true
                }
                Command::DeleteBackward => {
                    self.backspace_on_page();
                    true
                }
                Command::Indent => {
                    self.tab_on_page(text_area);
                    true
                }
                _ => false,
            };
            if handled {
                self.reveal(area);
                return;
            }
        }
        match command {
            Command::Move(direction, extend) => {
                let motion = match direction {
                    Direction::Left => Motion::Left,
                    Direction::Right => Motion::Right,
                    Direction::Up => Motion::Up,
                    Direction::Down => Motion::Down,
                    Direction::WordLeft => Motion::WordLeft,
                    Direction::WordRight => Motion::WordRight,
                    Direction::LineStart => Motion::LineStart,
                    Direction::LineColumnZero => Motion::LineColumnZero,
                    Direction::LineEnd => Motion::LineEnd,
                    Direction::Top => Motion::Top,
                    Direction::Bottom => Motion::Bottom,
                    Direction::PageUp => Motion::PageUp(page),
                    Direction::PageDown => Motion::PageDown(page),
                    Direction::NextWordStart => Motion::NextWordStart,
                    Direction::FindChar(c, forward, till) => Motion::FindChar(c, forward, till),
                };
                self.selections
                    .apply_motion(self.buffer.rope(), motion, extend);
            }
            Command::Newline => self.newline(),
            Command::Type(c) => self.type_char(c),
            Command::ToggleComment => self.toggle_comment(),
            Command::MoveLineUp => self.move_line(-1),
            Command::MoveLineDown => self.move_line(1),
            Command::DuplicateLine => self.duplicate_line(),
            Command::DeleteLine => self.delete_line(),
            Command::InsertLineBelow | Command::InsertLineAbove => {
                self.insert_line(command == Command::InsertLineAbove);
            }
            Command::SelectAllMatches => {
                // Next match until no new one comes.
                self.buffer.select_next_match(&mut self.selections);
                if !self.selections.primary().is_empty() {
                    loop {
                        let before = self.selections.len();
                        self.buffer.select_next_match(&mut self.selections);
                        if self.selections.len() == before {
                            break;
                        }
                    }
                }
            }
            Command::AddCursorAbove | Command::AddCursorBelow => {
                let rope = self.buffer.rope();
                let primary = self.selections.primary();
                let line = rope.char_to_line(primary.head);
                let column = primary.head - rope.line_to_char(line);
                let next = if command == Command::AddCursorAbove {
                    line.checked_sub(1)
                } else {
                    Some(line + 1).filter(|l| *l < rope.len_lines())
                };
                if let Some(next) = next {
                    let width = self.line_text(next).chars().count();
                    let at = rope.line_to_char(next) + column.min(width);
                    self.selections.add(Selection::cursor(at));
                }
            }
            Command::Indent => self.buffer.type_text(&mut self.selections, "    "),
            Command::Outdent => self.outdent(),
            Command::DeleteWordLeft => self.delete_to(Motion::WordLeft),
            Command::DeleteWordRight => self.delete_to(Motion::WordRight),
            Command::DeleteToLineStart => self.delete_to(Motion::LineColumnZero),
            Command::DeleteToLineEnd => self.delete_to(Motion::LineEnd),
            Command::DeleteBackward => self.backspace(),
            Command::DeleteForward => self.buffer.delete_forward(&mut self.selections),
            Command::Undo => {
                self.buffer.undo(&mut self.selections);
            }
            Command::Redo => {
                self.buffer.redo(&mut self.selections);
            }
            Command::SelectAll => {
                let len = self.buffer.rope().len_chars();
                self.selections.set_single(Selection {
                    anchor: 0,
                    head: len,
                    goal: None,
                });
            }
            Command::SelectLines(count) => {
                let rope = self.buffer.rope();
                let line = rope.char_to_line(self.selections.primary().head);
                let last = rope.len_lines();
                self.selections.set_single(Selection {
                    anchor: rope.line_to_char(line),
                    head: rope.line_to_char((line + count).min(last)),
                    goal: None,
                });
            }
            Command::SelectNextMatch => self.buffer.select_next_match(&mut self.selections),
            _ => return,
        }
        self.reveal(area);
    }

    /// Edits start from a visible place. Moves start wherever the caret is.
    fn prepare_for_command(&mut self, command: Command, area: Rect) {
        if matches!(
            command,
            Command::Newline | Command::DeleteBackward | Command::DeleteForward | Command::Indent
        ) {
            let primary = self.selections.primary();
            if primary.is_empty() {
                let at = rendered::snap(&self.page(area), primary.head);
                self.selections.set_single(Selection::cursor(at));
            }
        }
    }

    fn outdent(&mut self) {
        let head = self.selections.primary().head;
        let rope = self.buffer.rope();
        let start = rope.line_to_char(rope.char_to_line(head));
        let spaces = rope
            .line(rope.char_to_line(head))
            .chars()
            .take(TAB_WIDTH)
            .take_while(|c| *c == ' ')
            .count();
        if spaces > 0 {
            self.buffer
                .replace_range(&mut self.selections, start, start + spaces, "");
        }
    }

    fn char_at(&self, at: usize) -> Option<char> {
        let rope = self.buffer.rope();
        (at < rope.len_chars()).then(|| rope.char(at))
    }

    /// Keeps the line's indent, adds a level after an opening bracket, and puts a closing
    /// bracket on a line of its own.
    fn newline(&mut self) {
        let head = self.selections.primary().head;
        let rope = self.buffer.rope();
        let line = rope.line(rope.char_to_line(head));
        let indent: String = line
            .chars()
            .take_while(|c| matches!(c, ' ' | '\t'))
            .collect();
        let before = head.checked_sub(1).and_then(|at| self.char_at(at));
        let after = self.char_at(head);
        let opens = matches!(before, Some('{' | '(' | '['));
        let closes = matches!(
            (before, after),
            (Some('{'), Some('}')) | (Some('('), Some(')')) | (Some('['), Some(']'))
        );
        let inner = format!("{indent}{}", if opens { INDENT } else { "" });
        let text = if closes {
            format!("\n{inner}\n{indent}")
        } else {
            format!("\n{inner}")
        };
        self.buffer.type_text(&mut self.selections, &text);
        if closes {
            let at = head + 1 + inner.chars().count();
            self.selections.set_single(Selection::cursor(at));
        }
    }

    /// Types a character, closing brackets and quotes, wrapping a selection in them, and
    /// stepping over a closing one that's already there.
    pub fn type_char(&mut self, c: char) {
        if self.tracking && self.rendered {
            self.tracked_type(c.encode_utf8(&mut [0; 4]));
            return;
        }
        let primary = self.selections.primary();
        let close = match c {
            '(' => Some(')'),
            '[' => Some(']'),
            '{' => Some('}'),
            '"' | '`' | '\'' => Some(c),
            _ => None,
        };
        let pairs = !self.rendered && self.selections.len() == 1;
        let after = self.char_at(primary.head);
        let before = primary.head.checked_sub(1).and_then(|at| self.char_at(at));
        if pairs
            && primary.is_empty()
            && after == Some(c)
            && matches!(c, ')' | ']' | '}' | '"' | '`' | '\'')
        {
            self.selections
                .set_single(Selection::cursor(primary.head + 1));
            return;
        }
        let text = c.to_string();
        let Some(close) = close.filter(|_| pairs) else {
            self.buffer.type_text(&mut self.selections, &text);
            return;
        };
        if !primary.is_empty() {
            let (start, end) = (primary.start(), primary.end());
            let inner = self.buffer.rope().slice(start..end).to_string();
            let wrapped = format!("{c}{inner}{close}");
            self.buffer
                .replace_range(&mut self.selections, start, end, &wrapped);
            self.selections.set_single(Selection {
                anchor: start + 1,
                head: end + 1,
                goal: None,
            });
            return;
        }
        // Beside a word, a quote is an apostrophe or a lifetime, and a bracket wraps it later.
        let by_word = |c: Option<char>| c.is_some_and(|c| c.is_alphanumeric() || c == '_');
        if by_word(after) || (c == close && by_word(before)) {
            self.buffer.type_text(&mut self.selections, &text);
            return;
        }
        self.buffer
            .type_text(&mut self.selections, &format!("{c}{close}"));
        self.selections
            .set_single(Selection::cursor(primary.head + 1));
    }

    /// Deletes both halves of an empty pair.
    fn backspace(&mut self) {
        let primary = self.selections.primary();
        let pair = primary.head.checked_sub(1).and_then(|at| {
            let open = self.char_at(at)?;
            let close = self.char_at(primary.head)?;
            matches!(
                (open, close),
                ('(', ')') | ('[', ']') | ('{', '}') | ('"', '"') | ('`', '`') | ('\'', '\'')
            )
            .then_some(at)
        });
        match pair.filter(|_| primary.is_empty() && self.selections.len() == 1) {
            Some(at) => {
                self.buffer
                    .replace_range(&mut self.selections, at, at + 2, "");
                self.selections.set_single(Selection::cursor(at));
            }
            None => self.buffer.delete_backward(&mut self.selections),
        }
    }

    /// The whole lines the selection touches, as (first, last, start, end in chars).
    fn selected_lines(&self) -> (usize, usize, usize, usize) {
        let primary = self.selections.primary();
        let rope = self.buffer.rope();
        let first = rope.char_to_line(primary.start());
        let last = rope.char_to_line(primary.end());
        let start = rope.line_to_char(first);
        let end = rope.line_to_char(last) + self.line_text(last).chars().count();
        (first, last, start, end)
    }

    fn toggle_comment(&mut self) {
        let Some(mark) = self.buffer.path().and_then(Registry::line_comment) else {
            return;
        };
        let (first, last, start, end) = self.selected_lines();
        let lines: Vec<String> = (first..=last).map(|line| self.line_text(line)).collect();
        let written = |line: &&String| !line.trim().is_empty();
        let commented = lines
            .iter()
            .filter(written)
            .all(|line| line.trim_start().starts_with(mark));
        let margin = lines
            .iter()
            .filter(written)
            .map(|line| line.len() - line.trim_start().len())
            .min()
            .unwrap_or(0);
        let changed: Vec<String> = lines
            .iter()
            .map(|line| {
                if line.trim().is_empty() {
                    line.clone()
                } else if commented {
                    let at = line.len() - line.trim_start().len();
                    let rest = line[at + mark.len()..]
                        .strip_prefix(' ')
                        .unwrap_or(&line[at + mark.len()..]);
                    format!("{}{rest}", &line[..at])
                } else {
                    format!("{}{mark} {}", &line[..margin], &line[margin..])
                }
            })
            .collect();
        let text = changed.join("\n");
        self.buffer
            .replace_range(&mut self.selections, start, end, &text);
        self.selections
            .set_single(Selection::cursor(start + text.chars().count()));
    }

    fn move_line(&mut self, by: isize) {
        let (first, last, start, end) = self.selected_lines();
        let rope = self.buffer.rope();
        let count = rope.len_lines();
        let trailing = rope.line(count - 1).len_chars() == 0;
        let lines = count - usize::from(trailing);
        let other = if by < 0 {
            first.checked_sub(1)
        } else {
            (last + 1 < lines).then_some(last + 1)
        };
        let Some(other) = other else {
            return;
        };
        let head = self.selections.primary().head;
        let block = rope.slice(start..end).to_string();
        let neighbour = self.line_text(other);
        let (from, to, text, moved) = if by < 0 {
            let from = rope.line_to_char(other);
            (
                from,
                end,
                format!("{block}\n{neighbour}"),
                from + head - start,
            )
        } else {
            let to = rope.line_to_char(other) + neighbour.chars().count();
            let shift = neighbour.chars().count() + 1;
            (start, to, format!("{neighbour}\n{block}"), head + shift)
        };
        self.buffer
            .replace_range(&mut self.selections, from, to, &text);
        self.selections.set_single(Selection::cursor(moved));
    }

    /// Takes out the lines the selection touches, newline and all.
    fn delete_line(&mut self) {
        let (first, last, start, _) = self.selected_lines();
        let rope = self.buffer.rope();
        let lines = rope.len_lines();
        let end = if last + 1 < lines {
            rope.line_to_char(last + 1)
        } else {
            rope.len_chars()
        };
        // The last line takes the newline before it instead.
        let start = if end == rope.len_chars() && first > 0 && start > 0 {
            start - 1
        } else {
            start
        };
        self.buffer
            .replace_range(&mut self.selections, start, end, "");
        let rope = self.buffer.rope();
        let line = first.min(rope.len_lines().saturating_sub(1));
        self.selections
            .set_single(Selection::cursor(rope.line_to_char(line)));
    }

    /// Opens a line below this one, or above, at its indent, and goes there.
    fn insert_line(&mut self, above: bool) {
        let (first, last, start, end) = self.selected_lines();
        let line = if above { first } else { last };
        let indent: String = self
            .line_text(line)
            .chars()
            .take_while(|c| *c == ' ' || *c == '\t')
            .collect();
        let (at, text) = if above {
            (start, format!("{indent}\n"))
        } else {
            (end, format!("\n{indent}"))
        };
        self.buffer
            .replace_range(&mut self.selections, at, at, &text);
        let caret = if above {
            start + indent.chars().count()
        } else {
            at + text.chars().count()
        };
        self.selections.set_single(Selection::cursor(caret));
    }

    fn duplicate_line(&mut self) {
        let (_, _, start, end) = self.selected_lines();
        let block = self.buffer.rope().slice(start..end).to_string();
        let head = self.selections.primary().head;
        let copy = format!("\n{block}");
        self.buffer
            .replace_range(&mut self.selections, end, end, &copy);
        let at = head + block.chars().count() + 1;
        self.selections.set_single(Selection::cursor(at));
    }

    pub fn go_to_line(&mut self, line: usize, area: Rect) {
        let at = self.buffer.position(line.saturating_sub(1), 0);
        self.selections.set_single(Selection::cursor(at));
        self.reveal(area);
    }

    /// Where the caret is on screen, if it's in view.
    pub fn caret_cell(&self, area: Rect) -> Option<(f32, u16)> {
        self.cell_of(area, self.selections.primary().head)
    }

    /// Where a place in the text is on screen, if it's in view.
    pub fn cell_of(&self, area: Rect, head: usize) -> Option<(f32, u16)> {
        let area = self.text_area(area);
        let (row, x) = if self.rendered {
            let rows = self.page(area);
            let (row, x) = rendered::locate(&rows, head);
            (row, (area.x + Self::page_margin(area)) as f32 + x)
        } else {
            let row = self.buffer.rope().char_to_line(head);
            let column = self.column_of(head).checked_sub(self.left)?;
            let gutter = self.gutter_width(area.w);
            (row, (area.x + gutter) as f32 + column as f32)
        };
        let y = row.checked_sub(self.top)?;
        (y < area.h as usize).then_some((x, area.y + y as u16))
    }

    /// Takes a finished highlight. Returns whether there's something new to draw.
    pub fn collect_highlights(&self) -> bool {
        let Some((revision, spans)) = self.worker.get().and_then(Worker::finished) else {
            return false;
        };
        // It was asked for at this revision's size, or the next answer will correct it.
        if revision == self.buffer.revision() {
            self.spans_bytes.set(self.buffer.rope().len_bytes());
            *self.spans.borrow_mut() = Some((revision, spans));
            // Its bars were drawn without colours.
            *self.minimap.borrow_mut() = None;
            true
        } else {
            false
        }
    }

    /// Deletes from the cursor to where a motion lands, or just the selection if any.
    fn delete_to(&mut self, motion: Motion) {
        let primary = self.selections.primary();
        let (start, end) = if primary.is_empty() {
            let mut moved = self.selections.clone();
            moved.apply_motion(self.buffer.rope(), motion, false);
            let target = moved.primary().head;
            (target.min(primary.head), target.max(primary.head))
        } else {
            (primary.start(), primary.end())
        };
        if self.tracking && self.rendered {
            self.selections.set_single(Selection {
                anchor: start,
                head: end,
                goal: None,
            });
            self.tracked_delete(false);
            return;
        }
        self.buffer
            .replace_range(&mut self.selections, start, end, "");
        self.selections.set_single(Selection::cursor(start));
    }

    fn toggle_inline(&mut self, marker: &str) {
        let primary = self.selections.primary();
        let (start, end) = if primary.is_empty() {
            self.buffer.word_at(primary.head)
        } else {
            (primary.start(), primary.end())
        };
        let size = marker.chars().count();
        let rope = self.buffer.rope();
        let before = start
            .checked_sub(size)
            .filter(|from| rope.slice(*from..start) == marker);
        let after_end = end + size;
        let after = after_end <= rope.len_chars() && rope.slice(end..after_end) == marker;
        let inner = rope.slice(start..end).to_string();
        let (from, to, text, selected) = match before.filter(|_| after) {
            Some(from) => (from, after_end, inner, from),
            None => (start, end, format!("{marker}{inner}{marker}"), start + size),
        };
        let length = end - start;
        self.buffer
            .replace_range(&mut self.selections, from, to, &text);
        self.selections.set_single(Selection {
            anchor: selected,
            head: selected + length,
            goal: None,
        });
    }

    /// The prefix a selected line should get, given the one it has.
    fn next_prefix(format: Format, line: &str) -> (usize, String) {
        let hashes = line.chars().take_while(|c| *c == '#').count();
        let heading = hashes > 0 && line[hashes..].starts_with(' ');
        let simple = |prefix: &str| {
            if line.starts_with(prefix) {
                (prefix.len(), String::new())
            } else {
                (0, prefix.to_string())
            }
        };
        match format {
            Format::Heading if heading && hashes < MAX_HEADING_CYCLE => {
                (hashes + 1, format!("{} ", "#".repeat(hashes + 1)))
            }
            Format::Heading if heading => (hashes + 1, String::new()),
            Format::Heading => (0, "# ".to_string()),
            Format::Level(level) => {
                let drop = if heading { hashes + 1 } else { 0 };
                let add = "#".repeat(level as usize);
                (drop, if level == 0 { add } else { format!("{add} ") })
            }
            Format::Numbered => simple("1. "),
            Format::Bullet => simple("- "),
            Format::Task => simple("- [ ] "),
            Format::PlainText if heading => (hashes + 1, String::new()),
            Format::PlainText => {
                let markers = ["- [ ] ", "- [x] ", "- ", "* ", "> "];
                let found = markers.iter().find(|m| line.starts_with(**m));
                (found.map_or(0, |m| m.len()), String::new())
            }
            _ => simple("> "),
        }
    }

    fn toggle_lines(&mut self, format: Format) {
        let primary = self.selections.primary();
        let rope = self.buffer.rope();
        let first = rope.char_to_line(primary.start());
        let last = rope.char_to_line(primary.end());
        let start = rope.line_to_char(first);
        let end = rope.line_to_char(last) + self.line_text(last).chars().count();
        let mut text = Vec::new();
        for line in first..=last {
            let line = self.line_text(line);
            let (drop, add) = Self::next_prefix(format, &line);
            text.push(format!("{add}{}", &line[drop..]));
        }
        let text = text.join("\n");
        self.buffer
            .replace_range(&mut self.selections, start, end, &text);
        let at = start + text.chars().count();
        self.selections.set_single(Selection::cursor(at));
    }

    /// Wraps the word or selection in a command, or takes it back out.
    fn toggle_command(&mut self, open: &str) {
        let primary = self.selections.primary();
        let (start, end) = if primary.is_empty() {
            self.buffer.word_at(primary.head)
        } else {
            (primary.start(), primary.end())
        };
        let size = open.chars().count();
        let rope = self.buffer.rope();
        let inside = start >= size
            && rope.slice(start - size..start) == open
            && rope.get_char(end) == Some('}');
        let inner = rope.slice(start..end).to_string();
        let length = end - start;
        let selected = if inside {
            let (from, to) = (start - size, end + 1);
            self.buffer
                .replace_range(&mut self.selections, from, to, &inner);
            from
        } else {
            let text = format!("{open}{inner}}}");
            self.buffer
                .replace_range(&mut self.selections, start, end, &text);
            start + size
        };
        self.selections.set_single(Selection {
            anchor: selected,
            head: selected + length,
            goal: None,
        });
    }

    /// The caret's line and its text, with any section command taken off.
    fn line_as_text(&self) -> (usize, String) {
        let rope = self.buffer.rope();
        let line = rope.char_to_line(self.selections.primary().head);
        let text = self.line_text(line);
        let plain = text
            .split_once('{')
            .filter(|_| text.trim_start().starts_with("\\"))
            .and_then(|(_, rest)| rest.trim_end().strip_suffix('}'))
            .unwrap_or(&text)
            .to_string();
        (line, plain)
    }

    fn set_section(&mut self, level: u8) {
        let (line, plain) = self.line_as_text();
        let start = self.buffer.rope().line_to_char(line);
        let end = start + self.line_text(line).chars().count();
        let text = match level {
            1 => format!("\\section{{{plain}}}"),
            2 => format!("\\subsection{{{plain}}}"),
            3 => format!("\\subsubsection{{{plain}}}"),
            _ => plain,
        };
        self.buffer
            .replace_range(&mut self.selections, start, end, &text);
        let at = start + text.chars().count();
        self.selections.set_single(Selection::cursor(at));
    }

    /// Puts the selected lines in an environment, each one an `\\item` if it's a list.
    fn wrap_environment(&mut self, name: &str, items: bool) {
        let primary = self.selections.primary();
        let rope = self.buffer.rope();
        let first = rope.char_to_line(primary.start());
        let last = rope.char_to_line(primary.end());
        let start = rope.line_to_char(first);
        let end = rope.line_to_char(last) + self.line_text(last).chars().count();
        let mut text = format!("\\begin{{{name}}}\n");
        for line in first..=last {
            let lead = if items { "  \\item " } else { "  " };
            text.push_str(&format!("{lead}{}\n", self.line_text(line).trim()));
        }
        text.push_str(&format!("\\end{{{name}}}"));
        self.buffer
            .replace_range(&mut self.selections, start, end, &text);
        let at = start + text.chars().count() - name.len() - 7;
        self.selections.set_single(Selection::cursor(at));
    }

    fn format_latex(&mut self, format: Format) {
        match format {
            Format::Bold => self.toggle_command("\\textbf{"),
            Format::Italic => self.toggle_command("\\emph{"),
            Format::Code => self.toggle_command("\\texttt{"),
            Format::Strikethrough => {
                // `\\sout` is ulem's, which a paper needs to build.
                self.ensure_package("[normalem]{ulem}", "{ulem}");
                self.toggle_command("\\sout{");
            }
            Format::Link => self.make_link("https://"),
            Format::Bullet | Format::Task => self.wrap_environment("itemize", true),
            Format::Numbered => self.wrap_environment("enumerate", true),
            Format::Quote => self.wrap_environment("quote", false),
            Format::CodeBlock => self.wrap_environment("verbatim", false),
            Format::Level(level) => self.set_section(level),
            Format::Heading => self.set_section(1),
            Format::PlainText => self.set_section(0),
            Format::Rule => self.insert_block("\\bigskip\\hrule\\bigskip", 0),
            Format::Table => self.insert_table(3, 3),
        }
    }

    pub fn format(&mut self, format: Format, area: Rect) {
        self.prepare_edit(area);
        if self.dialect() == Dialect::Latex {
            self.format_latex(format);
            return;
        }
        match format {
            Format::Bold => self.toggle_inline("**"),
            Format::Italic => self.toggle_inline("*"),
            Format::Code => self.toggle_inline("`"),
            Format::Strikethrough => self.toggle_inline("~~"),
            Format::CodeBlock => self.fence_lines(),
            Format::Table => self.insert_table(3, 3),
            Format::Rule => self.insert_block("---", 3),
            Format::Link => self.make_link("https://"),
            _ => self.toggle_lines(format),
        }
    }

    /// Makes the word or selection a link to `address`.
    pub fn make_link(&mut self, address: &str) {
        let primary = self.selections.primary();
        let (start, end) = if primary.is_empty() {
            self.buffer.word_at(primary.head)
        } else {
            (primary.start(), primary.end())
        };
        let text = self.buffer.rope().slice(start..end).to_string();
        let link = if self.dialect() == Dialect::Latex {
            format!("\\href{{{address}}}{{{text}}}")
        } else {
            format!("[{text}]({address})")
        };
        self.buffer
            .replace_range(&mut self.selections, start, end, &link);
        let after = start + link.chars().count();
        self.selections.set_single(Selection::cursor(after));
    }

    /// Finds where each comment's quote is, for tinting it.
    pub fn set_quotes(&self, quotes: &[(usize, String)]) {
        let rope = self.buffer.rope();
        let mut ranges = Vec::new();
        for (line, quote) in quotes {
            let index = line.saturating_sub(1);
            if quote.is_empty() || index >= rope.len_lines() {
                continue;
            }
            let text = self.line_text(index);
            if let Some(at) = text.find(quote.as_str()) {
                let start = rope.line_to_char(index) + text[..at].chars().count();
                ranges.push((start, start + quote.chars().count()));
            }
        }
        *self.quoted.borrow_mut() = ranges;
    }

    /// Puts text in a block of its own below this line, with the caret `caret` chars in.
    pub fn insert_block(&mut self, block: &str, caret: usize) {
        let head = self.selections.primary().head;
        let rope = self.buffer.rope();
        let line = rope.char_to_line(head);
        let text = self.line_text(line);
        let at = rope.line_to_char(line) + text.chars().count();
        let gap = if text.trim().is_empty() { "" } else { "\n\n" };
        let insert = format!("{gap}{block}\n");
        self.buffer
            .replace_range(&mut self.selections, at, at, &insert);
        let spot = at + gap.chars().count() + caret;
        self.selections.set_single(Selection::cursor(spot));
    }

    /// Adds an image: a figure with a caption in a paper, a plain image in a note.
    pub fn insert_image(&mut self, path: &str) {
        if self.dialect() == Dialect::Latex {
            let figure = format!(
                "\\begin{{figure}}[h]\n  \\centering\n  \\includegraphics[width=\\linewidth]{{{path}}}\n  \\caption{{}}\n\\end{{figure}}"
            );
            let caret = figure.find("\\caption{}").unwrap_or(0) + "\\caption{".len();
            self.insert_block(&figure, caret);
        } else {
            self.insert_block(&format!("![]({path})"), 2);
        }
    }

    /// Writes this file, through its notebook if it has one.
    pub fn save(&mut self) -> io::Result<()> {
        match &self.notebook {
            Some(notebook) => {
                let written = notebook.write(&self.buffer.text());
                self.buffer.save_as(&written)
            }
            None => self.buffer.save(),
        }
    }

    /// Takes the file back from disk, through its notebook if it has one.
    pub fn reload(&mut self) -> io::Result<()> {
        let notebook = self.notebook.is_some();
        let read = self
            .buffer
            .path()
            .filter(|_| notebook)
            .map(std::fs::read_to_string);
        let Some(text) = read else {
            return self.buffer.reload(&mut self.selections);
        };
        let notebook = Notebook::read(&text?).map_err(io::Error::other)?;
        let page = notebook.page();
        self.notebook = Some(notebook);
        *self.page.borrow_mut() = None;
        self.buffer.reload_as(&mut self.selections, page)
    }

    /// The cell a source line is in, as its marker's line, what it says of itself and its
    /// code. None for a line that isn't in a code cell.
    pub fn code_cell_at(&self, line: usize) -> Option<(usize, Marker, String)> {
        self.notebook.as_ref()?;
        let last = self.buffer.rope().len_lines();
        let top = (0..=line.min(last.saturating_sub(1)))
            .rev()
            .find(|n| marker(&self.line_text(*n)).is_some())?;
        let mark = marker(&self.line_text(top)).filter(|m| m.code)?;
        let lines = (top + 1..last).map(|n| self.line_text(n));
        let lines: Vec<String> = lines.take_while(|text| marker(text).is_none()).collect();
        let open = lines.iter().position(|l| l.starts_with("```"))?;
        let close = lines.iter().rposition(|l| l.trim() == "```")?;
        (close > open).then(|| (top, mark, lines[open + 1..close].join("\n")))
    }

    /// Every code cell, top to bottom, as its number and code.
    pub fn code_cells(&self) -> Vec<(Marker, String)> {
        let lines = 0..self.buffer.rope().len_lines();
        let tops = lines.filter(|n| marker(&self.line_text(*n)).is_some_and(|m| m.code));
        let cells = tops.filter_map(|n| self.code_cell_at(n));
        cells.map(|(_, mark, code)| (mark, code)).collect()
    }

    /// Writes new cells into the file, so they have numbers of their own to run as.
    pub fn number_new_cells(&mut self) -> io::Result<()> {
        let head = self.selections.primary().head;
        self.save()?;
        self.reload()?;
        self.selections.set_single(Selection::cursor(head));
        Ok(())
    }

    /// Adds a cell after the one the caret is in, with the caret ready in it.
    pub fn insert_cell(&mut self, code: bool) {
        let language = self
            .notebook
            .as_ref()
            .map_or("python", |n| n.language.as_str());
        let cell = if code {
            cellix_doc::notebook::new_code_cell(language)
        } else {
            cellix_doc::notebook::new_text_cell()
        };
        let rope = self.buffer.rope();
        let line = rope.char_to_line(self.selections.primary().head);
        let next = (line + 1..rope.len_lines()).find(|n| marker(&self.line_text(*n)).is_some());
        let at = next.map_or(rope.len_chars(), |n| rope.line_to_char(n));
        let caret = if code {
            cell.find("\n\n").map_or(0, |gap| gap + 1)
        } else {
            cell.chars().count()
        };
        let lead = if at == rope.len_chars() && !self.buffer.text().ends_with("\n\n") {
            "\n"
        } else {
            ""
        };
        let block = format!("{lead}{cell}\n\n");
        self.buffer
            .replace_range(&mut self.selections, at, at, &block);
        let spot = at + lead.chars().count() + cell[..caret].chars().count();
        self.selections.set_single(Selection::cursor(spot));
    }

    /// Moves to the first line of the next code cell, if there is one.
    pub fn go_to_next_cell(&mut self, area: Rect) {
        let rope = self.buffer.rope();
        let line = rope.char_to_line(self.selections.primary().head);
        let next = (line + 1..rope.len_lines())
            .find(|n| marker(&self.line_text(*n)).is_some_and(|m| m.code));
        if let Some(next) = next {
            self.go_to_line(next + 3, area);
        }
    }

    /// What's wrong with this revision of the text.
    fn current_issues(&self) -> &[Issue] {
        if self.issues.0 == self.buffer.revision() {
            &self.issues.1
        } else {
            &[]
        }
    }

    /// What this document is written in, for the prose checker.
    pub fn prose_source(&self) -> Source {
        match self.dialect() {
            Dialect::Latex => Source::Latex,
            _ => Source::Markdown,
        }
    }

    /// The spelling or grammar issue at the caret.
    pub fn issue_at_caret(&self) -> Option<&Issue> {
        let head = self.selections.primary().head;
        let holds = |issue: &&Issue| issue.start <= head && head <= issue.end;
        self.current_issues().iter().find(holds)
    }

    /// The words of a range of the document.
    pub fn words_in(&self, start: usize, end: usize) -> String {
        self.buffer.rope().slice(start..end).to_string()
    }

    /// Puts a suggested word in place, and forgets the issue it settles.
    pub fn apply_fix(&mut self, start: usize, end: usize, text: &str) {
        self.buffer
            .replace_range(&mut self.selections, start, end, text);
        let at = start + text.chars().count();
        self.selections.set_single(Selection::cursor(at));
    }

    /// Takes a word off the list of what's wrong, wherever it stands.
    pub fn forget_issues(&mut self, word: &str) {
        let wanted = word.to_lowercase();
        let rope = self.buffer.rope().clone();
        self.issues.1.retain(|issue| {
            rope.slice(issue.start..issue.end)
                .to_string()
                .to_lowercase()
                != wanted
        });
    }

    fn syntax(&self) -> Syntax {
        match self.dialect() {
            Dialect::Latex => Syntax::Latex,
            _ => Syntax::Critic,
        }
    }

    fn changes(&self) -> Vec<Change> {
        changes::find(&self.buffer.text(), self.syntax())
    }

    /// The change the caret is in or right after.
    pub fn change_at_caret(&self) -> Option<Change> {
        let head = self.selections.primary().head;
        let holds = |c: &Change| c.start <= head && head <= c.end;
        self.changes().into_iter().find(holds)
    }

    /// Keeps or drops the change at the caret.
    pub fn settle_change(&mut self, accept: bool) {
        let Some(change) = self.change_at_caret() else {
            return;
        };
        let kept = if accept { &change.new } else { &change.old };
        self.buffer
            .replace_range(&mut self.selections, change.start, change.end, kept);
        let at = change.start + kept.chars().count();
        self.selections.set_single(Selection::cursor(at));
    }

    /// Keeps or drops every change, and says how many there were.
    pub fn settle_all_changes(&mut self, accept: bool) -> usize {
        let text = self.buffer.text();
        let count = changes::find(&text, self.syntax()).len();
        if count > 0 {
            let settled = changes::settle_all(&text, self.syntax(), accept);
            let total = self.buffer.rope().len_chars();
            self.buffer
                .replace_range(&mut self.selections, 0, total, &settled);
            let head = self.selections.primary().head.min(settled.chars().count());
            self.selections.set_single(Selection::cursor(head));
        }
        count
    }

    /// Adds `\\usepackage<what>` before `\\begin{document}`, unless `known` is already used.
    fn ensure_package(&mut self, what: &str, known: &str) {
        let text = self.buffer.text();
        if text.contains(known) {
            return;
        }
        let Some(at) = text.find("\\begin{document}") else {
            return;
        };
        let at = text[..at].chars().count();
        let mut selections = self.selections.clone();
        let line = format!("\\usepackage{what}\n");
        self.buffer.replace_range(&mut selections, at, at, &line);
        let length = line.chars().count();
        // The caret stays on the words it was on.
        let shifted: Vec<Selection> = self
            .selections
            .iter()
            .map(|s| Selection {
                anchor: s.anchor + length,
                head: s.head + length,
                goal: s.goal,
            })
            .collect();
        if let Some(first) = shifted.first() {
            self.selections.set_single(*first);
            for s in &shifted[1..] {
                self.selections.add(*s);
            }
        }
    }

    /// Turns tracking on or off. A paper needs the package that prints its marks.
    pub fn toggle_tracking(&mut self) -> bool {
        self.tracking = !self.tracking;
        let text = self.buffer.text();
        if self.tracking
            && self.dialect() == Dialect::Latex
            && !text.contains("{changes}")
            && let Some(at) = text.find("\\begin{document}")
        {
            let at = text[..at].chars().count();
            let mut selections = self.selections.clone();
            self.buffer
                .replace_range(&mut selections, at, at, "\\usepackage{changes}\n");
        }
        self.tracking
    }

    /// Where typing goes: a caret inside something crossed out stands after it.
    fn tracked_head(&self, changes: &[Change]) -> usize {
        let head = self.selections.primary().head;
        let deleted = changes
            .iter()
            .find(|c| c.kind == Kind::Delete && c.start < head && head < c.end);
        deleted.map_or(head, |c| c.end)
    }

    /// Takes out what's selected, as a suggestion if changes are being tracked.
    pub fn cut_selection(&mut self) {
        if self.tracking && self.rendered {
            self.tracked_delete(false);
        } else {
            self.buffer.replace(&mut self.selections, "");
        }
    }

    /// Pastes as a suggestion.
    pub fn tracked_paste(&mut self, text: &str) {
        self.tracked_type(text);
    }

    /// Adds text as a suggestion, joining the one the caret is at the end of.
    fn tracked_type(&mut self, text: &str) {
        if !self.selections.primary().is_empty() {
            self.tracked_delete(false);
            let changes = self.changes();
            let head = self.selections.primary().head;
            if let Some(change) = changes.iter().find(|c| c.start == head) {
                self.selections.set_single(Selection::cursor(change.end));
            }
        }
        let changes = self.changes();
        let head = self.tracked_head(&changes);
        let (open, close) = self.syntax().insertion();
        let closing = close.chars().count();
        let joins = changes
            .iter()
            .find(|c| c.kind == Kind::Insert && c.start < head && head <= c.end);
        let (at, typed) = match joins {
            Some(change) => (head.min(change.end - closing), text.to_string()),
            None => (head, format!("{open}{text}{close}")),
        };
        self.buffer
            .replace_range(&mut self.selections, at, at, &typed);
        let caret = match joins {
            Some(_) => at + text.chars().count(),
            None => at + open.chars().count() + text.chars().count(),
        };
        self.selections.set_single(Selection::cursor(caret));
    }

    /// Crosses out what's selected or next to the caret. Words the suggestion added simply
    /// go, since nobody has seen them yet.
    fn tracked_delete(&mut self, forward: bool) {
        let primary = self.selections.primary();
        let changes = self.changes();
        let (open, close) = self.syntax().deletion();
        if !primary.is_empty() {
            let text = self
                .buffer
                .rope()
                .slice(primary.start()..primary.end())
                .to_string();
            let lines: Vec<String> = text
                .split('\n')
                .map(|line| {
                    if line.is_empty() {
                        String::new()
                    } else {
                        format!("{open}{line}{close}")
                    }
                })
                .collect();
            let start = primary.start();
            self.buffer.replace_range(
                &mut self.selections,
                start,
                primary.end(),
                &lines.join("\n"),
            );
            self.selections.set_single(Selection::cursor(start));
            return;
        }
        let head = self.selections.primary().head;
        let total = self.buffer.rope().len_chars();
        // Step over what's already crossed out, as the page shows it as gone.
        let settled = changes.iter().find(|c| {
            c.kind == Kind::Delete
                && if forward {
                    c.start <= head && head < c.end
                } else {
                    c.start < head && head <= c.end
                }
        });
        let head = settled.map_or(head, |c| if forward { c.end } else { c.start });
        let at = if forward {
            (head < total).then_some(head)
        } else {
            head.checked_sub(1)
        };
        let Some(at) = at else {
            return;
        };
        let (added_open, added_close) = self.syntax().insertion();
        let added = changes.iter().find(|c| {
            let inner =
                (c.start + added_open.chars().count())..(c.end - added_close.chars().count());
            c.kind == Kind::Insert && (inner.contains(&at) || (!forward && head == c.end))
        });
        if let Some(change) = added {
            // The last of a suggestion takes its markers with it.
            if change.new.chars().count() <= 1 {
                self.buffer
                    .replace_range(&mut self.selections, change.start, change.end, "");
                self.selections.set_single(Selection::cursor(change.start));
                return;
            }
            let inner_end = change.end - added_close.chars().count();
            let at = if head == change.end {
                inner_end - 1
            } else {
                at
            };
            self.buffer
                .replace_range(&mut self.selections, at, at + 1, "");
            self.selections.set_single(Selection::cursor(at));
            return;
        }
        let Some(gone) = self.char_at(at) else {
            return;
        };
        if gone == '\n' {
            self.buffer
                .replace_range(&mut self.selections, at, at + 1, "");
            self.selections.set_single(Selection::cursor(at));
            return;
        }
        // Next to a crossing out, it joins that one.
        let opening = open.chars().count();
        if let (false, Some(change)) = (
            forward,
            changes
                .iter()
                .find(|c| c.kind == Kind::Delete && c.start == head),
        ) {
            let joined = format!("{open}{gone}");
            self.buffer
                .replace_range(&mut self.selections, at, change.start + opening, &joined);
            self.selections.set_single(Selection::cursor(at));
            return;
        }
        let crossed = format!("{open}{gone}{close}");
        self.buffer
            .replace_range(&mut self.selections, at, at + 1, &crossed);
        let caret = if forward {
            at + crossed.chars().count()
        } else {
            at
        };
        self.selections.set_single(Selection::cursor(caret));
    }

    /// The line that names the image the caret is on. A paper's figure counts as a whole.
    fn image_line_at_caret(&self) -> Option<usize> {
        let head = self.selections.primary().head;
        let rope = self.buffer.rope();
        let line = rope.char_to_line(head);
        if self.dialect() == Dialect::Markdown {
            return cellix_doc::figure::image_line(&self.line_text(line)).map(|_| line);
        }
        let names = |n: &usize| self.line_text(*n).contains("\\includegraphics");
        if names(&line) {
            return Some(line);
        }
        let opens = |n: &usize| self.line_text(*n).contains("\\begin{figure");
        let closes = |n: &usize| self.line_text(*n).contains("\\end{figure");
        let first = (0..=line).rev().find(opens)?;
        let last = (line..rope.len_lines()).find(closes)?;
        (first..=last).find(names)
    }

    pub fn image_at_caret(&self) -> bool {
        self.image_line_at_caret().is_some()
    }

    /// Puts a new line in place of the image's, leaving the caret where the line starts so
    /// the picture shows again.
    fn rewrite_image(&mut self, line: usize, text: &str) {
        let start = self.buffer.rope().line_to_char(line);
        let end = start + self.line_text(line).chars().count();
        self.buffer
            .replace_range(&mut self.selections, start, end, text);
        self.selections.set_single(Selection::cursor(start));
    }

    /// Sets how much of the page the image at the caret takes, in percent.
    pub fn set_image_width(&mut self, percent: u32) {
        let Some(line) = self.image_line_at_caret() else {
            return;
        };
        let text = self.line_text(line);
        let sized = if self.dialect() == Dialect::Latex {
            cellix_doc::figure::set_graphics_width(&text, percent)
        } else {
            cellix_doc::figure::set_width(&text, percent)
        };
        if let Some(sized) = sized {
            self.rewrite_image(line, &sized);
        }
    }

    /// Shows another file in the image's place, keeping its words and size.
    pub fn replace_image(&mut self, path: &str) {
        let Some(line) = self.image_line_at_caret() else {
            return;
        };
        let text = self.line_text(line);
        let replaced = if self.dialect() == Dialect::Latex {
            cellix_doc::figure::set_graphics_path(&text, path)
        } else {
            cellix_doc::figure::set_path(&text, path)
        };
        if let Some(replaced) = replaced {
            self.rewrite_image(line, &replaced);
        }
    }

    /// Takes the image out, with the figure around it in a paper.
    pub fn remove_image(&mut self) {
        let Some(line) = self.image_line_at_caret() else {
            return;
        };
        let rope = self.buffer.rope();
        let (mut first, mut last) = (line, line);
        if self.dialect() == Dialect::Latex {
            let opens = |n: &usize| self.line_text(*n).contains("\\begin{figure");
            let closes = |n: &usize| self.line_text(*n).contains("\\end{figure");
            if let (Some(a), Some(b)) = (
                (0..=line).rev().find(opens),
                (line..rope.len_lines()).find(closes),
            ) {
                (first, last) = (a, b);
            }
        }
        let start = rope.line_to_char(first);
        let end = rope.line_to_char((last + 1).min(rope.len_lines() - 1));
        let end = end.max(start + self.line_text(last).chars().count());
        self.buffer
            .replace_range(&mut self.selections, start, end, "");
        self.selections.set_single(Selection::cursor(start));
    }

    /// Adds a diagram below this line, with the caret in its source to go on writing.
    pub fn insert_diagram(&mut self, language: &str, source: &str) {
        let block = format!("```{language}\n{source}\n```");
        self.insert_block(&block, language.chars().count() + 4);
    }

    /// Types text where the caret is.
    pub fn insert_text(&mut self, text: &str) {
        self.buffer.replace(&mut self.selections, text);
    }

    /// Whether the caret is inside a formula showing as source.
    pub fn in_formula(&self) -> bool {
        self.page
            .borrow()
            .as_ref()
            .is_some_and(|p| p.open.is_some())
    }

    /// Inserts a symbol or template. Inside a formula it goes in where the caret is, and
    /// anywhere else it starts a new formula.
    pub fn insert_math(&mut self, symbol: &Symbol) {
        let display = if self.dialect() == Dialect::Latex {
            ("\\begin{equation}\n", "\n\\end{equation}")
        } else {
            ("$$\n", "\n$$")
        };
        let (open, close, block) = match (self.in_formula(), symbol.block) {
            (true, _) => ("", "", false),
            (false, true) => (display.0, display.1, true),
            (false, false) => ("$", "$", false),
        };
        let chars = symbol.text.chars().count();
        let caret = symbol.caret.min(chars) + open.chars().count();
        // What's selected goes in the template's first slot, rather than being replaced.
        let selected = self.selected_text().unwrap_or_default();
        let slot = symbol.caret < chars && !selected.is_empty();
        let body = if slot {
            let at: String = symbol.text.chars().take(symbol.caret).collect();
            let rest: String = symbol.text.chars().skip(symbol.caret).collect();
            format!("{at}{selected}{rest}")
        } else {
            symbol.text.to_string()
        };
        let text = format!("{open}{body}{close}");
        if block {
            self.insert_block(&text, caret);
            return;
        }
        let primary = self.selections.primary();
        let (start, end) = (primary.start(), primary.end());
        self.buffer
            .replace_range(&mut self.selections, start, end, &text);
        self.selections.set_single(Selection::cursor(start + caret));
    }

    /// The `tabular` the caret is in, read the same way as a Markdown table.
    fn tabular_at_caret(&self) -> Option<(usize, usize, usize, usize)> {
        let head = self.selections.primary().head;
        let rope = self.buffer.rope();
        let line = rope.char_to_line(head);
        let first = (0..=line)
            .rev()
            .find(|n| self.line_text(*n).contains("\\begin{tabular}"))?;
        let last =
            (line..rope.len_lines()).find(|n| self.line_text(*n).contains("\\end{tabular}"))?;
        let row = (first..line)
            .filter(|n| self.line_text(*n).contains("\\\\"))
            .count();
        let before: String = self
            .line_text(line)
            .chars()
            .take(head - rope.line_to_char(line))
            .collect();
        Some((first, last, row, before.matches('&').count()))
    }

    /// The table the caret is in, as its first and last lines and the caret's row and
    /// column, with the header as row zero and the rule left out.
    pub fn table_at_caret(&self) -> Option<(usize, usize, usize, usize)> {
        if self.dialect() == Dialect::Latex {
            return self.tabular_at_caret();
        }
        if let Dialect::Data(separator) = self.dialect() {
            let head = self.selections.primary().head;
            let rope = self.buffer.rope();
            let line = rope.char_to_line(head);
            let last = (0..rope.len_lines())
                .rev()
                .find(|n| !self.line_text(*n).is_empty())
                .unwrap_or(0);
            let before = rope.slice(rope.line_to_char(line)..head).to_string();
            return Some((
                0,
                last,
                line,
                cellix_doc::table::field_of(&before, separator),
            ));
        }
        let head = self.selections.primary().head;
        let rope = self.buffer.rope();
        let line = rope.char_to_line(head);
        let has_pipe = |n: usize| n < rope.len_lines() && self.line_text(n).contains('|');
        if !has_pipe(line) {
            return None;
        }
        let first = (0..=line)
            .rev()
            .take_while(|n| has_pipe(*n))
            .last()
            .unwrap_or(line);
        let last = (line..rope.len_lines())
            .take_while(|n| has_pipe(*n))
            .last()
            .unwrap_or(line);
        let rule = self.line_text(first + 1);
        let is_rule = |c: char| "|-: ".contains(c);
        if last == first || !rule.contains('-') || !rule.chars().all(is_rule) {
            return None;
        }
        let column = head - rope.line_to_char(line);
        let before: String = self.line_text(line).chars().take(column).collect();
        let pipes = before.matches('|').count().saturating_sub(1);
        let row = match line - first {
            0 => 0,
            1 => 1,
            below => below - 1,
        };
        Some((first, last, row, pipes))
    }

    /// Changes the table the caret is in, and leaves the caret in the cell that matters.
    pub fn edit_table(&mut self, edit: TableEdit) {
        let Some((first, last, row, column)) = self.table_at_caret() else {
            return;
        };
        let rope = self.buffer.rope();
        let start = rope.line_to_char(first);
        let end = rope.line_to_char(last) + self.line_text(last).chars().count();
        let text = rope.slice(start..end).to_string();
        if let Dialect::Data(separator) = self.dialect() {
            if let Some((table, at)) =
                cellix_doc::table::edit_rows(&text, separator, row, column, edit)
            {
                self.buffer
                    .replace_range(&mut self.selections, start, end, &table);
                self.selections.set_single(Selection::cursor(start + at));
            }
            return;
        }
        if self.dialect() == Dialect::Latex {
            let table = cellix_doc::latex::edit_tabular(&text, row, column, edit);
            let table = table.unwrap_or_default();
            self.buffer
                .replace_range(&mut self.selections, start, end, &table);
            let row = row + usize::from(edit == TableEdit::RowBelow);
            let column = column + usize::from(edit == TableEdit::ColumnRight);
            let at = cellix_doc::latex::tabular_cell_offset(&table, row, column);
            self.selections.set_single(Selection::cursor(start + at));
            return;
        }
        match cellix_doc::table::apply(&text, row, column, edit) {
            Some((table, row, column)) => {
                self.buffer
                    .replace_range(&mut self.selections, start, end, &table);
                let at = start + cellix_doc::table::cell_offset(&table, row, column);
                self.selections.set_single(Selection::cursor(at));
            }
            None => {
                // Whatever's after the table closes up behind it.
                let total = self.buffer.rope().len_chars();
                self.buffer
                    .replace_range(&mut self.selections, start, (end + 1).min(total), "");
                self.selections.set_single(Selection::cursor(start));
            }
        }
    }

    /// What kind of paragraph the caret is in, and which formats it's already in.
    pub fn caret_formats(&self) -> (&'static str, Vec<Format>) {
        let head = self.selections.primary().head;
        let rope = self.buffer.rope();
        let line = self.line_text(rope.char_to_line(head));
        let hashes = line.chars().take_while(|c| *c == '#').count();
        let trimmed = line.trim_start();
        let digits = trimmed.chars().take_while(char::is_ascii_digit).count();
        let mut active = Vec::new();
        if trimmed.starts_with("- [") {
            active.push(Format::Task);
        } else if trimmed.starts_with("- ") || trimmed.starts_with("* ") {
            active.push(Format::Bullet);
        } else if digits > 0 && trimmed[digits..].starts_with(". ") {
            active.push(Format::Numbered);
        } else if trimmed.starts_with("> ") {
            active.push(Format::Quote);
        }
        let cache = self.page.borrow();
        let rows = cache.as_ref().map(|p| p.rows.clone());
        let at = rows.iter().flat_map(|r| r.iter()).find_map(|row| {
            row.glyphs
                .iter()
                .find(|g| g.at == Some(head) || g.at.map(|a| a + 1) == Some(head))
        });
        if let Some(glyph) = at {
            if glyph.font.bold && hashes == 0 {
                active.push(Format::Bold);
            }
            if glyph.font.italic {
                active.push(Format::Italic);
            }
            if glyph.tone == Tone::Code {
                active.push(Format::Code);
            }
            if glyph.tone == Tone::Link {
                active.push(Format::Link);
            }
        }
        let style = match hashes {
            1 => "Heading 1",
            2 => "Heading 2",
            3 => "Heading 3",
            4 => "Heading 4",
            5 => "Heading 5",
            6 => "Heading 6",
            _ => "Text",
        };
        (style, active)
    }

    /// Puts the selected lines in a fenced block, with the caret on its first line.
    fn fence_lines(&mut self) {
        let primary = self.selections.primary();
        let rope = self.buffer.rope();
        let first = rope.char_to_line(primary.start());
        let last = rope.char_to_line(primary.end());
        let start = rope.line_to_char(first);
        let end = rope.line_to_char(last) + self.line_text(last).chars().count();
        let inner = rope.slice(start..end).to_string();
        let text = format!("```\n{inner}\n```");
        self.buffer
            .replace_range(&mut self.selections, start, end, &text);
        self.selections.set_single(Selection::cursor(start + 4));
    }

    /// Adds a new table below this line, with its first heading selected to type over.
    pub fn insert_table(&mut self, rows: usize, columns: usize) {
        if self.dialect() == Dialect::Latex {
            let table = cellix_doc::latex::new_table(rows, columns);
            let first = table.find("Column 1").unwrap_or(0);
            self.insert_block(&table, first);
            return;
        }
        let table = cellix_doc::table::new_table(rows, columns);
        self.insert_block(&table, 2);
        let head = self.selections.primary().head;
        self.selections.set_single(Selection {
            anchor: head,
            head: head + "Column 1".len(),
            goal: None,
        });
    }

    /// Selects the word at a place, as a double click does.
    pub fn select_word_at(&mut self, at: usize) {
        let (start, end) = self.buffer.word_at(at);
        self.selections.set_single(Selection {
            anchor: start,
            head: end,
            goal: None,
        });
    }

    /// Selects the whole line at a place, as a triple click does.
    pub fn select_line_at(&mut self, at: usize) {
        let rope = self.buffer.rope();
        let line = rope.char_to_line(at);
        let start = rope.line_to_char(line);
        let end = if line + 1 < rope.len_lines() {
            rope.line_to_char(line + 1)
        } else {
            rope.len_chars()
        };
        self.selections.set_single(Selection {
            anchor: start,
            head: end,
            goal: None,
        });
    }

    /// Whether this file has a page to show, whichever view it's in.
    pub fn has_page(&self) -> bool {
        self.buffer.path().is_some_and(|p| Dialect::of(p).is_some())
    }

    pub fn selected_text(&self) -> Option<String> {
        let rope = self.buffer.rope();
        let parts: Vec<String> = self
            .selections
            .iter()
            .filter(|s| !s.is_empty())
            .map(|s| rope.slice(s.start()..s.end()).to_string())
            .collect();
        (!parts.is_empty()).then(|| parts.join("\n"))
    }

    pub fn draw(
        &self,
        grid: &mut Grid,
        area: Rect,
        theme: &Theme,
        focused: bool,
        registry: &Registry,
        marks: &Marks,
    ) {
        let full = area;
        let area = self.text_area(full);
        self.view_rows.set(area.h);
        self.view_width.set(area.w);
        let dark = !theme.is_light();
        if self.dark.replace(dark) != dark {
            *self.page.borrow_mut() = None;
            *self.library.borrow_mut() = Library::default();
        }
        if self.rendered {
            self.draw_page(grid, area, theme, focused, marks);
            if self.shows_minimap(full) {
                self.draw_minimap(grid, full, theme);
            } else {
                let track = Rect::new(area.right(), area.y, 1, area.h);
                self.draw_scrollbar(grid, track, self.page(area).len(), theme);
            }
            return;
        }
        let lines = self.buffer.rope().len_lines();
        let track = Rect::new(area.right(), area.y, 1, area.h);
        if !self.shows_minimap(full) {
            self.draw_scrollbar(grid, track, lines, theme);
        }
        grid.fill(area, theme.text);
        let gutter = self.gutter_width(area.w);
        let rope = self.buffer.rope();
        let current = rope.char_to_line(self.selections.primary().head);
        for row in 0..area.h {
            let line = self.top + row as usize;
            if line >= rope.len_lines() {
                break;
            }
            let y = area.y + row;
            let active = focused && line == current;
            let bg = if active { theme.line } else { theme.text.bg };
            let text_style = cellix_render::Style { bg, ..theme.text };
            grid.fill(Rect::new(area.x, y, area.w, 1), text_style);
            let number_fg = if marks.errors.contains(&(line + 1)) {
                theme.error
            } else if active {
                theme.number_active
            } else {
                theme.gutter.fg
            };
            let gutter_style = cellix_render::Style {
                fg: number_fg,
                bg,
                ..theme.gutter
            };
            let number = format!("{:>w$} ", line + 1, w = gutter as usize - 1);
            grid.put_str(area.x, y, &number, gutter_style, area.x + gutter);
            if marks.comments.contains(&(line + 1)) {
                let mark = cellix_render::Style {
                    fg: theme.accent,
                    bg,
                    ..theme.gutter
                };
                grid.put_str(area.x, y, "◆", mark, area.x + gutter);
            }

            let mut column = 0;
            let mut text = String::new();
            for c in self.line_text(line).chars() {
                let width = display_width(c, column);
                if column + width > self.left {
                    if c == '\t' {
                        text.extend(std::iter::repeat_n(' ', width));
                    } else {
                        text.push(c);
                    }
                }
                column += width;
            }
            grid.put_str(area.x + gutter, y, &text, text_style, area.right());
            let guide = cellix_render::Style {
                fg: theme.guide,
                bg,
                ..theme.text
            };
            let indent = text.chars().take_while(|c| *c == ' ').count();
            for at in (0..indent).step_by(TAB_WIDTH) {
                let x = area.x + gutter + at as u16;
                if x < area.right() {
                    grid.put_str(x, y, "│", guide, area.right());
                }
            }
        }
        self.draw_highlights(grid, area, gutter, theme, registry);
        if self.shows_minimap(full) {
            self.draw_minimap(grid, full, theme);
        }
        for (line, change) in &self.line_changes().1 {
            if *line >= self.top && *line < self.top + area.h as usize {
                let y = area.y + (*line - self.top) as u16;
                self.draw_change(grid, area.x as f32 + gutter as f32 - 0.7, y, *change, theme);
            }
        }
        for (start, end) in self.found.iter().copied() {
            let s = Selection {
                anchor: start,
                head: end,
                goal: None,
            };
            self.draw_range(grid, area, gutter, &s, theme.found);
        }
        for s in self.selections.iter().filter(|s| !s.is_empty()) {
            self.draw_range(grid, area, gutter, s, theme.selection);
        }
        if focused {
            let head = self.selections.primary().head;
            let line = rope.char_to_line(head);
            let column = self.column_of(head);
            if line >= self.top && column >= self.left {
                let x = area.x as usize + gutter as usize + column - self.left;
                let y = area.y as usize + line - self.top;
                if x < area.right() as usize && y < area.bottom() as usize {
                    grid.cursor = Some((x as u16, y as u16));
                }
            }
        }
    }

    fn draw_scrollbar(&self, grid: &mut Grid, track: Rect, lines: usize, theme: &Theme) {
        grid.fill(track, theme.text);
        let rows = track.h as usize;
        if lines <= rows {
            return;
        }
        let height = (rows * rows / lines).max(1);
        let top = self.top * rows / lines;
        // A slim rounded thumb on the pixel tiers, and a block of colour on cells.
        let cells = Rect::new(track.x, track.y + top as u16, 1, height as u16);
        let thumb = cellix_render::Style {
            fg: theme.scrollbar,
            painted: true,
            ..theme.text
        };
        for y in cells.y..cells.bottom() {
            grid.put_str(cells.x, y, "▐", thumb, cells.right());
        }
        let bounds =
            cellix_render::Bounds::cells(cells.x as f32 + 0.3, cells.y as f32, 0.5, cells.h as f32);
        grid.panel(bounds, 20, Some(theme.scrollbar), None);
    }

    /// Backgrounds of code blocks and tables, drawn before the text on top of them.
    fn draw_page_blocks(&self, grid: &mut Grid, area: Rect, rows: &[Row], theme: &Theme) {
        let left = (area.x + Self::page_margin(area)) as f32;
        let visible = self.top..(self.top + area.h as usize).min(rows.len());
        let y_of = |index: usize| area.y as f32 + (index - self.top) as f32;
        let mut index = visible.start;
        while index < visible.end {
            let row = &rows[index];
            let group =
                |r: &Row| (row.shaded && r.shaded) || (row.table.is_some() && r.table.is_some());
            if !group(row) {
                index += 1;
                continue;
            }
            let end = (index..visible.end)
                .find(|i| !group(&rows[*i]))
                .unwrap_or(visible.end);
            let (top, height) = (y_of(index), (end - index) as f32);
            if row.shaded {
                let width = area.right() as f32 - left - 1.0;
                let bounds = Bounds::cells(left, top - 0.08, width, height + 0.2);
                grid.panel(bounds, 22, Some(theme.code_bg), Some(theme.border));
            } else if let Some(table) = &row.table {
                let width = table.columns.last().copied().unwrap_or(0.0);
                let bounds = Bounds::cells(left, top + 0.02, width, height - 0.04);
                if table.header {
                    let head = Bounds::cells(left, top + 0.02, width, 1.0);
                    grid.panel(head, 14, Some(theme.code_bg), None);
                }
                for at in index + 1..end {
                    let line = Bounds::cells(left, y_of(at), width, 0.0);
                    grid.hairline(line, theme.popup_border);
                }
                let inner = table.columns.len().saturating_sub(2);
                for x in table.columns.iter().skip(1).take(inner) {
                    let line = Bounds::cells(left + x, top + 0.02, 0.0, height - 0.04);
                    grid.hairline(line, theme.popup_border);
                }
                grid.panel(bounds, 14, None, Some(theme.popup_border));
            }
            index = end;
        }
    }

    fn draw_page(&self, grid: &mut Grid, area: Rect, theme: &Theme, focused: bool, marks: &Marks) {
        grid.fill(area, theme.text);
        let rows = self.page(area);
        let proportional = !self.measure.in_cells();
        let left = area.x + Self::page_margin(area);
        let primary = self.selections.primary();
        let (caret_row, caret_x) = rendered::locate(&rows, primary.head);
        if proportional {
            self.draw_page_blocks(grid, area, &rows, theme);
            self.draw_figures(grid, area, &rows, theme);
        }
        let changes = self.line_changes();
        for (index, row) in rows.iter().enumerate().skip(self.top) {
            let y = area.y + (index - self.top) as u16;
            if y >= area.bottom() {
                break;
            }
            let bg = if row.shaded && !proportional {
                theme.code_bg
            } else {
                theme.text.bg
            };
            if !proportional {
                grid.fill(
                    Rect::new(area.x, y, area.w, 1),
                    cellix_render::Style { bg, ..theme.text },
                );
            }
            // A tall heading's spacer shares its line, and takes no mark.
            let mut earlier = rows[..index]
                .iter()
                .rev()
                .take_while(|r| r.line == row.line);
            let first_row = row.stop && !earlier.any(|r| r.stop);
            if first_row && let Some(change) = changes.1.get(&row.line) {
                // Beside a tall heading's text, which sits in the row below its own.
                let mark = if row.tall { y + 1 } else { y };
                self.draw_change(grid, left as f32 - 1.4, mark, *change, theme);
            }
            if first_row && marks.comments.contains(&(row.line + 1)) {
                let mark = cellix_render::Style {
                    fg: theme.accent,
                    bg,
                    ..theme.text
                };
                grid.put_str(area.x + 1, y, "◆", mark, left);
            }
            // Xcode shows a heading's level in the margin, beside the caret.
            let commented = marks.comments.contains(&(row.line + 1));
            if proportional && focused && index == caret_row && row.heading > 0 && !commented {
                let dim = cellix_render::Style {
                    fg: theme.gutter.fg,
                    ..theme.text
                };
                self.draw_heading_mark(grid, row, (area.x, left, y), dim.fg);
            }
            if row.rule && proportional {
                let line = Bounds::cells(left as f32, y as f32 + 0.5, 60.0, 0.0);
                grid.hairline(line, theme.popup_border);
            }
            if row.quote {
                // Whole rows, so a block's bar runs on unbroken from row to row.
                let bar = Bounds::cells(left as f32 + 0.1, y as f32, 0.3, 1.0);
                let colour = row.callout.map(|kind| callout_colour(kind, theme));
                grid.panel(bar, 0, Some(colour.unwrap_or(theme.popup_border)), None);
            }
            // A tall heading on the last row would reach into the status bar, which paint
            // can't be clipped by, so it waits until it fits.
            if !(proportional && row.tall && y + 2 >= area.bottom()) {
                self.draw_page_row(grid, row, (left, y), area, bg, theme);
            }
        }
        if focused && primary.is_empty() && caret_row >= self.top {
            let y = area.y as usize + caret_row - self.top;
            let x = left as f32 + caret_x;
            self.draw_preview(grid, area, (x, y as f32), theme);
            if y < area.bottom() as usize && x < area.right() as f32 {
                let row = &rows[caret_row];
                let size = row.glyphs.first().map_or(100, |g| g.font.size) as f32 / 100.0;
                grid.set_caret(x, y as u16, (0.62 * size).min(1.6));
                if row.tall
                    && let Some(caret) = &mut grid.caret
                {
                    caret.y += units(TALL_DROP);
                }
            }
        }
    }

    /// Shows what the formula under the caret looks like, just below its line.
    fn draw_preview(&self, grid: &mut Grid, area: Rect, (x, y): (f32, f32), theme: &Theme) {
        let cache = self.page.borrow();
        let Some(page) = cache.as_ref() else {
            return;
        };
        let Some(figure) = page.preview.and_then(|index| page.figures.get(index)) else {
            return;
        };
        const PADDING: (f32, f32) = (1.0, 0.3);
        let (width, height) = (
            figure.width + PADDING.0 * 2.0,
            figure.height + PADDING.1 * 2.0,
        );
        let left = x.min(area.right() as f32 - width - 1.0).max(area.x as f32);
        let top = y + 1.2;
        let clip = Bounds::cells(area.x as f32, area.y as f32, area.w as f32, area.h as f32);
        grid.popup(
            Bounds::cells(left, top, width, height),
            22,
            theme.popup,
            theme.popup_border,
        );
        grid.paint.push(Paint::Image {
            bounds: Bounds::cells(
                left + PADDING.0,
                top + PADDING.1,
                figure.width,
                figure.height,
            ),
            clip,
            bitmap: figure.bitmap.clone(),
            color: theme.text.fg,
        });
    }

    /// The small `#` beside a heading, sitting on the heading's baseline.
    fn draw_heading_mark(
        &self,
        grid: &mut Grid,
        row: &Row,
        (x, left, y): (u16, u16, u16),
        color: Rgb,
    ) {
        /// How far a font's baseline sits below the middle of its line, in font sizes.
        const BASELINE: f32 = 0.35;
        const MARK_SIZE: u8 = 85;
        let size = row.glyphs.first().map_or(100, |g| g.font.size);
        let scale = (size as f32 - MARK_SIZE as f32) / 100.0;
        let drop = BASELINE * INTERFACE_SIZE * scale;
        let top = y as f32 + if row.tall { TALL_DROP } else { 0.0 } + drop;
        let font = Font {
            size: MARK_SIZE,
            ..Font::default()
        };
        grid.text_run(
            (x as f32 + 0.7, top),
            left as f32,
            "#",
            (color, false),
            font,
        );
    }

    fn draw_page_row(
        &self,
        grid: &mut Grid,
        row: &Row,
        (left, y): (u16, u16),
        area: Rect,
        bg: Rgb,
        theme: &Theme,
    ) {
        let proportional = !self.measure.in_cells();
        let origin = left as f32;
        let fg_of = |tone: Tone| match tone {
            Tone::Text | Tone::Block(None) => theme.text.fg,
            Tone::Heading => theme.heading,
            Tone::Code => theme.string,
            Tone::Block(Some(token)) => theme.token(token),
            Tone::Link | Tone::Check(_) => theme.accent,
            Tone::Dim | Tone::TableLine => theme.gutter.fg,
            Tone::Formula { .. } | Tone::Source | Tone::Marked => theme.text.fg,
            Tone::Struck | Tone::Raised | Tone::Lowered => theme.text.fg,
            Tone::Footnote => theme.accent,
            Tone::Callout(kind) => callout_colour(kind, theme),
            Tone::Inserted => theme.inserted(),
            Tone::Deleted | Tone::Failed => theme.deleted(),
            Tone::Margin(_) => theme.gutter.fg,
        };
        let selected = |at: Option<usize>| {
            at.is_some_and(|at| {
                self.selections
                    .iter()
                    .any(|s| at >= s.start() && at < s.end())
            })
        };
        let found = |at: Option<usize>| {
            at.is_some_and(|at| self.found.iter().any(|f| at >= f.0 && at < f.1))
        };
        let quoted = |at: Option<usize>| {
            at.is_some_and(|at| self.quoted.borrow().iter().any(|q| at >= q.0 && at < q.1))
        };
        let mark = |at: Option<usize>| {
            if selected(at) {
                Some(theme.selection)
            } else if found(at) {
                Some(theme.found)
            } else {
                quoted(at).then_some(theme.pick)
            }
        };
        let issues = self.current_issues();
        let flagged = |at: Option<usize>| {
            at.is_some_and(|at| issues.iter().any(|i| at >= i.start && at < i.end))
        };
        // Cells carry the text for the cell tier, and for tests. Pixels come from paint.
        for g in &row.glyphs {
            let style = cellix_render::Style {
                fg: fg_of(g.tone),
                bg: mark(g.at).filter(|_| !proportional).unwrap_or(bg),
                bold: g.font.bold,
                italic: g.font.italic,
                underline: g.underline || flagged(g.at),
                painted: proportional,
            };
            let x = left + g.x as u16;
            grid.put_str(x, y, g.ch.encode_utf8(&mut [0; 4]), style, area.right());
        }
        if !proportional {
            return;
        }
        let text_y = y as f32 + if row.tall { TALL_DROP } else { 0.0 };
        let right = area.right() as f32;
        for issue in issues {
            let inside = |g: &&Glyph| g.at.is_some_and(|at| at >= issue.start && at < issue.end);
            let mut word = row.glyphs.iter().filter(inside);
            let Some(first) = word.next() else {
                continue;
            };
            let last = word.next_back().unwrap_or(first);
            let x = origin + first.x;
            let width = last.x + last.advance - first.x;
            let colour = if issue.spelling {
                theme.deleted()
            } else {
                theme.accent
            };
            // Dots, since a solid line means a link, or text a change added.
            const DOT: f32 = 0.18;
            const STEP: f32 = 0.4;
            for dot in 0..(width / STEP).ceil() as usize {
                let spot = Bounds::cells(x + dot as f32 * STEP, text_y + 0.9, DOT, 0.07);
                grid.panel(spot, 0, Some(colour), None);
            }
        }
        let mut i = 0;
        while i < row.glyphs.len() {
            let g = row.glyphs[i];
            let same = |o: &Glyph| {
                o.font == g.font
                    && o.tone == g.tone
                    && o.underline == g.underline
                    && mark(o.at) == mark(g.at)
                    && (g.tone != Tone::TableLine || o.x == g.x)
            };
            // A table's cells are apart, so a run stops at a jump in position.
            let mut end = i + 1;
            while end < row.glyphs.len() && same(&row.glyphs[end]) {
                let prior = row.glyphs[end - 1];
                if (prior.x + prior.advance - row.glyphs[end].x).abs() > 0.01 {
                    break;
                }
                end += 1;
            }
            let run = &row.glyphs[i..end];
            let text: String = run.iter().map(|g| g.ch).collect();
            let x = origin + g.x;
            let width: f32 = run.iter().map(|g| g.advance).sum();
            if let Some(color) = mark(g.at) {
                let bounds = Bounds::cells(x, text_y + 0.06, width, 0.88);
                grid.panel(bounds, 6, Some(color), None);
            }
            match g.tone {
                Tone::TableLine => {}
                Tone::Formula { index, .. } => {
                    if let Some(figure) = self.figures().get(index as usize) {
                        // Text sits on a baseline below the row's middle: stand on it.
                        let line = text_y + 0.5 + 0.35 * cellix_render::INTERFACE_SIZE;
                        let top = line - figure.baseline;
                        grid.paint.push(Paint::Image {
                            bounds: Bounds::cells(x, top, figure.width, figure.height),
                            clip: Bounds::cells(
                                area.x as f32,
                                area.y as f32,
                                area.w as f32,
                                area.h as f32,
                            ),
                            bitmap: figure.bitmap.clone(),
                            color: theme.text.fg,
                        });
                    }
                }
                Tone::Check(checked) => {
                    // Square on screen: a row is about twice as tall as a column is wide.
                    const SIDE: f32 = 0.56;
                    let cell = self.measure.cell_pixels().unwrap_or((1, 2));
                    let across = SIDE * cell.1 as f32 / cell.0.max(1) as f32;
                    let left = x + ((width - across) / 2.0).max(0.0);
                    let top = y as f32 + (1.0 - SIDE) / 2.0;
                    let bounds = Bounds::cells(left, top, across, SIDE);
                    let (fill, edge) = if checked {
                        (Some(theme.accent), Some(theme.accent))
                    } else {
                        (None, Some(theme.gutter.fg))
                    };
                    grid.panel(bounds, 22, fill, edge);
                    if checked {
                        grid.paint.push(cellix_render::Paint::Icon {
                            bounds,
                            icon: Icon::Check,
                            color: theme.button_text,
                        });
                    }
                }
                _ => {
                    if matches!(g.tone, Tone::Code | Tone::Source) {
                        let pill = Bounds::cells(x - 0.2, text_y + 0.14, width + 0.4, 0.72);
                        grid.panel(pill, 14, Some(theme.code_bg), None);
                    }
                    if g.tone == Tone::Marked {
                        let pill = Bounds::cells(x - 0.1, text_y + 0.1, width + 0.2, 0.8);
                        grid.panel(pill, 8, Some(theme.highlighter()), None);
                    }
                    let shift = match g.tone {
                        Tone::Raised | Tone::Footnote => -0.2,
                        Tone::Lowered => 0.2,
                        Tone::Margin(moved) => moved as f32 / 64.0,
                        _ => 0.0,
                    };
                    let inked = g.underline || g.tone == Tone::Inserted;
                    let color = (fg_of(g.tone), inked);
                    grid.text_run((x, text_y + shift), right, &text, color, g.font);
                    if matches!(g.tone, Tone::Struck | Tone::Deleted) {
                        let line = Bounds::cells(x, text_y + 0.5, width, 0.0);
                        grid.hairline(line, fg_of(g.tone));
                    }
                }
            }
            i = end;
        }
    }

    fn draw_highlights(
        &self,
        grid: &mut Grid,
        area: Rect,
        gutter: u16,
        theme: &Theme,
        registry: &Registry,
    ) {
        let Some(path) = self.buffer.path() else {
            return;
        };
        let revision = self.buffer.revision();
        let mut cache = self.spans.borrow_mut();
        if !cache.as_ref().is_some_and(|(r, _)| *r == revision) {
            let bytes = self.buffer.rope().len_bytes();
            if bytes <= MAX_INLINE_HIGHLIGHT {
                *cache = Some((revision, registry.highlight(path, &self.buffer.text())));
                self.spans_bytes.set(bytes);
            } else if self.asked.get() != revision {
                self.asked.set(revision);
                let worker = self.worker.get_or_init(Worker::default);
                worker.request(path, self.buffer.text(), revision);
            }
        }
        let Some((cached, spans)) = cache.as_ref() else {
            return;
        };
        let rope = self.buffer.rope();
        // Until the worker answers, text after the edit keeps its colours by moving with it.
        let moved = rope.len_bytes() as isize - self.spans_bytes.get() as isize;
        let edit = rope.char_to_byte(self.selections.primary().head.min(rope.len_chars()));
        let shifted: Vec<Span>;
        let spans = if *cached == revision || moved == 0 {
            spans
        } else {
            let shift = |at: usize| {
                if at >= edit.saturating_sub(moved.max(0) as usize) {
                    at.saturating_add_signed(moved).min(rope.len_bytes())
                } else {
                    at
                }
            };
            shifted = spans
                .iter()
                .map(|s| Span {
                    start: shift(s.start),
                    end: shift(s.end),
                    token: s.token,
                })
                .collect();
            &shifted
        };
        let last_line = (self.top + area.h as usize).min(rope.len_lines());
        let from = rope.line_to_byte(self.top);
        let to = rope.line_to_byte(last_line);
        let first = spans.partition_point(|s| s.end <= from);
        for span in spans[first..].iter().take_while(|s| s.start < to) {
            let fg = theme.token(span.token);
            let (mut at, end) = (rope.byte_to_char(span.start), rope.byte_to_char(span.end));
            while at < end {
                let line = rope.char_to_line(at);
                let line_end =
                    end.min(rope.line_to_char(line) + self.line_text(line).chars().count());
                if line >= self.top && line < last_line && at < line_end {
                    let (a, b) = (self.column_of(at), self.column_of(line_end));
                    let x_from = a.max(self.left) - self.left;
                    let x_to = b.max(self.left) - self.left;
                    let rect = Rect::new(
                        (area.x + gutter).saturating_add(x_from as u16),
                        area.y + (line - self.top) as u16,
                        (x_to - x_from) as u16,
                        1,
                    );
                    let clipped = Rect::new(
                        rect.x,
                        rect.y,
                        rect.w.min(area.right().saturating_sub(rect.x)),
                        1,
                    );
                    grid.set_style(clipped, |style| cellix_render::Style { fg, ..style });
                }
                let next = rope.len_lines().min(line + 1);
                if next == line + 1 && next < rope.len_lines() {
                    at = rope.line_to_char(next);
                } else {
                    break;
                }
            }
        }
    }

    fn draw_range(&self, grid: &mut Grid, area: Rect, gutter: u16, s: &Selection, bg: Rgb) {
        let rope = self.buffer.rope();
        let (first, last) = (rope.char_to_line(s.start()), rope.char_to_line(s.end()));
        for line in first.max(self.top)..=last {
            if line >= self.top + area.h as usize {
                break;
            }
            let start = rope.line_to_char(line);
            let from = self.column_of(s.start().max(start));
            let line_end = start + self.line_text(line).chars().count();
            let to = self.column_of(s.end().min(line_end));
            let x_from = (area.x + gutter) as usize + from.saturating_sub(self.left);
            let x_to = (area.x + gutter) as usize + to.saturating_sub(self.left);
            let y = area.y + (line - self.top) as u16;
            let rect = Rect::new(
                x_from as u16,
                y,
                (x_to - x_from.min(x_to)).max(usize::from(line < last)) as u16,
                1,
            );
            grid.set_style(rect, |style| cellix_render::Style { bg, ..style });
        }
    }
}