T21a round 1 fixes

- Enter over a selection drops it and splits at its start, one undo step
- undo and redo keep where the caret was and put it back
- a format unwraps only its own markers, so italic inside bold keeps the bold
- keys into a merged table cell land on the cell that shows
- undo rebuilds a block whose kind needs other fields, so styling comes back
- tests activate their window, as focus events need
- undo bookkeeping moves to blocks/edit.rs

AGENTS.md

@@ -114,6 +114,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - A component that fills its container says `w_full` on its root. As a window's root, or in a flex row, it would shrink to its text's narrowest width.
 - A field's `Highlight` sets color, wash, weight, slant and strike per span; measuring and painting share one set of runs, so wrapped heights match.
 - A block editor keeps one undo for the document: text and structure together, as snapshots through `forms::History`; it takes Undo and Redo before its fields do.
+- gpui calls focus listeners only while the window is active. A test that needs focus events activates its window with `window.activate_window()`.
 - A terminal takes Tab and Shift-Tab through bindings under `ElyTerminal`, ahead of focus moves; other keys go through `terminal::keys` in a key listener, and typed text through its input handler.
 - A shell in the gallery starts without startup files and with a fixed prompt, so captures show no user or host.
 - gpui 0.2.2 has no `(ElementId, usize)` id. A child id names its index: `(self.id.clone(), format!("row-{ix}"))`.

TASKS.md

@@ -247,4 +247,5 @@ Per-component lines live in `tasks/`. Tags there:
 | T19 | 3 | PASS | |
 | T20 | 1 | FAIL | the demo picked past memory's end; an empty gutter had no height to press; a breakpoint's box also opened it; zoomed ticks lost their places; docs promised bound keys and batch actions |
 | T20 | 2 | PASS | |
-| T21a | 1 | — | pending review |
+| T21a | 1 | FAIL | Enter kept a selection; undo of a split left focus on a gone field; italic inside bold ate the bold; keys went into merged cells; undo kept a changed kind's highlighter |
+| T21a | 2 | — | pending review |

src/documents/blocks/edit.rs

@@ -1,6 +1,6 @@
 use std::collections::HashMap;
 
-use gpui::{Context, Window};
+use gpui::{App, Context, Focusable, Window};
 
 use super::{Block, BlockData, BlockEditor, BlockEvent, BlockKind, Snapshot};
 
@@ -17,13 +17,13 @@ impl BlockEditor {
             self.turn_into(key, BlockKind::Paragraph, window, cx);
             return;
         }
-        let caret = field.read(cx).cursor();
-        let tail = field.read(cx).text()[caret..].to_string();
-        log::info!("block editor: block {key} splits at {caret}");
+        let selection = field.read(cx).selection();
+        let tail = field.read(cx).text()[selection.end..].to_string();
+        log::info!("block editor: block {key} splits over {selection:?}");
         self.before_change(cx);
         field.update(cx, |field, cx| {
             let end = field.text().len();
-            field.select(caret..end, cx);
+            field.select(selection.start..end, cx);
             field.insert("", cx);
         });
         let key = self.fresh_key();
@@ -73,29 +73,39 @@ impl BlockEditor {
 
     pub(crate) fn undo(&mut self, window: &mut Window, cx: &mut Context<Self>) {
         self.settle(true, cx);
-        if let Some(previous) = self.history.undo(self.current.clone()) {
+        if let Some(previous) = self.history.undo(self.now(cx)) {
             log::info!("block editor: undo");
             self.restore(previous, window, cx);
         }
     }
 
     pub(crate) fn redo(&mut self, window: &mut Window, cx: &mut Context<Self>) {
-        if let Some(next) = self.history.redo(self.current.clone()) {
+        if let Some(next) = self.history.redo(self.now(cx)) {
             log::info!("block editor: redo");
             self.restore(next, window, cx);
         }
     }
 
-    /// Puts the document back as `snapshot` holds it, keeping fields whose blocks stay.
+    /// The current document with the caret where it is now.
+    fn now(&self, cx: &Context<Self>) -> Snapshot {
+        let mut now = self.current.clone();
+        now.caret = self.snapshot(cx).caret;
+        now
+    }
+
+    /// Puts the document back as `snapshot` holds it, keeping fields whose blocks stay, and the caret where it was.
     fn restore(&mut self, snapshot: Snapshot, window: &mut Window, cx: &mut Context<Self>) {
         let mut kept: HashMap<u64, Block> = self
             .blocks
             .drain(..)
             .map(|block| (block.key, block))
             .collect();
-        for (key, data) in &snapshot.0 {
+        for (key, data) in &snapshot.blocks {
             let block = match kept.remove(key) {
-                Some(mut block) if block.fields.len() == data.texts.len() => {
+                Some(mut block)
+                    if block.fields.len() == data.texts.len()
+                        && block.kind.same_fields(&data.kind) =>
+                {
                     block.kind = data.kind.clone();
                     for (field, text) in block.fields.iter().zip(&data.texts) {
                         if field.read(cx).text() != text {
@@ -108,8 +118,70 @@ impl BlockEditor {
             };
             self.blocks.push(block);
         }
+        if let Some((key, field, selection)) = snapshot.caret.clone()
+            && let Some(block) = self.blocks.iter().find(|block| block.key == key)
+            && let Some(field) = block.fields.get(field)
+        {
+            field.update(cx, |field, cx| field.select(selection, cx));
+            window.focus(&field.focus_handle(cx));
+        }
         self.current = snapshot;
         cx.emit(BlockEvent::Changed);
         cx.notify();
     }
+
+    pub(super) fn snapshot(&self, cx: &App) -> Snapshot {
+        let caret = self.caret.and_then(|(key, field)| {
+            let block = self.blocks.iter().find(|block| block.key == key)?;
+            let selection = block.fields.get(field)?.read(cx).selection();
+            Some((key, field, selection))
+        });
+        Snapshot {
+            caret,
+            blocks: self
+                .blocks
+                .iter()
+                .map(|block| {
+                    let texts = block
+                        .fields
+                        .iter()
+                        .map(|field| field.read(cx).text().to_string())
+                        .collect();
+                    (
+                        block.key,
+                        BlockData {
+                            kind: block.kind.clone(),
+                            texts,
+                        },
+                    )
+                })
+                .collect(),
+        }
+    }
+
+    /// Records what changed as one undo step, a burst of typing as one, and tells the owner.
+    pub(super) fn settle(&mut self, typing: bool, cx: &mut Context<Self>) {
+        let now = self.snapshot(cx);
+        if now.blocks != self.current.blocks {
+            let before = std::mem::replace(&mut self.current, now);
+            self.history.record(before, typing);
+            cx.emit(BlockEvent::Changed);
+            cx.notify();
+        }
+    }
+
+    /// Records the document before a change to its blocks.
+    pub(crate) fn before_change(&mut self, cx: &mut Context<Self>) {
+        self.settle(true, cx);
+        let mut before = self.current.clone();
+        before.caret = self.snapshot(cx).caret;
+        self.history.record(before, false);
+    }
+
+    /// After a change to the blocks: the new state is current.
+    pub(crate) fn after_change(&mut self, cx: &mut Context<Self>) {
+        self.current = self.snapshot(cx);
+        cx.emit(BlockEvent::Changed);
+        cx.notify();
+    }
 }

src/documents/blocks/keys.rs

@@ -1,6 +1,6 @@
 use gpui::{Context, Window};
 
-use super::{BlockData, BlockEditor, BlockKind};
+use super::{BlockData, BlockEditor, BlockKind, table::shown_cell};
 use crate::documents::modes::at_edge;
 
 /// A new empty paragraph.
@@ -43,12 +43,13 @@ impl BlockEditor {
             BlockKind::Image(_) | BlockKind::Video(_) | BlockKind::Embed(_) => {
                 self.insert(Some(key), paragraph(), window, cx);
             }
-            BlockKind::Table { columns, .. } => {
+            BlockKind::Table { columns, merged } => {
                 let below = field + columns;
                 if below >= self.blocks[self.index(key)].fields.len() {
                     self.add_row(key, window, cx);
                 }
-                self.focus_field(key, below, None, window, cx);
+                let shown = shown_cell(&merged, columns, below);
+                self.focus_field(key, shown, None, window, cx);
             }
             _ => return false,
         }
@@ -95,14 +96,15 @@ impl BlockEditor {
         if !at_edge(block.fields[field].read(cx), down) {
             return false;
         }
-        if let BlockKind::Table { columns, .. } = block.kind {
+        if let BlockKind::Table { columns, merged } = &block.kind {
             let row = if down {
                 Some(field + columns)
             } else {
-                field.checked_sub(columns)
+                field.checked_sub(*columns)
             };
             if let Some(row) = row.filter(|row| *row < block.fields.len()) {
-                self.focus_field(key, row, None, window, cx);
+                let shown = shown_cell(merged, *columns, row);
+                self.focus_field(key, shown, None, window, cx);
                 return true;
             }
         }

src/documents/blocks/kind.rs

@@ -150,6 +150,16 @@ impl BlockKind {
         }
     }
 
+    /// Whether a field made for this kind serves `other` unchanged: the same highlighter and hint.
+    pub(crate) fn same_fields(&self, other: &Self) -> bool {
+        std::mem::discriminant(self) == std::mem::discriminant(other)
+            && match (self, other) {
+                (Self::Heading(one), Self::Heading(two)) => one == two,
+                (Self::Synced(one), Self::Synced(two)) => one == two,
+                _ => true,
+            }
+    }
+
     /// Prose: Enter splits it, Backspace at its start turns it back into text or joins it up.
     pub fn is_prose(&self) -> bool {
         matches!(

src/documents/blocks/mod.rs

@@ -61,9 +61,12 @@ pub(crate) struct Block {
     _changes: Vec<Subscription>,
 }
 
-/// The whole document, as undo keeps it.
-#[derive(Clone, Debug, PartialEq)]
-struct Snapshot(Vec<(u64, BlockData)>);
+/// The whole document as undo keeps it, and where the caret was: block, field and selection.
+#[derive(Clone, Debug)]
+struct Snapshot {
+    blocks: Vec<(u64, BlockData)>,
+    caret: Option<(u64, usize, std::ops::Range<usize>)>,
+}
 
 /// A document of blocks, Notion-like: prose, lists and to-dos, quotes and callouts, toggles, code, math, diagrams, dividers, tables, media, columns and synced blocks. Markdown at a paragraph's start changes its kind; `/` offers every kind; `@` and `:` suggest people and emoji; blocks drag by their handle. Undo covers text and structure alike.
 pub struct BlockEditor {
@@ -71,6 +74,8 @@ pub struct BlockEditor {
     next: u64,
     history: History<Snapshot>,
     current: Snapshot,
+    /// The field that last held focus, as block and field.
+    caret: Option<(u64, usize)>,
     pub(crate) people: Vec<SharedString>,
     pub(crate) diagram: Option<RenderDiagram>,
     _synced: Subscription,
@@ -85,7 +90,11 @@ impl BlockEditor {
             blocks: Vec::new(),
             next: 0,
             history: History::default(),
-            current: Snapshot(Vec::new()),
+            current: Snapshot {
+                blocks: Vec::new(),
+                caret: None,
+            },
+            caret: None,
             people: Vec::new(),
             diagram: None,
             _synced: synced,
@@ -114,34 +123,12 @@ impl BlockEditor {
     /// The document as it stands.
     pub fn blocks(&self, cx: &App) -> Vec<BlockData> {
         self.snapshot(cx)
-            .0
+            .blocks
             .into_iter()
             .map(|(_, data)| data)
             .collect()
     }
 
-    fn snapshot(&self, cx: &App) -> Snapshot {
-        Snapshot(
-            self.blocks
-                .iter()
-                .map(|block| {
-                    let texts = block
-                        .fields
-                        .iter()
-                        .map(|field| field.read(cx).text().to_string())
-                        .collect();
-                    (
-                        block.key,
-                        BlockData {
-                            kind: block.kind.clone(),
-                            texts,
-                        },
-                    )
-                })
-                .collect(),
-        )
-    }
-
     /// A field for `kind`: inline markdown styled as typed, or code colored as code.
     fn field(
         kind: &BlockKind,
@@ -300,9 +287,16 @@ impl BlockEditor {
                 }
                 self.settle(true, cx);
             }
-            InputEvent::Focus if field.read(cx).is_empty() => {
-                let hint = self.blocks[ix].kind.placeholder();
-                field.update(cx, |field, cx| field.set_placeholder(hint, cx));
+            InputEvent::Focus => {
+                self.caret = self.blocks[ix]
+                    .fields
+                    .iter()
+                    .position(|each| each == field)
+                    .map(|at| (key, at));
+                if field.read(cx).is_empty() {
+                    let hint = self.blocks[ix].kind.placeholder();
+                    field.update(cx, |field, cx| field.set_placeholder(hint, cx));
+                }
             }
             InputEvent::Blur if self.blocks[ix].kind == BlockKind::Paragraph => {
                 field.update(cx, |field, cx| field.set_placeholder("", cx));
@@ -332,30 +326,6 @@ impl BlockEditor {
         }
     }
 
-    /// Records what changed as one undo step, a burst of typing as one, and tells the owner.
-    fn settle(&mut self, typing: bool, cx: &mut Context<Self>) {
-        let now = self.snapshot(cx);
-        if now != self.current {
-            let before = std::mem::replace(&mut self.current, now);
-            self.history.record(before, typing);
-            cx.emit(BlockEvent::Changed);
-            cx.notify();
-        }
-    }
-
-    /// Records the document before a change to its blocks.
-    pub(crate) fn before_change(&mut self, cx: &mut Context<Self>) {
-        self.settle(true, cx);
-        self.history.record(self.current.clone(), false);
-    }
-
-    /// After a change to the blocks: the new state is current.
-    pub(crate) fn after_change(&mut self, cx: &mut Context<Self>) {
-        self.current = self.snapshot(cx);
-        cx.emit(BlockEvent::Changed);
-        cx.notify();
-    }
-
     pub(crate) fn focus_field(
         &self,
         key: u64,
@@ -453,7 +423,7 @@ impl BlockEditor {
 
     pub fn duplicate(&mut self, key: u64, window: &mut Window, cx: &mut Context<Self>) {
         let ix = self.index(key);
-        let data = self.snapshot(cx).0[ix].1.clone();
+        let data = self.snapshot(cx).blocks[ix].1.clone();
         self.insert(Some(key), data, window, cx);
     }
 

src/documents/blocks/table.rs

@@ -25,6 +25,15 @@ pub(crate) fn span(merged: &[(usize, usize)], columns: usize, row: usize, column
         .count()
 }
 
+/// The cell that shows for `cell`: itself, or the one a merge folds it into.
+pub(crate) fn shown_cell(merged: &[(usize, usize)], columns: usize, cell: usize) -> usize {
+    let (row, mut column) = (cell / columns, cell % columns);
+    while column > 0 && merged.contains(&(row, column)) {
+        column -= 1;
+    }
+    row * columns + column
+}
+
 /// Merges after `column` is removed: its own go, the ones right of it move left.
 pub(crate) fn without_column(merged: &[(usize, usize)], column: usize) -> Vec<(usize, usize)> {
     merged

src/documents/format.rs

@@ -2,7 +2,7 @@ use std::ops::Range;
 
 use gpui::{App, Entity};
 
-use super::markdown::{Mark, marks};
+use super::markdown::{Mark, edge, marks};
 use crate::forms::TextInput;
 
 /// A format the toolbars apply to markdown.
@@ -32,6 +32,17 @@ impl Format {
         }
     }
 
+    /// The mark an inline format writes.
+    fn mark(self) -> Option<Mark> {
+        match self {
+            Self::Bold => Some(Mark::Strong),
+            Self::Italic => Some(Mark::Emphasis),
+            Self::Strike => Some(Mark::Strike),
+            Self::Code => Some(Mark::Code),
+            _ => None,
+        }
+    }
+
     /// The prefix a line format starts its lines with; numbered lines count up from one.
     fn prefix(self, nth: usize) -> String {
         match self {
@@ -81,6 +92,28 @@ fn line_prefix(line: &str) -> usize {
     number_prefix(line)
 }
 
+/// The span of `format` that `selection` fills, but for other styles' markers: where it is written, and where its text sits.
+fn own(
+    text: &str,
+    selection: &Range<usize>,
+    format: Format,
+) -> Option<(Range<usize>, Range<usize>)> {
+    let wanted = format.mark().expect("an inline format has a mark");
+    let only_markers = |part: &str| part.chars().all(|ch| matches!(ch, '*' | '_' | '~' | '`'));
+    marks(text).into_iter().find_map(|(range, mark)| {
+        if mark != wanted || selection.start < range.start || range.end < selection.end {
+            return None;
+        }
+        let edge = edge(&text[range.clone()], mark);
+        let inner = range.start + edge..range.end - edge;
+        let fills = inner.start <= selection.start
+            && selection.end <= inner.end
+            && only_markers(&text[inner.start..selection.start])
+            && only_markers(&text[selection.end..inner.end]);
+        fills.then_some((range, inner))
+    })
+}
+
 /// `text` with `format` applied over `selection`, and where the selection lands. An inline format wraps the selection, or unwraps it when already wrapped; an empty one leaves the caret between its markers. A line format prefixes each touched line, or takes the prefix off when every line has it.
 pub fn apply(text: &str, selection: Range<usize>, format: Format) -> (String, Range<usize>) {
     assert!(
@@ -97,11 +130,21 @@ pub fn apply(text: &str, selection: Range<usize>, format: Format) -> (String, Ra
     if let Some(marker) = format.marker() {
         let (before, after) = (&text[..selection.start], &text[selection.end..]);
         let inner = &text[selection.clone()];
-        if before.ends_with(marker) && after.starts_with(marker) {
+        if selection.is_empty() && before.ends_with(marker) && after.starts_with(marker) {
             let start = selection.start - marker.len();
             let unwrapped = format!("{}{inner}{}", &before[..start], &after[marker.len()..]);
             return (unwrapped, start..selection.end - marker.len());
         }
+        if let Some((range, text_range)) = own(text, &selection, format) {
+            let shift = text_range.start - range.start;
+            let unwrapped = format!(
+                "{}{}{}",
+                &text[..range.start],
+                &text[text_range],
+                &text[range.end..]
+            );
+            return (unwrapped, selection.start - shift..selection.end - shift);
+        }
         let shift = marker.len();
         let wrapped = format!("{before}{marker}{inner}{marker}{after}");
         return (wrapped, selection.start + shift..selection.end + shift);
@@ -204,6 +247,19 @@ mod tests {
         );
     }
 
+    #[test]
+    fn a_format_unwraps_only_its_own_markers() {
+        assert_eq!(
+            apply("**bold**", 2..6, Format::Italic),
+            ("***bold***".into(), 3..7)
+        );
+        assert_eq!(
+            apply("a *it* b", 3..5, Format::Italic),
+            ("a it b".into(), 2..4)
+        );
+        assert_eq!(apply("**bold**", 2..6, Format::Bold), ("bold".into(), 0..4));
+    }
+
     #[test]
     fn line_formats_prefix_every_touched_line_or_take_it_off() {
         let text = "one\ntwo\nthree";

src/documents/markdown.rs

@@ -37,6 +37,16 @@ fn leading(text: &str, ch: char) -> usize {
         .sum()
 }
 
+/// How many bytes at each end of `span` write `mark`: its asterisks, tildes or backticks.
+pub(crate) fn edge(span: &str, mark: Mark) -> usize {
+    let first = span.chars().next().expect("a styled span has its marker");
+    let run = leading(span, first).min(span.len() / 2);
+    match mark {
+        Mark::Emphasis => run.min(first.len_utf8()),
+        _ => run,
+    }
+}
+
 /// The marks over `text`'s bytes: styles, code, links, headings and quotes, and the syntax that writes each.
 pub fn marks(text: &str) -> Vec<(Range<usize>, Mark)> {
     let mut out = Vec::new();
@@ -50,18 +60,13 @@ pub fn marks(text: &str) -> Vec<(Range<usize>, Mark)> {
                     Event::Start(Tag::Emphasis) => Mark::Emphasis,
                     _ => Mark::Strike,
                 };
-                let first = span.chars().next().expect("a styled span has its marker");
-                let edge = leading(span, first).min(span.len() / 2);
-                let edge = match mark {
-                    Mark::Emphasis => edge.min(first.len_utf8()),
-                    _ => edge,
-                };
+                let edge = edge(span, mark);
                 out.push((range.clone(), mark));
                 out.push((range.start..range.start + edge, Mark::Syntax));
                 out.push((range.end - edge..range.end, Mark::Syntax));
             }
             Event::Code(_) => {
-                let ticks = leading(span, '`');
+                let ticks = edge(span, Mark::Code);
                 out.push((range.clone(), Mark::Code));
                 out.push((range.start..range.start + ticks, Mark::Syntax));
                 out.push((range.end - ticks..range.end, Mark::Syntax));

src/documents/tests.rs

@@ -13,6 +13,7 @@ fn open(
         super::bind_keys(cx);
     });
     let (editor, cx) = cx.add_window_view(|window, cx| BlockEditor::new(blocks, window, cx));
+    cx.update(|window, _| window.activate_window());
     settle(cx);
     (editor, cx)
 }
@@ -182,3 +183,125 @@ fn a_press_beside_a_blocks_text_puts_the_caret_at_its_end(cx: &mut TestAppContex
         Some((editor.read_with(cx, |editor, _| editor.blocks[0].key), 0))
     );
 }
+
+/// Puts block `ix`'s field `field` in focus with `range` selected.
+fn select(
+    editor: &Entity<BlockEditor>,
+    ix: usize,
+    field: usize,
+    range: std::ops::Range<usize>,
+    cx: &mut VisualTestContext,
+) {
+    cx.update(|window, cx| {
+        let field = editor.read(cx).blocks[ix].fields[field].clone();
+        field.update(cx, |field, cx| field.select(range, cx));
+        window.focus(&field.focus_handle(cx));
+    });
+    settle(cx);
+}
+
+#[gpui::test]
+fn enter_over_a_selection_drops_it_and_splits_there(cx: &mut TestAppContext) {
+    let (editor, cx) = open(vec![BlockData::new(BlockKind::Paragraph, ["abcdef"])], cx);
+    select(&editor, 0, 0, 2..4, cx);
+    cx.simulate_keystrokes("enter");
+    settle(cx);
+    assert_eq!(
+        blocks(&editor, cx),
+        [
+            text(BlockKind::Paragraph, "ab"),
+            text(BlockKind::Paragraph, "ef")
+        ]
+    );
+    cx.simulate_keystrokes("cmd-z");
+    settle(cx);
+    assert_eq!(
+        blocks(&editor, cx),
+        [text(BlockKind::Paragraph, "abcdef")],
+        "one step undoes it"
+    );
+}
+
+#[gpui::test]
+fn undo_puts_the_caret_back_where_the_change_began(cx: &mut TestAppContext) {
+    let (editor, cx) = open(vec![BlockData::new(BlockKind::Paragraph, ["abcdef"])], cx);
+    caret(&editor, 0, 3, cx);
+    cx.simulate_keystrokes("enter");
+    settle(cx);
+    cx.simulate_keystrokes("cmd-z");
+    settle(cx);
+    let first = cx.update(|_, cx| editor.read(cx).blocks[0].key);
+    let focused = cx.update(|window, cx| editor.read(cx).focused(window, cx));
+    assert_eq!(focused, Some((first, 0)));
+    cx.simulate_keystrokes("cmd-shift-z");
+    settle(cx);
+    assert_eq!(
+        blocks(&editor, cx),
+        [
+            text(BlockKind::Paragraph, "abc"),
+            text(BlockKind::Paragraph, "def")
+        ],
+        "redo splits again"
+    );
+    cx.simulate_keystrokes("cmd-z");
+    settle(cx);
+    cx.simulate_input("x");
+    settle(cx);
+    assert_eq!(
+        blocks(&editor, cx),
+        [text(BlockKind::Paragraph, "abcxdef")],
+        "typing lands at the caret undo put back"
+    );
+}
+
+#[gpui::test]
+fn keys_into_a_merged_cell_land_on_the_cell_that_shows(cx: &mut TestAppContext) {
+    let table = BlockKind::Table {
+        columns: 2,
+        merged: vec![(1, 1)],
+    };
+    let (editor, cx) = open(vec![BlockData::new(table, ["A", "B", "c", ""])], cx);
+    select(&editor, 0, 0, 1..1, cx);
+    cx.update(|window, cx| {
+        let field = editor.read(cx).blocks[0].fields[1].clone();
+        window.focus(&field.focus_handle(cx));
+    });
+    settle(cx);
+    cx.simulate_keystrokes("down");
+    settle(cx);
+    let key = cx.update(|_, cx| editor.read(cx).blocks[0].key);
+    let focused = cx.update(|window, cx| editor.read(cx).focused(window, cx));
+    assert_eq!(
+        focused,
+        Some((key, 2)),
+        "down from B lands on the cell c spans"
+    );
+}
+
+#[gpui::test]
+fn undoing_a_kind_change_brings_back_its_styling(cx: &mut TestAppContext) {
+    let (editor, cx) = open(vec![BlockData::new(BlockKind::Paragraph, ["**lift**"])], cx);
+    cx.update(|window, cx| {
+        let key = editor.read(cx).blocks[0].key;
+        editor.update(cx, |editor, cx| {
+            editor.turn_into(key, BlockKind::Code, window, cx)
+        });
+    });
+    settle(cx);
+    caret(&editor, 0, 0, cx);
+    cx.simulate_keystrokes("cmd-z");
+    settle(cx);
+    let bold = cx.update(|_, cx| {
+        let field = editor.read(cx).blocks[0].fields[0].clone();
+        field
+            .read(cx)
+            .highlights(cx)
+            .iter()
+            .any(|(_, highlight)| highlight.weight.is_some())
+    });
+    assert_eq!(
+        blocks(&editor, cx),
+        [text(BlockKind::Paragraph, "**lift**")]
+    );
+    assert!(bold, "the paragraph styles its markdown again");
+}