gpuikit 0.9.0

//! Markdown rendering for gpuikit
//!
//! This module provides markdown parsing and rendering using GPUI elements.
//! It supports CommonMark and GitHub Flavored Markdown.
//!
//! # Example
//!
//! ```
//! # use gpui::{Context, IntoElement, Render, Window, div, prelude::*};
//! use gpuikit::markdown::{MarkdownStyle, markdown};
//! # struct D;
//! # impl Render for D { fn render(&mut self, window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
//! # let _ =
//! // Simple usage - create markdown element inline
//! div().child(markdown("# Hello\n\nThis is **bold** text.", cx))
//! # ;
//!
//! // With custom style
//! div().child(
//!     markdown("# Hello", cx)
//!         .style(MarkdownStyle::new().code_font("Monaco"))
//! )
//! # }}
//! # let mut tcx = gpui::TestAppContext::single();
//! # tcx.update(gpuikit::init);
//! # let _ = tcx.add_window_view(|_, _| D);
//! ```

mod code_highlight;
mod elements;
mod inline_style;
mod parser;
mod selectable_text;
mod selection;
mod stitch;
mod style;

pub use code_highlight::normalize_language;
#[cfg(feature = "editor")]
pub use code_highlight::{
    CodeHighlightTheme, DEFAULT_DARK_THEME, DEFAULT_LIGHT_THEME, code_highlight_themes,
    init_code_highlighting, set_code_highlight_theme,
};
pub use elements::*;
pub use inline_style::*;
pub use parser::*;
pub use selectable_text::{RunRole, SelectableText};
pub use selection::{MarkdownSelection, SelectionPosition};
pub use stitch::preprocessing_available;
pub use style::*;

use crate::a11y::{A11y, Announce};
use crate::theme::{ActiveTheme, Themeable};
use gpui::{
    App, Context, ElementId, Entity, EntityId, IntoElement, ParentElement, Role, SharedString,
    Styled, Task, Window, div, prelude::*, rems,
};
use pulldown_cmark::{Alignment, Event, Tag, TagEnd};

/// The id of the element every run of one document hangs under.
///
/// gpui hashes an element's *whole* id path into an accessibility node id and
/// refuses duplicates, so run ids only need to be unique within a document if
/// the document itself is uniquely identified. Keyed on the entity so it is
/// also stable across frames: assistive technology reads a changed node id as
/// a different element.
fn document_element_id(entity_id: EntityId) -> ElementId {
    ElementId::NamedInteger("md-doc".into(), entity_id.as_u64())
}

/// The id of one text run, in document order. Only unique underneath a
/// [`document_element_id`].
fn run_element_id(index: usize) -> ElementId {
    ElementId::NamedInteger("md-run".into(), index as u64)
}

/// A markdown document that can be rendered as a GPUI element.
///
/// This entity parses and holds markdown content, ready for rendering.
///
/// # Streaming
///
/// Content arriving a piece at a time — an LLM reply, a log tail — goes in
/// through [`append`](Self::append), which extends the source and re-parses on
/// a background thread. The previously parsed events keep rendering until the
/// new parse lands, so the document never blanks, and deltas arriving during a
/// parse coalesce into a single follow-up parse rather than one each.
///
/// ```
/// # use gpui::AppContext as _;
/// use gpuikit::markdown::Markdown;
/// # let mut tcx = gpui::TestAppContext::single();
/// # tcx.update(gpuikit::init);
/// # let markdown = tcx.update(|cx| cx.new(|cx| Markdown::new("# Hello", cx)));
/// # let delta = String::from(" world");
/// # tcx.update(|cx| {
/// markdown.update(cx, |markdown, cx| markdown.append(&delta, cx));
/// # });
/// # // The re-parse runs on the background executor; a test drives it by hand.
/// # tcx.run_until_parked();
/// ```
pub struct Markdown {
    source: SharedString,
    /// The source the current [`events`](Self::events) were parsed from. Lags
    /// [`source`](Self::source) while a parse is in flight.
    parsed_source: SharedString,
    events: Vec<MarkdownEvent>,
    /// Which code block, if any, the parsed source leaves unclosed, as an
    /// ordinal among the document's code blocks.
    ///
    /// Published together with [`events`](Self::events) so it can never
    /// describe a different parse than the one being rendered.
    unclosed_code_block: Option<usize>,
    /// Document-wide text selection, shared with every rendered run.
    selection: MarkdownSelection,
    parse_state: ParseState,
    /// Handle to the running parse loop. Held here so that dropping the
    /// document cancels a parse in flight.
    parse_task: Option<Task<()>>,
    preprocess_partial: bool,
}

/// Whether a background parse is running, and whether the source changed again
/// while it ran.
///
/// This pair is the whole coalescing rule: a request that arrives mid-parse
/// sets `dirty` instead of spawning a second parse, and the running loop
/// re-runs once against the newest source when it lands.
enum ParseState {
    Idle,
    Parsing { dirty: bool },
}

/// One finished parse: the events, and what the source said about the last
/// code block.
///
/// The two travel together on purpose. A flag published separately from the
/// events it describes would, for one frame, describe the previous parse.
struct ParsedDocument {
    events: Vec<MarkdownEvent>,
    /// The ordinal — among this document's code blocks, in document order —
    /// of the block whose fence never closed, if there is one.
    ///
    /// That block is rendered as plain monospace: a fence that is still
    /// arriving is a different cache key on every delta, so highlighting it
    /// costs a full syntect pass per frame and evicts every settled block's
    /// entry. It highlights once, and for good, when its closer lands.
    unclosed_code_block: Option<usize>,
}

/// Parsed markdown event with source range information.
#[derive(Clone, Debug)]
pub struct MarkdownEvent {
    /// The pulldown-cmark event.
    pub event: Event<'static>,
    /// Where the event came from in the text handed to the parser.
    ///
    /// With [partial-syntax preprocessing](Markdown::preprocess_partial) on
    /// and something to close, that text is the *preprocessed* source, so
    /// offsets past the first insertion do not line up with
    /// [`Markdown::source`]. Rendering does not read this field.
    pub source_range: std::ops::Range<usize>,
}

impl Markdown {
    /// Create a new Markdown instance from source text.
    ///
    /// This first parse is synchronous, so a document is never empty on its
    /// first frame; later ones go to a background thread.
    pub fn new(source: impl Into<SharedString>, _cx: &mut Context<Self>) -> Self {
        let source: SharedString = source.into();
        let preprocess_partial = true;
        let parsed = Self::parse(&source, preprocess_partial);
        Self {
            parsed_source: source.clone(),
            source,
            events: parsed.events,
            unclosed_code_block: parsed.unclosed_code_block,
            selection: MarkdownSelection::new(),
            parse_state: ParseState::Idle,
            parse_task: None,
            preprocess_partial,
        }
    }

    /// Get the source text.
    ///
    /// While a parse is in flight this is ahead of what is rendered — see
    /// [`parsed_source`](Self::parsed_source).
    pub fn source(&self) -> &str {
        &self.source
    }

    /// The source the currently rendered events came from.
    ///
    /// Equal to [`source`](Self::source) once the latest parse has landed.
    pub fn parsed_source(&self) -> &str {
        &self.parsed_source
    }

    /// Update the markdown content. Drops any selection — its positions
    /// belong to the old text.
    ///
    /// The re-parse happens on a background thread: the previous events keep
    /// rendering until it lands, so [`events`](Self::events) still reports the
    /// old parse when read in the same turn. Setting the source it already has
    /// does nothing at all.
    pub fn set_source(&mut self, source: impl Into<SharedString>, cx: &mut Context<Self>) {
        let source = source.into();
        if source == self.source {
            return;
        }

        self.source = source;
        self.selection.clear();
        self.request_parse(cx);
    }

    /// Append to the markdown content — one delta of a streaming document.
    ///
    /// Unlike [`set_source`](Self::set_source) this keeps the selection: a
    /// selection is a pair of `(run, byte offset)` positions, so text arriving
    /// at the end of the document cannot disturb one made earlier in it.
    ///
    /// Appending nothing does nothing.
    pub fn append(&mut self, text: &str, cx: &mut Context<Self>) {
        if text.is_empty() {
            return;
        }

        let mut source = String::with_capacity(self.source.len() + text.len());
        source.push_str(&self.source);
        source.push_str(text);
        self.source = source.into();
        self.request_parse(cx);
    }

    /// Whether a background parse is in flight — i.e. whether what is rendered
    /// is still one source behind.
    pub fn is_parsing(&self) -> bool {
        matches!(self.parse_state, ParseState::Parsing { .. })
    }

    /// Whether syntax a partial document leaves open is closed before parsing.
    /// On by default, and inert unless the crate was built with the `stitch`
    /// feature — see [`preprocessing_available`].
    pub fn preprocess_partial(&self) -> bool {
        self.preprocess_partial
    }

    /// Turn [partial-syntax preprocessing](Self::preprocess_partial) on or
    /// off. Re-parses the current source unless nothing changed.
    pub fn set_preprocess_partial(&mut self, preprocess_partial: bool, cx: &mut Context<Self>) {
        if self.preprocess_partial == preprocess_partial {
            return;
        }

        self.preprocess_partial = preprocess_partial;
        self.request_parse(cx);
    }

    /// The document's selection handle — for clearing it from outside (e.g.
    /// when another document in the same view starts a selection).
    pub fn selection(&self) -> MarkdownSelection {
        self.selection.clone()
    }

    /// The currently selected text, if any — what ⌘C should copy. Routing
    /// the copy binding is the embedding app's job; this is the value.
    pub fn selected_text(&self) -> Option<String> {
        self.selection.selected_text()
    }

    /// Re-parse the current source in the background.
    ///
    /// While a parse is running this only marks the document dirty — the
    /// running loop picks the newest source up when it lands. Ten deltas
    /// during one parse therefore cost one extra parse, not ten.
    fn request_parse(&mut self, cx: &mut Context<Self>) {
        if let ParseState::Parsing { dirty } = &mut self.parse_state {
            *dirty = true;
            return;
        }

        self.parse_state = ParseState::Parsing { dirty: false };

        let mut pending = (self.source.clone(), self.preprocess_partial);
        // One task looping, rather than a task that re-spawns itself: the
        // handle lives on the entity, so a task assigning `parse_task` from
        // inside itself would drop the very task doing the assigning.
        self.parse_task = Some(cx.spawn(async move |this, cx| {
            loop {
                let (source, preprocess_partial) = pending;
                let parse = {
                    let source = source.clone();
                    cx.background_executor()
                        .spawn(async move { Self::parse(&source, preprocess_partial) })
                };
                let parsed = parse.await;

                match this.update(cx, |this, cx| this.parse_landed(source, parsed, cx)) {
                    Ok(Some(next)) => pending = next,
                    // Nothing more to parse, or the document is gone.
                    Ok(None) | Err(_) => break,
                }
            }
        }));
    }

    /// Publish a finished parse and decide whether to run another.
    ///
    /// Publishing and deciding happen in this one update, so a delta arriving
    /// around it cannot be lost: it either set `dirty` before this ran and is
    /// picked up here, or it lands afterwards, finds [`ParseState::Idle`], and
    /// starts a parse of its own.
    fn parse_landed(
        &mut self,
        parsed_source: SharedString,
        parsed: ParsedDocument,
        cx: &mut Context<Self>,
    ) -> Option<(SharedString, bool)> {
        self.events = parsed.events;
        self.unclosed_code_block = parsed.unclosed_code_block;
        self.parsed_source = parsed_source;
        cx.notify();

        match self.parse_state {
            ParseState::Parsing { dirty: true } => {
                self.parse_state = ParseState::Parsing { dirty: false };
                Some((self.source.clone(), self.preprocess_partial))
            }
            _ => {
                self.parse_state = ParseState::Idle;
                None
            }
        }
    }

    fn parse(source: &str, preprocess_partial: bool) -> ParsedDocument {
        // Asked of the *raw* source, before preprocessing: `close_open_syntax`
        // rewrites inline markers and its output is what the events describe,
        // but a fence is a property of the text the caller actually appended
        // to. (mdstitch closes no fences, so the two agree here regardless —
        // asking the raw source is what keeps that true if it ever starts to.)
        let fence_open = parser::has_open_code_fence(source);

        let source = if preprocess_partial {
            stitch::close_open_syntax(source)
        } else {
            std::borrow::Cow::Borrowed(source)
        };

        let parser = Parser::new_ext(&source, parser::default_options());

        let events: Vec<MarkdownEvent> = parser
            .into_offset_iter()
            .map(|(event, range)| MarkdownEvent {
                event: event.into_static(),
                source_range: range,
            })
            .collect();

        // The ordinal comes from the events rather than from the scan, so the
        // two can never point at a block the renderer will not reach. A scan
        // saying "open" with no code block in the events yields `None`.
        let unclosed_code_block = if fence_open {
            events
                .iter()
                .filter(|event| matches!(event.event, Event::Start(Tag::CodeBlock(_))))
                .count()
                .checked_sub(1)
        } else {
            None
        };

        ParsedDocument {
            events,
            unclosed_code_block,
        }
    }

    /// Get the parsed events.
    ///
    /// These are the events of [`parsed_source`](Self::parsed_source), which
    /// is one source behind while a parse is in flight.
    pub fn events(&self) -> &[MarkdownEvent] {
        &self.events
    }
}

/// Element for rendering markdown content.
#[derive(IntoElement)]
pub struct MarkdownElement {
    markdown: Entity<Markdown>,
    style: MarkdownStyle,
    element_id: Option<ElementId>,
}

/// Create a markdown element from source text.
///
/// This is a convenience function that creates the entity and element in one step.
/// For more control, use `Markdown::new()` and `MarkdownElement::new()` separately.
///
/// Note that this mints a *new* entity on every call. Called from a `render`,
/// the document gets a new element id every frame, which a screen reader reads
/// as the whole document being replaced. Hold an `Entity<Markdown>` — which
/// text selection needs anyway — for anything longer-lived than a one-shot.
pub fn markdown(source: impl Into<SharedString>, cx: &mut App) -> MarkdownElement {
    let entity = cx.new(|cx| Markdown::new(source, cx));
    MarkdownElement::new(entity)
}

impl MarkdownElement {
    /// Create a new markdown element with default styling.
    pub fn new(markdown: Entity<Markdown>) -> Self {
        Self {
            markdown,
            style: MarkdownStyle::default(),
            element_id: None,
        }
    }

    /// Set a custom style for the markdown.
    pub fn style(mut self, style: MarkdownStyle) -> Self {
        self.style = style;
        self
    }

    /// Override the element id the document — and therefore all of its text
    /// runs — is scoped under.
    ///
    /// The default is derived from the `Markdown` entity, which is unique and
    /// stable already. Set this only when the same entity is rendered more
    /// than once in a frame; each copy then needs its own id. (Note that two
    /// live copies of one entity still share a single selection.)
    pub fn id(mut self, id: impl Into<ElementId>) -> Self {
        self.element_id = Some(id.into());
        self
    }

    /// The id this element renders under — the explicit [`Self::id`] if one
    /// was given, otherwise the entity-derived default.
    pub fn element_id(&self) -> ElementId {
        self.element_id
            .clone()
            .unwrap_or_else(|| document_element_id(self.markdown.entity_id()))
    }
}

impl RenderOnce for MarkdownElement {
    fn render(self, _window: &mut Window, cx: &mut App) -> impl IntoElement {
        let document_id = self.element_id();
        let markdown = self.markdown.read(cx);
        let events = markdown.events.clone();
        let unclosed_code_block = markdown.unclosed_code_block;
        let selection = markdown.selection.clone();
        let style = self.style.clone();

        // New frame: the previous frame's run layouts are about to be
        // dropped and must not be hit-tested.
        selection.begin_frame();

        let renderer = MarkdownRenderer::new(style, selection, unclosed_code_block);
        renderer.render_events(&events, document_id, cx)
    }
}

/// Internal renderer that builds the element tree from markdown events.
struct MarkdownRenderer {
    style: MarkdownStyle,
    elements: Vec<gpui::AnyElement>,

    // State tracking
    in_heading: Option<HeadingLevel>,
    in_code_block: bool,
    /// The language token of the fence currently open, normalized from its
    /// info string. `None` for an indented block or a bare fence.
    code_block_language: Option<String>,
    /// The ordinal of the code block the source leaves unclosed, if any —
    /// see [`ParsedDocument::unclosed_code_block`].
    unclosed_code_block: Option<usize>,
    /// How many code blocks this renderer has opened. Counted here rather than
    /// taken from the parse so that the ordinal is compared against the blocks
    /// actually being drawn.
    code_blocks_seen: usize,
    /// Whether the block currently open is that unclosed one, i.e. whether it
    /// draws plain.
    code_block_is_unclosed: bool,
    in_block_quote: bool,
    in_image: Option<ImageContext>,
    list_stack: Vec<ListContext>,
    /// Document-order index for selectable text runs. A run's index is its
    /// element id, its selection identity and its slot in the per-frame
    /// registry all at once, so there is only ever one counter to keep in
    /// step.
    run_counter: usize,
    /// Shared selection state, handed to every run.
    selection: MarkdownSelection,

    // Table state
    in_table: bool,
    table_alignments: Vec<Alignment>,
    table_rows: Vec<Vec<RichText>>,
    current_row: Vec<RichText>,
    in_table_head: bool,

    // Rich text tracking
    current_text: RichText,
    active_style: InlineStyle,

    /// Every list row this renderer emitted, in document order.
    ///
    /// A rendered element only reports its height, which cannot say which
    /// marker a row got, what it was indented by, or which row a piece of text
    /// landed in — exactly what nesting scrambles.
    #[cfg(test)]
    emitted_list_items: Vec<ListRow>,

    /// The language every code block was flushed with, in document order.
    ///
    /// A drawn element reports its height, not its colors, so this is how the
    /// tests read the plain-versus-highlighted decision. Mirrors
    /// [`Self::emitted_list_items`].
    #[cfg(test)]
    emitted_code_blocks: Vec<Option<String>>,
}

/// One row `flush_list_item` emitted, as the tests read it.
#[cfg(test)]
#[derive(Clone, Debug, PartialEq, Eq)]
struct ListRow {
    marker: elements::ItemMarker,
    indent_level: usize,
    text: String,
}

#[derive(Clone, Debug)]
struct ImageContext {
    url: String,
    alt: String,
}

#[derive(Clone, Debug)]
struct ListContext {
    ordered: bool,
    current_index: u64,
    /// The item of *this* list that is currently open, if any. Only the
    /// innermost list may be asked: a nested list opens while its parent's
    /// item is still open, so a child's blocks would otherwise be attributed
    /// to the parent's item.
    item: Option<ItemContext>,
}

/// One open list item.
#[derive(Clone, Debug, Default)]
struct ItemContext {
    /// The marker this item's first block drew, once it has drawn one. Every
    /// later block of the same item reserves the width of this same marker
    /// without painting it, so their text lines up under the first block's.
    marker: Option<String>,
}

impl MarkdownRenderer {
    fn new(
        style: MarkdownStyle,
        selection: MarkdownSelection,
        unclosed_code_block: Option<usize>,
    ) -> Self {
        Self {
            style,
            selection,
            elements: Vec::new(),
            in_heading: None,
            in_code_block: false,
            code_block_language: None,
            unclosed_code_block,
            code_blocks_seen: 0,
            code_block_is_unclosed: false,
            in_block_quote: false,
            in_image: None,
            list_stack: Vec::new(),
            run_counter: 0,
            in_table: false,
            table_alignments: Vec::new(),
            table_rows: Vec::new(),
            current_row: Vec::new(),
            in_table_head: false,
            current_text: RichText::new(),
            active_style: InlineStyle::default(),
            #[cfg(test)]
            emitted_list_items: Vec::new(),
            #[cfg(test)]
            emitted_code_blocks: Vec::new(),
        }
    }

    fn render_events(
        mut self,
        events: &[MarkdownEvent],
        document_id: ElementId,
        cx: &App,
    ) -> impl IntoElement + use<> {
        for event in events {
            self.handle_event(&event.event, cx);
        }

        // The id is what scopes the run ids below it; the role is what puts
        // the document into the accessibility tree. `.announce()` is only
        // reachable on a div that already has an `.id()`, so the two cannot
        // drift apart. A bare `.id()` adds no hitbox, so selection
        // hit-testing and link clicks are untouched. A document is named by
        // its contents, so it takes no accessible name — see
        // `crate::a11y::role_requires_a_name`.
        div()
            .id(document_id)
            .announce(A11y::new(Role::Document))
            .w_full()
            .flex()
            .flex_col()
            .gap(rems(self.style.block_spacing))
            .children(self.elements)
    }

    fn handle_event(&mut self, event: &Event<'static>, cx: &App) {
        match event {
            Event::Start(tag) => self.handle_start_tag(tag, cx),
            Event::End(tag) => self.handle_end_tag(tag, cx),
            Event::Text(text) => self.handle_text(text),
            Event::Code(code) => self.handle_inline_code(code),
            // Agent/LLM output often uses single newlines as real breaks;
            // `soft_break_as_hard_break` opts into honoring them.
            Event::SoftBreak => {
                if self.style.soft_break_as_hard_break {
                    self.current_text.push("\n", self.active_style)
                } else {
                    self.current_text.push(" ", self.active_style)
                }
            }
            Event::HardBreak => self.current_text.push("\n", self.active_style),
            Event::Rule => self.push_divider(cx),
            Event::TaskListMarker(checked) => self.handle_task_marker(*checked),
            Event::Html(_) | Event::InlineHtml(_) => {}
            Event::FootnoteReference(_) | Event::InlineMath(_) | Event::DisplayMath(_) => {}
        }
    }

    /// Takes `cx` because a nested list flushes its parent's item, and
    /// flushing builds an element.
    fn handle_start_tag(&mut self, tag: &Tag<'static>, cx: &App) {
        match tag {
            Tag::Paragraph => {}
            Tag::Heading { level, .. } => {
                self.in_heading = Some((*level).into());
            }
            Tag::BlockQuote(_) => {
                self.in_block_quote = true;
            }
            Tag::CodeBlock(kind) => {
                self.in_code_block = true;
                // Compared before the increment, so the flag is set on the
                // block whose ordinal this is rather than on the next one.
                self.code_block_is_unclosed =
                    self.unclosed_code_block == Some(self.code_blocks_seen);
                self.code_blocks_seen += 1;
                self.code_block_language =
                    parser::code_block_language(kind).and_then(code_highlight::normalize_language);
            }
            Tag::List(start) => {
                // A list nested under an item opens *before* that item ends,
                // so the parent's text is still buffered here. Left there, the
                // first child item's flush would pick it up and emit both as
                // one row, and the parent's own `End(Item)` would then find an
                // empty buffer and emit nothing.
                //
                // Flushed before the push, so the parent's row gets its own
                // indent, its own marker and — for an ordered list — its own
                // ordinal rather than the child's first one. Guarded on a
                // non-empty stack because a top-level list has no parent item
                // to attribute a row to.
                if !self.list_stack.is_empty() {
                    self.flush_list_item(cx);
                }

                self.list_stack.push(ListContext {
                    ordered: start.is_some(),
                    current_index: start.unwrap_or(1),
                    item: None,
                });
            }
            Tag::Item => {
                // Opened on the innermost list, which is the one this item
                // belongs to. A loose item's blocks each end with
                // `End(Paragraph)`, and this is what tells that ending it is
                // inside an item rather than in body text.
                if let Some(list_ctx) = self.list_stack.last_mut() {
                    list_ctx.item = Some(ItemContext::default());
                }
            }
            Tag::Emphasis => {
                self.active_style.italic = true;
            }
            Tag::Strong => {
                self.active_style.bold = true;
            }
            Tag::Strikethrough => {
                self.active_style.strikethrough = true;
            }
            Tag::Link { dest_url, .. } => {
                // Inline: the link is a styled, clickable range of the
                // surrounding text run, not its own block element.
                self.active_style.link = Some(self.current_text.add_link(dest_url.to_string()));
            }
            Tag::Image {
                dest_url, title, ..
            } => {
                self.in_image = Some(ImageContext {
                    url: dest_url.to_string(),
                    alt: title.to_string(),
                });
            }
            Tag::Table(alignments) => {
                self.in_table = true;
                self.table_alignments = alignments.clone();
                self.table_rows.clear();
            }
            Tag::TableHead => {
                self.in_table_head = true;
                self.current_row.clear();
            }
            Tag::TableRow => {
                self.current_row.clear();
            }
            Tag::TableCell => {
                self.current_text.clear();
            }
            // Listed rather than folded into a `_ => {}` wildcard on purpose:
            // an exhaustive match is what turns a pulldown-cmark upgrade into a
            // compile error instead of silently dropped styling.
            //
            // Superscript and Subscript are inert because the options that
            // produce them are off (see `parser::default_options`) *and*
            // because `InlineStyle` has nowhere to put them.
            Tag::FootnoteDefinition(_)
            | Tag::MetadataBlock(_)
            | Tag::DefinitionList
            | Tag::DefinitionListTitle
            | Tag::DefinitionListDefinition
            | Tag::Superscript
            | Tag::Subscript
            | Tag::HtmlBlock => {}
        }
    }

    fn handle_end_tag(&mut self, tag: &TagEnd, cx: &App) {
        match tag {
            TagEnd::Paragraph => {
                if self.in_block_quote {
                    self.flush_block_quote(cx);
                } else if self.in_list_item() {
                    // A loose list wraps each item's content in a paragraph.
                    // Flushed as body text it would lose its marker, its
                    // indent and its number, and the item's `End(Item)` would
                    // then find an empty buffer and emit nothing at all.
                    self.flush_list_item(cx);
                } else {
                    self.flush_paragraph(cx);
                }
            }
            TagEnd::Heading(level) => {
                let heading_level: elements::HeadingLevel = (*level).into();
                self.in_heading = None;
                self.flush_heading(heading_level, cx);
            }
            TagEnd::BlockQuote(_) => {
                self.in_block_quote = false;
            }
            TagEnd::CodeBlock => {
                self.in_code_block = false;
                self.flush_code_block(cx);
            }
            TagEnd::List(_) => {
                self.list_stack.pop();
            }
            TagEnd::Item => {
                // Flushed *before* the item closes: a tight item's text is
                // still buffered here, and closing first would leave it with
                // no item to take its marker from.
                self.flush_list_item(cx);
                if let Some(list_ctx) = self.list_stack.last_mut() {
                    list_ctx.item = None;
                }
            }
            TagEnd::Emphasis => {
                self.active_style.italic = false;
            }
            TagEnd::Strong => {
                self.active_style.bold = false;
            }
            TagEnd::Strikethrough => {
                self.active_style.strikethrough = false;
            }
            TagEnd::Link => {
                self.active_style.link = None;
            }
            TagEnd::Image => {
                self.flush_image(cx);
            }
            TagEnd::Table => {
                self.flush_table(cx);
                self.in_table = false;
            }
            TagEnd::TableHead => {
                self.in_table_head = false;
                if !self.current_row.is_empty() {
                    self.table_rows.push(std::mem::take(&mut self.current_row));
                }
            }
            TagEnd::TableRow => {
                if !self.current_row.is_empty() {
                    self.table_rows.push(std::mem::take(&mut self.current_row));
                }
            }
            TagEnd::TableCell => {
                self.current_row
                    .push(std::mem::take(&mut self.current_text));
            }
            TagEnd::FootnoteDefinition
            | TagEnd::MetadataBlock(_)
            | TagEnd::DefinitionList
            | TagEnd::DefinitionListTitle
            | TagEnd::DefinitionListDefinition
            | TagEnd::Superscript
            | TagEnd::Subscript
            | TagEnd::HtmlBlock => {}
        }
    }

    fn handle_text(&mut self, text: &str) {
        if let Some(ref mut img_ctx) = self.in_image {
            img_ctx.alt = text.to_string();
        } else {
            self.current_text.push(text, self.active_style);
        }
    }

    fn handle_inline_code(&mut self, code: &str) {
        // A styled span (background wash via the palette), not literal
        // backticks in the text.
        let mut style = self.active_style;
        style.code = true;
        self.current_text.push(code, style);
    }

    fn handle_task_marker(&mut self, checked: bool) {
        let marker = if checked { "☑ " } else { "☐ " };
        self.current_text.push(marker, self.active_style);
    }

    /// Theme-resolved colors for inline code and link spans.
    fn palette(&self, cx: &App) -> InlinePalette {
        let theme = cx.theme();
        InlinePalette {
            code_background: Some(self.style.inline_code_bg.unwrap_or(theme.surface())),
            link_color: Some(self.style.link_color.unwrap_or(theme.accent())),
        }
    }

    /// Id and selection context for the next text run, in document order.
    /// The counter must tick once per selectable run and nowhere else — it
    /// is the identity the element id, the per-frame registry and the
    /// highlights all agree on.
    fn next_run(&mut self, cx: &App) -> (ElementId, elements::RunContext) {
        let run = self.run_counter;
        self.run_counter += 1;
        let theme = cx.theme();
        let run_cx = elements::RunContext {
            selection: self.selection.clone(),
            run,
            selection_background: self
                .style
                .selection_background
                .unwrap_or_else(|| theme.accent().opacity(0.25)),
        };
        (run_element_id(run), run_cx)
    }

    fn flush_paragraph(&mut self, cx: &App) {
        if self.current_text.is_empty() {
            return;
        }

        let rich_text = std::mem::take(&mut self.current_text);
        let palette = self.palette(cx);
        let (id, run_cx) = self.next_run(cx);
        let element =
            elements::rich_paragraph(id, &rich_text, &self.style.body, &palette, run_cx, cx);
        self.elements.push(element.into_any_element());
    }

    fn flush_heading(&mut self, level: HeadingLevel, cx: &App) {
        if self.current_text.is_empty() {
            return;
        }

        let rich_text = std::mem::take(&mut self.current_text);
        let heading_style = match level {
            elements::HeadingLevel::H1 => &self.style.h1,
            elements::HeadingLevel::H2 => &self.style.h2,
            elements::HeadingLevel::H3 => &self.style.h3,
            elements::HeadingLevel::H4 => &self.style.h4,
            elements::HeadingLevel::H5 => &self.style.h5,
            elements::HeadingLevel::H6 => &self.style.h6,
        }
        .clone();

        let palette = self.palette(cx);
        let (id, run_cx) = self.next_run(cx);
        let element =
            elements::rich_heading(id, &rich_text, level, &heading_style, &palette, run_cx, cx);
        self.elements.push(element.into_any_element());
    }

    fn flush_block_quote(&mut self, cx: &App) {
        if self.current_text.is_empty() {
            return;
        }

        let rich_text = std::mem::take(&mut self.current_text);
        let palette = self.palette(cx);
        let (id, run_cx) = self.next_run(cx);
        let element = elements::rich_block_quote(
            id,
            &rich_text,
            &self.style.body,
            self.style.block_quote_border,
            self.style.block_quote_text,
            &palette,
            run_cx,
            cx,
        );
        self.elements.push(element.into_any_element());
    }

    fn flush_code_block(&mut self, cx: &App) {
        // Both taken before the early return: an empty fence still closes, and
        // neither its language nor its unclosed-ness must leak into the next
        // block.
        //
        // A block whose fence has not closed yet is drawn with no language,
        // which is the path a bare fence already takes: no syntect pass, and
        // no cache entry for text that is about to change. It highlights once
        // its closer arrives.
        let is_unclosed = std::mem::take(&mut self.code_block_is_unclosed);
        let language = self.code_block_language.take().filter(|_| !is_unclosed);

        #[cfg(test)]
        self.emitted_code_blocks.push(language.clone());

        if self.current_text.is_empty() {
            return;
        }

        let text = self.current_text.to_plain_text();
        self.current_text.clear();

        let (id, run_cx) = self.next_run(cx);
        let element = elements::code_block(
            id,
            text,
            language.as_deref(),
            &self.style.code,
            &self.style.code_font_family,
            self.style.code_block_bg,
            self.style.code_block_border,
            run_cx,
            cx,
        );
        self.elements.push(element.into_any_element());
    }

    /// Whether the innermost list has an item open — i.e. whether a block
    /// ending right now is one of that item's blocks.
    ///
    /// Only the innermost is asked. A nested list opens while its parent's
    /// item is still open, so "any item on the stack" would route a child's
    /// blocks against the parent's context.
    fn in_list_item(&self) -> bool {
        self.list_stack
            .last()
            .is_some_and(|list_ctx| list_ctx.item.is_some())
    }

    /// The marker the row about to be emitted draws.
    ///
    /// An item's first block takes the list's next ordinal and remembers what
    /// it drew; every later block of the same item redraws that marker
    /// hidden, advancing nothing — an ordinal belongs to an item, not to a
    /// flush, so `1. a` with a second paragraph must not leave the next item
    /// at `3.`.
    fn take_item_marker(&mut self) -> elements::ItemMarker {
        let Some(list_ctx) = self.list_stack.last_mut() else {
            return elements::ItemMarker::Shown(elements::unordered_marker());
        };

        if let Some(marker) = list_ctx.item.as_ref().and_then(|item| item.marker.clone()) {
            return elements::ItemMarker::Hidden(marker);
        }

        let marker = if list_ctx.ordered {
            let marker = elements::ordered_marker(list_ctx.current_index);
            list_ctx.current_index += 1;
            marker
        } else {
            elements::unordered_marker()
        };

        if let Some(item) = list_ctx.item.as_mut() {
            item.marker = Some(marker.clone());
        }

        elements::ItemMarker::Shown(marker)
    }

    fn flush_list_item(&mut self, cx: &App) {
        if self.current_text.is_empty() {
            return;
        }

        let rich_text = std::mem::take(&mut self.current_text);
        let marker = self.take_item_marker();
        let indent_level = self.list_stack.len().saturating_sub(1);

        #[cfg(test)]
        self.emitted_list_items.push(ListRow {
            marker: marker.clone(),
            indent_level,
            text: rich_text.to_plain_text(),
        });

        let palette = self.palette(cx);
        let (id, run_cx) = self.next_run(cx);
        let element = elements::rich_list_item(
            id,
            &rich_text,
            marker,
            indent_level,
            &self.style.body,
            &palette,
            run_cx,
            cx,
        );
        self.elements.push(element.into_any_element());
    }

    fn flush_image(&mut self, cx: &App) {
        let img_ctx = match self.in_image.take() {
            Some(ctx) => ctx,
            None => return,
        };

        self.current_text.clear();

        let alt = if img_ctx.alt.is_empty() {
            None
        } else {
            Some(img_ctx.alt.as_str())
        };

        let element = elements::image(img_ctx.url, alt, cx);
        self.elements.push(element.into_any_element());
    }

    fn flush_table(&mut self, cx: &App) {
        if self.table_rows.is_empty() {
            return;
        }

        let rows = std::mem::take(&mut self.table_rows);
        let alignments = std::mem::take(&mut self.table_alignments);

        let element = self.render_table(rows, alignments, cx);
        self.elements.push(element.into_any_element());
    }

    fn render_table(
        &self,
        rows: Vec<Vec<RichText>>,
        alignments: Vec<Alignment>,
        cx: &App,
    ) -> impl IntoElement + use<> {
        let theme = cx.theme();
        let border_color = theme.border();

        div()
            .flex()
            .flex_col()
            .border_1()
            .border_color(border_color)
            .rounded_sm()
            .overflow_hidden()
            .children(rows.into_iter().enumerate().map(|(row_idx, row)| {
                let is_header = row_idx == 0;
                let bg = if is_header {
                    theme.surface()
                } else if row_idx % 2 == 0 {
                    theme.bg()
                } else {
                    theme.surface().opacity(0.5)
                };

                div()
                    .flex()
                    .flex_row()
                    .bg(bg)
                    .when(row_idx > 0, |el| el.border_t_1().border_color(border_color))
                    .children(row.into_iter().enumerate().map(|(col_idx, cell)| {
                        let alignment = alignments.get(col_idx).copied().unwrap_or(Alignment::None);
                        // Styled (code wash, link color) but not clickable —
                        // cells sit in a custom layout without element ids.
                        let (text, highlights) = cell.to_highlights_with(&self.palette(cx));
                        let styled_text: SharedString = text.into();

                        div()
                            .flex_1()
                            // Same shape as a list item: without `min_w_0` the
                            // cell can't shrink below one unbroken line.
                            .min_w_0()
                            .px_2()
                            .py_1()
                            .text_size(rems(self.style.body.size))
                            .when(col_idx > 0, |el| el.border_l_1().border_color(border_color))
                            .when(is_header, |el| el.font_weight(gpui::FontWeight::SEMIBOLD))
                            .map(|el| match alignment {
                                Alignment::Left | Alignment::None => el,
                                Alignment::Center => el.text_center(),
                                Alignment::Right => el.text_right(),
                            })
                            .child(gpui::StyledText::new(styled_text).with_highlights(highlights))
                    }))
            }))
    }

    fn push_divider(&mut self, cx: &App) {
        let element = elements::divider(self.style.rule_color, cx);
        self.elements.push(element.into_any_element());
    }
}

#[cfg(test)]
mod tests {
    use super::selectable_text::recorder::{self, RecordedRun};
    use super::*;
    use gpui::{AnyElement, Pixels, Render, TestAppContext, VisualTestContext, point, px, size};
    use std::cell::Cell;
    use std::collections::HashSet;
    use std::rc::Rc;

    /// Narrow enough that every sample text below has to wrap.
    const WIDTH: Pixels = px(240.);

    /// Long enough to take several lines at that width — and exactly one line
    /// if whatever contains it refuses to wrap.
    const LONG: &str = "This one is deliberately long enough that it has to wrap onto several lines inside a narrow container.";

    struct TestView {
        source: SharedString,
    }

    impl Render for TestView {
        fn render(&mut self, _window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
            // The root element is stretched to the window, so the div we
            // measure has to be a child of it — inside a column it keeps its
            // content height, which is the signal these tests read.
            div().flex().flex_col().child(
                div()
                    .w(WIDTH)
                    .debug_selector(|| "measured".into())
                    .child(markdown(self.source.clone(), cx)),
            )
        }
    }

    /// Render `source` into a [`WIDTH`]-wide container and report how tall it
    /// got. Text that will not wrap stays one line tall however long it is.
    fn height(cx: &mut TestAppContext, source: &str) -> Pixels {
        cx.update(crate::theme::init);

        let source = SharedString::from(source.to_string());
        let (_view, cx) = cx.add_window_view(move |_window, _cx| TestView { source });

        cx.debug_bounds("measured")
            .expect("the measured container was never drawn")
            .size
            .height
    }

    /// The height of a single line of body text.
    fn line(cx: &mut TestAppContext) -> Pixels {
        height(cx, "x")
    }

    /// A list item's text column is narrower than a paragraph's by the marker
    /// and the gap (and, when nested, the indent), so it is allowed to take a
    /// couple of lines more than the same text set as a paragraph.
    #[track_caller]
    fn assert_wrapped_like_a_paragraph(item: Pixels, paragraph: Pixels, line: Pixels) {
        assert!(
            paragraph > line,
            "the baseline paragraph did not wrap ({paragraph:?}), so this measures nothing"
        );
        assert!(
            item > line,
            "the text stayed on one line ({item:?}) instead of wrapping"
        );
        assert!(
            item <= paragraph + line * 2.,
            "{item:?} is much taller than the same text as a paragraph ({paragraph:?})"
        );
    }

    /// One of every kind of run this renderer emits, in this order: heading,
    /// paragraph, quote, list item, code.
    const EVERY_RUN_KIND: &str = concat!(
        "# Title\n",
        "\n",
        "A paragraph.\n",
        "\n",
        "> A quote.\n",
        "\n",
        "- An item\n",
        "\n",
        "```\n",
        "let x = 1;\n",
        "```\n",
    );

    /// Draw one frame of whatever `build` produces, and report every run the
    /// way gpui's accessibility walk would have seen it.
    fn draw(
        cx: &mut VisualTestContext,
        build: impl FnOnce() -> Vec<MarkdownElement>,
    ) -> Vec<RecordedRun> {
        recorder::clear();
        cx.draw(
            point(px(0.), px(0.)),
            size(px(800.), px(600.)),
            |_window, _cx| -> AnyElement { div().children(build()).into_any_element() },
        );
        recorder::take()
    }

    /// The last two segments of a run's id path: the document it belongs to,
    /// and the run itself.
    fn doc_and_run(run: &RecordedRun) -> (&str, &str) {
        match run.id_segments.as_slice() {
            [.., doc, this] => (doc.as_str(), this.as_str()),
            other => panic!("a run's id path should have at least two segments: {other:?}"),
        }
    }

    fn id_paths(runs: &[RecordedRun]) -> Vec<String> {
        runs.iter().map(|run| run.id_path.clone()).collect()
    }

    /// The fence's info string used to be dropped at `Tag::CodeBlock` and a
    /// literal `None` passed on to the element, so fixing the code block
    /// element alone would have changed nothing.
    #[gpui::test]
    fn a_fence_carries_its_language_without_leaking_it(cx: &mut TestAppContext) {
        use pulldown_cmark::CodeBlockKind;

        cx.update(crate::theme::init);
        cx.update(|cx| {
            let mut renderer =
                MarkdownRenderer::new(MarkdownStyle::default(), MarkdownSelection::new(), None);

            let fence = |info: &'static str| Tag::CodeBlock(CodeBlockKind::Fenced(info.into()));

            renderer.handle_start_tag(&fence("rust,ignore"), cx);
            assert_eq!(
                renderer.code_block_language.as_deref(),
                Some("rust"),
                "the info string should reach the renderer, normalized"
            );

            // An empty fence still closes, and must not hand its language to
            // whatever block comes next.
            renderer.flush_code_block(cx);
            assert_eq!(renderer.code_block_language, None);

            for no_language in [
                fence(""),
                fence("text"),
                Tag::CodeBlock(CodeBlockKind::Indented),
            ] {
                renderer.handle_start_tag(&no_language, cx);
                assert_eq!(
                    renderer.code_block_language, None,
                    "{no_language:?} names no language"
                );
            }
        });
    }

    #[gpui::test]
    fn two_documents_in_one_frame_get_disjoint_run_ids(cx: &mut TestAppContext) {
        cx.update(crate::theme::init);
        let (first, second) = (document(cx, EVERY_RUN_KIND), document(cx, EVERY_RUN_KIND));
        let cx = cx.add_empty_window();

        let runs = draw(cx, || {
            vec![
                MarkdownElement::new(first.clone()),
                MarkdownElement::new(second.clone()),
            ]
        });

        assert_eq!(runs.len(), 10, "five runs per document, twice over");

        // The whole point: gpui hashes the *whole* path into a node id, and
        // two documents used to mint the same `md-run-N` under the same
        // ancestors.
        let paths = id_paths(&runs);
        let distinct: HashSet<&String> = paths.iter().collect();
        assert_eq!(distinct.len(), 10, "colliding id paths: {paths:?}");

        let mut documents = HashSet::new();
        for run in &runs {
            let (doc, this) = doc_and_run(run);
            assert!(doc.starts_with("md-doc-"), "unscoped run: {}", run.id_path);
            assert!(this.starts_with("md-run-"), "odd run id: {}", run.id_path);
            documents.insert(doc.to_string());
        }
        assert_eq!(
            documents.len(),
            2,
            "two documents, two scopes: {documents:?}"
        );
    }

    #[gpui::test]
    fn a_long_unordered_list_item_wraps(cx: &mut TestAppContext) {
        let line = line(cx);
        let paragraph = height(cx, LONG);
        let item = height(cx, &format!("- {LONG}"));

        assert_wrapped_like_a_paragraph(item, paragraph, line);
    }

    #[gpui::test]
    fn a_long_ordered_list_item_wraps(cx: &mut TestAppContext) {
        let line = line(cx);
        let paragraph = height(cx, LONG);
        let item = height(cx, &format!("1. {LONG}"));

        assert_wrapped_like_a_paragraph(item, paragraph, line);
    }

    #[gpui::test]
    fn a_long_list_item_with_inline_styles_wraps(cx: &mut TestAppContext) {
        // Bold, code and a link put a different element in the same flex
        // container — the `rich_list_item` path rather than `list_item`.
        let styled = format!("**Bold** and `code` and a [link](#) — {LONG}");

        let line = line(cx);
        let paragraph = height(cx, &styled);
        let item = height(cx, &format!("- {styled}"));

        assert_wrapped_like_a_paragraph(item, paragraph, line);
    }

    #[gpui::test]
    fn a_long_nested_list_item_wraps(cx: &mut TestAppContext) {
        // An indented item, so the row is rendered at `indent_level > 0`. The
        // parent now emits a row of its own, so subtract it to measure the
        // nested one — what is left is the nested row plus the block gap.
        let line = line(cx);
        let paragraph = height(cx, LONG);
        let parent = height(cx, "- parent");
        let item = height(cx, &format!("- parent\n    - {LONG}")) - parent;

        assert_wrapped_like_a_paragraph(item, paragraph, line);
    }

    // --- nested lists ---

    /// Drive the renderer over `source` and report the list rows it emitted,
    /// as `(marker, indent level, text)` in document order. No window is
    /// needed: this reads what `flush_list_item` built, not what it laid out.
    fn emitted_rows(cx: &mut TestAppContext, source: &str) -> Vec<(ItemMarker, usize, String)> {
        cx.update(crate::theme::init);
        let events = Markdown::parse(source, false).events;

        cx.update(|cx: &mut App| {
            let mut renderer =
                MarkdownRenderer::new(MarkdownStyle::default(), MarkdownSelection::new(), None);
            for event in &events {
                renderer.handle_event(&event.event, cx);
            }
            renderer
                .emitted_list_items
                .iter()
                .map(|row| (row.marker.clone(), row.indent_level, row.text.clone()))
                .collect()
        })
    }

    /// The same rows, carrying only the marker a reader actually sees: a
    /// continuation block of a multi-block item draws none, so it reads as
    /// empty here.
    fn list_rows(cx: &mut TestAppContext, source: &str) -> Vec<(String, usize, String)> {
        emitted_rows(cx, source)
            .into_iter()
            .map(|(marker, indent_level, text)| {
                let marker = match marker {
                    ItemMarker::Shown(marker) => marker,
                    ItemMarker::Hidden(_) => String::new(),
                };
                (marker, indent_level, text)
            })
            .collect()
    }

    /// Draw `source` and report the role each of its runs announced, in
    /// document order — what a screen reader would be told the document is.
    fn run_roles(cx: &mut TestAppContext, source: &str) -> Vec<Option<Role>> {
        cx.update(crate::theme::init);
        let doc = document(cx, source);
        let cx = cx.add_empty_window();

        draw(cx, || vec![MarkdownElement::new(doc.clone())])
            .iter()
            .map(|run| run.role)
            .collect()
    }

    /// A nested document and the same items un-indented must lay out to the
    /// same height: nesting changes a row's indent, never whether it exists.
    /// Comparing against the flat form hardcodes neither the line height nor
    /// the block spacing.
    #[track_caller]
    fn assert_same_row_count(cx: &mut TestAppContext, nested: &str, flat: &str) {
        let nested_height = height(cx, nested);
        let flat_height = height(cx, flat);

        assert_eq!(
            nested_height, flat_height,
            "{nested:?} laid out to {nested_height:?}, but the same items flat \
             ({flat:?}) came to {flat_height:?}"
        );
    }

    #[gpui::test]
    fn a_nested_list_does_not_swallow_its_parents_text(cx: &mut TestAppContext) {
        // The whole bug: `- x` with an indented `- y` under it used to render
        // as one row reading "xy", because the child's flush picked up the
        // parent's still-buffered text.
        assert_eq!(
            list_rows(cx, "- x\n    - y"),
            vec![
                ("•".to_string(), 0, "x".to_string()),
                ("•".to_string(), 1, "y".to_string()),
            ]
        );
    }

    #[gpui::test]
    fn an_ordered_list_nested_in_an_unordered_one(cx: &mut TestAppContext) {
        assert_eq!(
            list_rows(cx, "- x\n    1. y\n    2. z"),
            vec![
                ("•".to_string(), 0, "x".to_string()),
                ("1.".to_string(), 1, "y".to_string()),
                ("2.".to_string(), 1, "z".to_string()),
            ]
        );
    }

    #[gpui::test]
    fn an_unordered_list_nested_in_an_ordered_one(cx: &mut TestAppContext) {
        assert_eq!(
            list_rows(cx, "1. x\n    - y"),
            vec![
                ("1.".to_string(), 0, "x".to_string()),
                ("•".to_string(), 1, "y".to_string()),
            ]
        );
    }

    #[gpui::test]
    fn three_levels_of_nesting_each_keep_their_own_row(cx: &mut TestAppContext) {
        assert_eq!(
            list_rows(cx, "- a\n    - b\n        - c"),
            vec![
                ("•".to_string(), 0, "a".to_string()),
                ("•".to_string(), 1, "b".to_string()),
                ("•".to_string(), 2, "c".to_string()),
            ]
        );
    }

    #[gpui::test]
    fn a_parent_with_inline_styles_keeps_its_own_row(cx: &mut TestAppContext) {
        // The `rich_list_item` path: bold, code and a link put spans in the
        // parent's buffer, which is what the child used to inherit.
        assert_eq!(
            list_rows(cx, "- **bold** and `code` and a [link](#)\n    - child"),
            vec![
                ("•".to_string(), 0, "bold and code and a link".to_string()),
                ("•".to_string(), 1, "child".to_string()),
            ]
        );
    }

    #[gpui::test]
    fn a_nested_list_does_not_renumber_its_parents_siblings(cx: &mut TestAppContext) {
        // The parent's row is emitted from inside the *child's* `Start(List)`,
        // which is where the parent's ordinal is taken. Flush after pushing
        // the child's context and `x` burns the child's `1.` while `z`
        // silently becomes `1.` too.
        assert_eq!(
            list_rows(cx, "1. x\n    - y\n2. z"),
            vec![
                ("1.".to_string(), 0, "x".to_string()),
                ("•".to_string(), 1, "y".to_string()),
                ("2.".to_string(), 0, "z".to_string()),
            ]
        );
    }

    #[gpui::test]
    fn nested_task_items_keep_their_own_checkboxes(cx: &mut TestAppContext) {
        // Checkboxes are pushed into `current_text`, so they travelled with
        // whatever the flush picked up: both used to land on one row.
        assert_eq!(
            list_rows(cx, "- [ ] parent\n    - [x] child"),
            vec![
                ("•".to_string(), 0, "☐ parent".to_string()),
                ("•".to_string(), 1, "☑ child".to_string()),
            ]
        );
    }

    #[gpui::test]
    fn a_parent_with_no_text_of_its_own_emits_no_row(cx: &mut TestAppContext) {
        // Nothing buffered to flush, so the guard in `flush_list_item` has to
        // keep the extra flush from inventing a blank row.
        assert_eq!(
            list_rows(cx, "-\n    - child"),
            vec![("•".to_string(), 1, "child".to_string())]
        );
    }

    #[gpui::test]
    fn a_list_that_follows_a_paragraph_does_not_absorb_it(cx: &mut TestAppContext) {
        // A top-level `Tag::List` has no parent item to attribute a row to,
        // and the paragraph already flushed at `End(Paragraph)`. An unguarded
        // flush here could only ever invent a row.
        assert_eq!(
            list_rows(cx, "A paragraph.\n\n- item"),
            vec![("•".to_string(), 0, "item".to_string())]
        );
    }

    #[gpui::test]
    fn a_nested_list_lays_out_as_many_rows_as_a_flat_one(cx: &mut TestAppContext) {
        assert_same_row_count(cx, "- x\n    - y", "- x\n- y");
    }

    #[gpui::test]
    fn a_deeply_nested_list_lays_out_as_many_rows_as_a_flat_one(cx: &mut TestAppContext) {
        assert_same_row_count(
            cx,
            "1. a\n    - b\n        - c\n2. d",
            "1. a\n- b\n- c\n2. d",
        );
    }

    // --- loose lists ---

    /// A list whose items are separated by a blank line, and the same list
    /// without them. CommonMark calls the first *loose* and wraps each item's
    /// content in a paragraph; both must render as the same list.
    #[track_caller]
    fn assert_loose_matches_tight(cx: &mut TestAppContext, loose: &str, tight: &str) {
        let loose_rows = list_rows(cx, loose);
        let tight_rows = list_rows(cx, tight);

        assert!(
            !tight_rows.is_empty(),
            "{tight:?} emitted no rows at all, so this measures nothing"
        );
        assert_eq!(
            loose_rows, tight_rows,
            "{loose:?} emitted {loose_rows:?}, but the same items tight ({tight:?}) \
             emitted {tight_rows:?}"
        );
    }

    #[gpui::test]
    fn a_loose_list_keeps_its_markers(cx: &mut TestAppContext) {
        // The whole bug: the blank line makes each item's content a paragraph,
        // which was flushed as body text — no marker, no indent, no number —
        // and the item's `End(Item)` then found an empty buffer.
        assert_eq!(
            list_rows(cx, "- one\n\n- two\n"),
            vec![
                ("•".to_string(), 0, "one".to_string()),
                ("•".to_string(), 0, "two".to_string()),
            ]
        );
    }

    #[gpui::test]
    fn a_loose_ordered_list_keeps_its_numbers(cx: &mut TestAppContext) {
        assert_eq!(
            list_rows(cx, "1. one\n\n2. two\n\n3. three\n"),
            vec![
                ("1.".to_string(), 0, "one".to_string()),
                ("2.".to_string(), 0, "two".to_string()),
                ("3.".to_string(), 0, "three".to_string()),
            ]
        );
    }

    #[gpui::test]
    fn loose_and_tight_lists_emit_the_same_rows(cx: &mut TestAppContext) {
        for (loose, tight) in [
            ("- one\n\n- two\n", "- one\n- two\n"),
            ("1. one\n\n2. two\n", "1. one\n2. two\n"),
            (
                "- **bold** and `code` and a [link](#)\n\n- plain\n",
                "- **bold** and `code` and a [link](#)\n- plain\n",
            ),
            ("- [ ] todo\n\n- [x] done\n", "- [ ] todo\n- [x] done\n"),
            // Looseness is per list: the outer list here is loose and the
            // inner one tight, and both routes have to agree.
            ("- parent\n\n    - child\n", "- parent\n    - child\n"),
        ] {
            assert_loose_matches_tight(cx, loose, tight);
        }
    }

    #[gpui::test]
    fn a_nested_loose_list_still_indents(cx: &mut TestAppContext) {
        assert_eq!(
            list_rows(cx, "- parent\n\n    - child\n"),
            vec![
                ("•".to_string(), 0, "parent".to_string()),
                ("•".to_string(), 1, "child".to_string()),
            ]
        );
    }

    #[gpui::test]
    fn a_loose_item_is_announced_as_a_list_item(cx: &mut TestAppContext) {
        assert_eq!(
            run_roles(cx, "- one\n\n- two\n"),
            vec![Some(Role::ListItem), Some(Role::ListItem)],
            "a loose list used to be announced as a sequence of paragraphs"
        );
    }

    #[gpui::test]
    fn an_item_with_two_blocks_draws_one_marker(cx: &mut TestAppContext) {
        assert_eq!(
            emitted_rows(cx, "- first block\n\n  second block\n"),
            vec![
                (
                    ItemMarker::Shown("•".to_string()),
                    0,
                    "first block".to_string()
                ),
                (
                    ItemMarker::Hidden("•".to_string()),
                    0,
                    "second block".to_string()
                ),
            ]
        );
    }

    #[gpui::test]
    fn an_items_second_paragraph_does_not_burn_a_number(cx: &mut TestAppContext) {
        // The ordinal belongs to the item, not to the flush: taken per flush,
        // `still one` would have eaten `2.` and left the next item at `3.`.
        assert_eq!(
            list_rows(cx, "1. one\n\n   still one\n\n2. two\n"),
            vec![
                ("1.".to_string(), 0, "one".to_string()),
                (String::new(), 0, "still one".to_string()),
                ("2.".to_string(), 0, "two".to_string()),
            ]
        );
    }

    #[gpui::test]
    fn a_continuation_block_is_announced_as_a_paragraph(cx: &mut TestAppContext) {
        // The runs are a flat list, so a second `ListItem` here would tell a
        // screen reader this one-item list has two items.
        assert_eq!(
            run_roles(cx, "- first block\n\n  second block\n"),
            vec![Some(Role::ListItem), Some(Role::Paragraph)]
        );
    }

    #[gpui::test]
    fn a_paragraph_after_a_list_is_still_a_paragraph(cx: &mut TestAppContext) {
        assert_eq!(
            run_roles(cx, "- one\n\n- two\n\nAfter the list.\n"),
            vec![
                Some(Role::ListItem),
                Some(Role::ListItem),
                Some(Role::Paragraph)
            ]
        );
    }

    #[gpui::test]
    fn a_block_after_a_nested_list_belongs_to_the_item_that_held_it(cx: &mut TestAppContext) {
        // The inner `End(List)` arrives first, so this block lands back at the
        // parent's indent under a hidden marker — which is what alignment with
        // the parent's own text demands.
        assert_eq!(
            emitted_rows(cx, "- parent\n\n    - child\n\n  after the child\n"),
            vec![
                (ItemMarker::Shown("•".to_string()), 0, "parent".to_string()),
                (ItemMarker::Shown("•".to_string()), 1, "child".to_string()),
                (
                    ItemMarker::Hidden("•".to_string()),
                    0,
                    "after the child".to_string()
                ),
            ]
        );
    }

    #[gpui::test]
    fn a_loose_list_lays_out_like_a_tight_one(cx: &mut TestAppContext) {
        assert_eq!(height(cx, "- one\n\n- two\n"), height(cx, "- one\n- two\n"));
    }

    #[gpui::test]
    fn a_continuation_block_starts_in_the_items_text_column(cx: &mut TestAppContext) {
        // A hidden marker still takes up its width, so an item's second block
        // wraps exactly like a second item would. Drop that width and the
        // block gets a wider column and comes out shorter.
        let two_blocks = height(cx, &format!("- {LONG}\n\n  {LONG}"));
        let two_items = height(cx, &format!("- {LONG}\n- {LONG}"));

        assert_eq!(
            two_blocks, two_items,
            "an item's second block ({two_blocks:?}) did not wrap like a second \
             item ({two_items:?})"
        );
    }

    #[gpui::test]
    fn a_long_table_cell_wraps(cx: &mut TestAppContext) {
        let line = line(cx);
        let short = height(cx, "| A | B |\n| --- | --- |\n| one | two |");
        let long = height(cx, &format!("| A | B |\n| --- | --- |\n| {LONG} | two |"));

        assert!(
            long >= short + line,
            "the cell stayed one line tall ({long:?} against {short:?}) instead of wrapping"
        );
    }

    #[gpui::test]
    fn a_document_keeps_its_run_ids_across_frames(cx: &mut TestAppContext) {
        // Assistive technology reads a changed node id as a different
        // element, so redrawing an unchanged document must not renumber it.
        cx.update(crate::theme::init);
        let doc = document(cx, EVERY_RUN_KIND);
        let cx = cx.add_empty_window();

        let first = draw(cx, || vec![MarkdownElement::new(doc.clone())]);
        let second = draw(cx, || vec![MarkdownElement::new(doc.clone())]);

        assert_eq!(id_paths(&first), id_paths(&second));
        assert!(!first.is_empty());
    }

    #[gpui::test]
    fn an_explicit_id_separates_two_elements_over_one_entity(cx: &mut TestAppContext) {
        cx.update(crate::theme::init);
        let doc = document(cx, EVERY_RUN_KIND);
        let cx = cx.add_empty_window();

        let runs = draw(cx, || {
            vec![
                MarkdownElement::new(doc.clone()).id("left"),
                MarkdownElement::new(doc.clone()).id("right"),
            ]
        });

        let paths = id_paths(&runs);
        let distinct: HashSet<&String> = paths.iter().collect();
        assert_eq!(distinct.len(), 10, "colliding id paths: {paths:?}");

        let documents: HashSet<&str> = runs.iter().map(|run| doc_and_run(run).0).collect();
        assert_eq!(
            documents,
            HashSet::from(["left", "right"]),
            "the override should replace the entity-derived scope"
        );
    }

    // --- streaming ---

    /// Everything the parse produced as text, so a test can say what the
    /// document currently reads as without spelling out an event list.
    fn rendered_text(markdown: &Markdown) -> String {
        markdown
            .events()
            .iter()
            .filter_map(|event| match &event.event {
                Event::Text(text) | Event::Code(text) => Some(text.to_string()),
                _ => None,
            })
            .collect()
    }

    fn document(cx: &mut TestAppContext, source: &str) -> Entity<Markdown> {
        let source = source.to_string();
        cx.new(|cx| Markdown::new(source, cx))
    }

    /// Count parses by counting notifications: a landed parse is the only
    /// thing this entity notifies for.
    fn count_parses(cx: &mut TestAppContext, markdown: &Entity<Markdown>) -> Rc<Cell<usize>> {
        let parses = Rc::new(Cell::new(0));
        let counter = parses.clone();
        cx.update(|cx| {
            cx.observe(markdown, move |_, _| counter.set(counter.get() + 1))
                .detach()
        });
        parses
    }

    #[gpui::test]
    fn the_first_parse_is_synchronous(cx: &mut TestAppContext) {
        // A document must not be empty on its first frame, so `new` parses
        // before it returns rather than scheduling like every later parse.
        let markdown = document(cx, "# Hello");

        markdown.read_with(cx, |markdown, _| {
            assert_eq!(rendered_text(markdown), "Hello");
            assert_eq!(markdown.parsed_source(), "# Hello");
            assert!(!markdown.is_parsing());
        });
    }

    #[gpui::test]
    fn append_extends_the_document(cx: &mut TestAppContext) {
        let markdown = document(cx, "Hello");
        markdown.update(cx, |markdown, cx| markdown.append(", world", cx));
        cx.run_until_parked();

        markdown.read_with(cx, |markdown, _| {
            assert_eq!(markdown.source(), "Hello, world");
            assert_eq!(markdown.parsed_source(), "Hello, world");
            assert_eq!(rendered_text(markdown), "Hello, world");
        });
    }

    #[gpui::test]
    fn appending_nothing_is_inert(cx: &mut TestAppContext) {
        let markdown = document(cx, "Hello");
        let parses = count_parses(cx, &markdown);

        markdown.update(cx, |markdown, cx| markdown.append("", cx));
        cx.run_until_parked();

        assert_eq!(parses.get(), 0, "an empty delta scheduled a parse");
        markdown.read_with(cx, |markdown, _| assert_eq!(markdown.source(), "Hello"));
    }

    #[gpui::test]
    fn the_old_parse_keeps_rendering_until_the_new_one_lands(cx: &mut TestAppContext) {
        // The point of parsing off the UI thread is that the view never goes
        // blank: the previous events stay up while the new parse runs.
        let markdown = document(cx, "before");
        markdown.update(cx, |markdown, cx| markdown.append(" and after", cx));

        markdown.read_with(cx, |markdown, _| {
            assert!(markdown.is_parsing());
            assert_eq!(markdown.source(), "before and after");
            assert_eq!(markdown.parsed_source(), "before");
            assert_eq!(rendered_text(markdown), "before");
        });

        cx.run_until_parked();

        markdown.read_with(cx, |markdown, _| {
            assert!(!markdown.is_parsing());
            assert_eq!(rendered_text(markdown), "before and after");
        });
    }

    #[gpui::test]
    fn deltas_arriving_during_a_parse_coalesce(cx: &mut TestAppContext) {
        let markdown = document(cx, "one");
        let parses = count_parses(cx, &markdown);

        // Five deltas with no chance for the executor to run in between: the
        // first schedules a parse, the other four only mark it dirty.
        markdown.update(cx, |markdown, cx| {
            for delta in [" two", " three", " four", " five", " six"] {
                markdown.append(delta, cx);
            }
        });
        cx.run_until_parked();

        assert_eq!(
            parses.get(),
            2,
            "five deltas during one parse should cost one extra parse, not four"
        );
        markdown.read_with(cx, |markdown, _| {
            assert_eq!(rendered_text(markdown), "one two three four five six")
        });
    }

    #[gpui::test]
    fn a_long_stream_lands_on_the_final_source(cx: &mut TestAppContext) {
        let markdown = document(cx, "");
        let mut expected = String::new();

        for i in 0..200 {
            let delta = format!("{i} ");
            expected.push_str(&delta);
            markdown.update(cx, |markdown, cx| markdown.append(&delta, cx));
            // Let some deltas land mid-parse and others find the document idle.
            if i % 7 == 0 {
                cx.run_until_parked();
            }
        }
        cx.run_until_parked();

        markdown.read_with(cx, |markdown, _| {
            assert_eq!(markdown.source(), expected);
            assert_eq!(markdown.parsed_source(), expected);
            assert!(!markdown.is_parsing());
            assert_eq!(rendered_text(markdown), expected.trim_end());
        });
    }

    #[gpui::test]
    fn setting_the_same_source_does_not_reparse(cx: &mut TestAppContext) {
        let markdown = document(cx, "# Same");
        let parses = count_parses(cx, &markdown);

        markdown.update(cx, |markdown, cx| markdown.set_source("# Same", cx));
        cx.run_until_parked();
        assert_eq!(parses.get(), 0, "an unchanged source scheduled a parse");

        markdown.update(cx, |markdown, cx| markdown.set_source("# Different", cx));
        cx.run_until_parked();
        assert_eq!(parses.get(), 1);
    }

    #[gpui::test]
    fn append_keeps_the_selection(cx: &mut TestAppContext) {
        // Selection positions are `(run, byte offset within the run)`, so text
        // arriving at the end of the document cannot disturb one made earlier.
        let markdown = document(cx, "First block\n\nSecond block");
        let selection = markdown.read_with(cx, |markdown, _| markdown.selection());
        selection.select_in_run(0, 0..5);

        markdown.update(cx, |markdown, cx| markdown.append("\n\nThird block", cx));
        cx.run_until_parked();

        assert!(!selection.is_empty(), "append dropped the selection");
        let (start, end) = selection.range().expect("the selection went away");
        assert_eq!((start.run, start.offset), (0, 0));
        assert_eq!((end.run, end.offset), (0, 5));
    }

    #[gpui::test]
    fn set_source_drops_the_selection(cx: &mut TestAppContext) {
        let markdown = document(cx, "First block\n\nSecond block");
        let selection = markdown.read_with(cx, |markdown, _| markdown.selection());
        selection.select_in_run(0, 0..5);

        markdown.update(cx, |markdown, cx| markdown.set_source("Something else", cx));

        assert!(
            selection.is_empty(),
            "the selection survived a source it no longer indexes"
        );
    }

    #[gpui::test]
    fn the_default_document_id_follows_the_entity(cx: &mut TestAppContext) {
        cx.update(crate::theme::init);
        let doc = document(cx, "Hello.");
        let expected = document_element_id(doc.entity_id());

        assert_eq!(MarkdownElement::new(doc.clone()).element_id(), expected);
        assert_eq!(
            MarkdownElement::new(doc).id("mine").element_id(),
            ElementId::Name("mine".into())
        );
    }

    #[gpui::test]
    fn every_run_kind_reports_a_role_and_its_text(cx: &mut TestAppContext) {
        cx.update(crate::theme::init);
        let doc = document(cx, EVERY_RUN_KIND);
        let cx = cx.add_empty_window();

        let runs = draw(cx, || vec![MarkdownElement::new(doc.clone())]);

        let reported: Vec<(Option<Role>, Option<&str>, Option<usize>)> = runs
            .iter()
            .map(|run| (run.role, run.label.as_deref(), run.level))
            .collect();

        assert_eq!(
            reported,
            vec![
                (Some(Role::Heading), Some("Title"), Some(1)),
                (Some(Role::Paragraph), Some("A paragraph."), None),
                (Some(Role::Blockquote), Some("A quote."), None),
                (Some(Role::ListItem), Some("An item"), None),
                (Some(Role::Code), Some("let x = 1;\n"), None),
            ]
        );
    }

    #[gpui::test]
    fn headings_report_their_level(cx: &mut TestAppContext) {
        cx.update(crate::theme::init);
        let doc = document(cx, "# One\n\n### Three\n");
        let cx = cx.add_empty_window();

        let runs = draw(cx, || vec![MarkdownElement::new(doc.clone())]);

        let levels: Vec<_> = runs.iter().map(|run| run.level).collect();
        assert_eq!(levels, vec![Some(1), Some(3)]);
    }

    #[test]
    fn heading_levels_are_numbered_one_through_six() {
        let levels: Vec<u8> = [
            HeadingLevel::H1,
            HeadingLevel::H2,
            HeadingLevel::H3,
            HeadingLevel::H4,
            HeadingLevel::H5,
            HeadingLevel::H6,
        ]
        .into_iter()
        .map(HeadingLevel::level)
        .collect();

        assert_eq!(levels, vec![1, 2, 3, 4, 5, 6]);
    }

    #[gpui::test]
    fn dropping_the_document_mid_parse_is_harmless(cx: &mut TestAppContext) {
        let markdown = document(cx, "start");
        markdown.update(cx, |markdown, cx| markdown.append(" more", cx));
        drop(markdown);

        // The parse task is owned by the entity, so this drives an in-flight
        // parse whose document is already gone.
        cx.run_until_parked();
    }

    #[cfg(feature = "stitch")]
    #[gpui::test]
    fn partial_emphasis_renders_as_emphasis(cx: &mut TestAppContext) {
        // Mid-stream `**bold` has no closer yet. Parsed as-is it draws literal
        // asterisks that turn into bold one delta later — the flicker.
        let markdown = document(cx, "A **partially written");

        markdown.read_with(cx, |markdown, _| {
            assert_eq!(rendered_text(markdown), "A partially written");
            assert!(
                markdown
                    .events()
                    .iter()
                    .any(|event| matches!(event.event, Event::Start(Tag::Strong)))
            );
        });
    }

    #[gpui::test]
    #[cfg(feature = "stitch")]
    fn a_partial_link_is_not_a_link_yet(cx: &mut TestAppContext) {
        // The label reads as plain text until the URL completes — a live link
        // to a placeholder URL would be worse than no link.
        let markdown = document(cx, "See [the docs](htt");

        markdown.read_with(cx, |markdown, _| {
            assert_eq!(rendered_text(markdown), "See the docs");
            assert!(
                !markdown
                    .events()
                    .iter()
                    .any(|event| matches!(event.event, Event::Start(Tag::Link { .. }))),
                "an incomplete URL became a clickable link"
            );
        });
    }

    #[cfg(feature = "stitch")]
    #[gpui::test]
    fn a_complete_document_parses_the_same_either_way(cx: &mut TestAppContext) {
        let source = "# Title\n\nA **bold** word, `code`, and a [link](https://example.com).\n";
        let markdown = document(cx, source);
        let with_preprocessing = markdown.read_with(cx, |markdown, _| rendered_text(markdown));

        markdown.update(cx, |markdown, cx| {
            markdown.set_preprocess_partial(false, cx)
        });
        cx.run_until_parked();

        markdown.read_with(cx, |markdown, _| {
            assert_eq!(rendered_text(markdown), with_preprocessing)
        });
    }

    #[cfg(feature = "stitch")]
    #[gpui::test]
    fn preprocessing_can_be_turned_off(cx: &mut TestAppContext) {
        let markdown = document(cx, "A **partially written");
        markdown.update(cx, |markdown, cx| {
            assert!(markdown.preprocess_partial());
            markdown.set_preprocess_partial(false, cx);
        });
        cx.run_until_parked();

        markdown.read_with(cx, |markdown, _| {
            assert!(!markdown.preprocess_partial());
            assert_eq!(rendered_text(markdown), "A **partially written");
        });
    }

    // --- a code fence that has not closed yet ---
    //
    // A streaming fence is a different cache key on every delta, so
    // highlighting it costs a full syntect pass per frame and fills the cache
    // with prefixes that evict every settled block. The fix is to draw it
    // plain until its closer arrives, which these read as `language == None`
    // on the block — the same thing a bare fence gets, and the path that skips
    // syntect entirely.

    /// The language each code block was flushed with, in document order.
    fn emitted_code_block_languages(cx: &mut TestAppContext, source: &str) -> Vec<Option<String>> {
        cx.update(crate::theme::init);
        let parsed = Markdown::parse(source, false);

        cx.update(|cx: &mut App| {
            let mut renderer = MarkdownRenderer::new(
                MarkdownStyle::default(),
                MarkdownSelection::new(),
                parsed.unclosed_code_block,
            );
            for event in &parsed.events {
                renderer.handle_event(&event.event, cx);
            }
            renderer.emitted_code_blocks.clone()
        })
    }

    #[gpui::test]
    fn an_unclosed_fence_is_drawn_plain(cx: &mut TestAppContext) {
        assert_eq!(
            emitted_code_block_languages(cx, "```rust\nfn main() {\n"),
            vec![None],
            "a fence still arriving must not be highlighted"
        );
    }

    #[gpui::test]
    fn a_closed_fence_keeps_its_language(cx: &mut TestAppContext) {
        assert_eq!(
            emitted_code_block_languages(cx, "```rust\nfn main() {}\n```\n"),
            vec![Some("rust".to_string())],
            "a settled block highlights, once, forever"
        );
    }

    /// Only the block that is actually open loses its language. Earlier blocks
    /// in the same document are settled and keep theirs.
    #[gpui::test]
    fn only_the_open_block_loses_its_language(cx: &mut TestAppContext) {
        let source = "```rust\nfn a() {}\n```\n\nThen:\n\n```python\ndef b():\n";

        assert_eq!(
            emitted_code_block_languages(cx, source),
            vec![Some("rust".to_string()), None],
        );
    }

    /// The issue proposed keying this on `stitch::close_open_syntax` returning
    /// `Cow::Owned`. It cannot be: an open `**bold` after a *settled* fence
    /// makes that signal fire and would strip a finished block's colors.
    #[gpui::test]
    fn an_open_inline_marker_does_not_disturb_a_settled_fence(cx: &mut TestAppContext) {
        let source = "```rust\nfn a() {}\n```\n\n**bold";

        assert_eq!(
            emitted_code_block_languages(cx, source),
            vec![Some("rust".to_string())],
            "the block closed; what happens after it is not its business"
        );
    }

    /// The entity end to end: a fence streamed in through `append` is plain
    /// for every delta until the closer lands, and highlighted from then on.
    #[gpui::test]
    fn a_streamed_fence_highlights_when_it_finishes(cx: &mut TestAppContext) {
        let deltas = [
            "Here you go:\n\n",
            "```ru",
            "st\n",
            "fn main() {\n",
            "    println!(\"hi\");\n",
            "}\n",
            "``",
            "`\n",
            "\nDone.",
        ];

        let markdown = document(cx, "");

        for (index, delta) in deltas.iter().enumerate() {
            markdown.update(cx, |markdown, cx| markdown.append(delta, cx));
            cx.run_until_parked();

            let source = markdown.read_with(cx, |markdown, _| markdown.source().to_string());
            let languages = emitted_code_block_languages(cx, &source);
            let closed = index >= 7;

            if index < 1 {
                assert!(languages.is_empty(), "no code block yet: {source:?}");
            } else if closed {
                assert_eq!(
                    languages,
                    vec![Some("rust".to_string())],
                    "after delta {index} the fence has closed: {source:?}"
                );
            } else {
                assert_eq!(
                    languages,
                    vec![None],
                    "after delta {index} the fence is still open: {source:?}"
                );
            }
        }
    }
}