Navigate to multibuffer headers (WIP)

Issue #5129

crates/editor/src/editor.rs

@@ -9533,6 +9533,56 @@ impl Editor {
         })
     }
 
+    pub fn move_to_previous_multibuffer_header(
+        &mut self,
+        _: &MoveToStartOfExcerpt,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) {
+        if matches!(self.mode, EditorMode::SingleLine { .. }) {
+            cx.propagate();
+            return;
+        }
+
+        self.change_selections(Some(Autoscroll::fit()), window, cx, |s| {
+            s.move_with(|map, selection| {
+                selection.collapse_to(
+                    movement::multibuffer_header(
+                        map,
+                        selection.head(),
+                        workspace::searchable::Direction::Prev,
+                    ),
+                    SelectionGoal::None,
+                )
+            });
+        })
+    }
+
+    pub fn move_to_next_multibuffer_header(
+        &mut self,
+        _: &MoveToEndOfExcerpt,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) {
+        if matches!(self.mode, EditorMode::SingleLine { .. }) {
+            cx.propagate();
+            return;
+        }
+
+        self.change_selections(Some(Autoscroll::fit()), window, cx, |s| {
+            s.move_with(|map, selection| {
+                selection.collapse_to(
+                    movement::multibuffer_header(
+                        map,
+                        selection.head(),
+                        workspace::searchable::Direction::Next,
+                    ),
+                    SelectionGoal::None,
+                )
+            });
+        })
+    }
+
     pub fn select_to_end_of_paragraph(
         &mut self,
         _: &SelectToEndOfParagraph,

crates/editor/src/movement.rs

@@ -467,6 +467,31 @@ pub fn end_of_excerpt(
     }
 }
 
+pub fn multibuffer_header(
+    map: &DisplaySnapshot,
+    display_point: DisplayPoint,
+    direction: Direction,
+) -> DisplayPoint {
+    let point = map.display_point_to_point(display_point, Bias::Left);
+    // It seems likely that the better way to implement this is to reuse block logic via
+    // `map.blocks_in_range` and add support for `reversed_blocks_in_range`. I haven't evaluated in
+    // depth whether this will work.
+    //
+    // Before thinking of that, implementation plan was to:
+    //
+    // * For `Direction::Prev`, use `reversed_excerpts_at` to iterate over the excerpts in reverse
+    // order.
+    //
+    // * For `Direction::Next`, use `excerpts_at` to iterate over the excerpts.
+    //
+    // * Find boundaries by checking when `buffer_id` changes similar to the block_map logic
+    // [here](https://github.com/zed-industries/zed/blob/e5b61949148cd87e08ae38e80949bce9b4ede9f7/crates/editor/src/display_map/block_map.rs#L845).
+    //
+    // Another alternative might be to use `excerpt_before` and `excerpt_after` methods to walk the
+    // excerpts.
+    todo!();
+}
+
 /// Scans for a boundary preceding the given start point `from` until a boundary is found,
 /// indicated by the given predicate returning true.
 /// The predicate is called with the character to the left and right of the candidate boundary location.

crates/multi_buffer/src/multi_buffer.rs

@@ -4968,6 +4968,23 @@ impl MultiBufferSnapshot {
         }
     }
 
+    pub fn excerpts_at<T>(&self, position: T) -> impl Iterator<Item = MultiBufferExcerpt<'_>>
+    where
+        T: ToOffset,
+    {
+        todo!()
+    }
+
+    pub fn reversed_excerpts_at<T>(
+        &self,
+        position: T,
+    ) -> impl Iterator<Item = MultiBufferExcerpt<'_>>
+    where
+        T: ToOffset,
+    {
+        todo!()
+    }
+
     pub fn excerpt_before(&self, id: ExcerptId) -> Option<MultiBufferExcerpt<'_>> {
         let start_locator = self.excerpt_locator_for_id(id);
         let mut excerpts = self