Allow folding buffers inside multi buffers (#22046)

Closes https://github.com/zed-industries/zed/issues/4925


https://github.com/user-attachments/assets/e7b87375-893f-41ae-a2d9-d501499e40d1


Allows to fold any buffer inside multi buffers, either by clicking the
chevron icon on the header, or by using
`editor::Fold`/`editor::UnfoldLines`/`editor::ToggleFold`/`editor::FoldAll`
and `editor::UnfoldAll` actions inside the multi buffer (those were noop
there before).

Every fold has a fake line inside it, so it's possible to navigate into
that via the keyboard and unfold it with the corresponding editor
action.

The state is synchronized with the outline panel state: any fold inside
multi buffer folds the corresponding file entry; any file entry fold
inside the outline panel folds the corresponding buffer inside the multi
buffer, any directory fold inside the outline panel folds the
corresponding buffers inside the multi buffer for each nested file entry
in the panel.


Release Notes:

- Added a possibility to fold buffers inside multi buffers

---------

Co-authored-by: Antonio Scandurra <antonio@zed.dev>
Co-authored-by: Max Brunsfeld <max@zed.dev>
Co-authored-by: Cole Miller <cole@zed.dev>

crates/editor/src/display_map.rs

@@ -269,7 +269,7 @@ impl DisplayMap {
                     let start = buffer_snapshot.anchor_before(range.start);
                     let end = buffer_snapshot.anchor_after(range.end);
                     BlockProperties {
-                        placement: BlockPlacement::Replace(start..end),
+                        placement: BlockPlacement::Replace(start..=end),
                         render,
                         height,
                         style,
@@ -336,6 +336,38 @@ impl DisplayMap {
         block_map.remove_intersecting_replace_blocks(offset_ranges, inclusive);
     }
 
+    pub fn fold_buffer(&mut self, buffer_id: language::BufferId, cx: &mut ModelContext<Self>) {
+        let snapshot = self.buffer.read(cx).snapshot(cx);
+        let edits = self.buffer_subscription.consume().into_inner();
+        let tab_size = Self::tab_size(&self.buffer, cx);
+        let (snapshot, edits) = self.inlay_map.sync(snapshot, edits);
+        let (snapshot, edits) = self.fold_map.read(snapshot, edits);
+        let (snapshot, edits) = self.tab_map.sync(snapshot, edits, tab_size);
+        let (snapshot, edits) = self
+            .wrap_map
+            .update(cx, |map, cx| map.sync(snapshot, edits, cx));
+        let mut block_map = self.block_map.write(snapshot, edits);
+        block_map.fold_buffer(buffer_id, self.buffer.read(cx), cx)
+    }
+
+    pub fn unfold_buffer(&mut self, buffer_id: language::BufferId, cx: &mut ModelContext<Self>) {
+        let snapshot = self.buffer.read(cx).snapshot(cx);
+        let edits = self.buffer_subscription.consume().into_inner();
+        let tab_size = Self::tab_size(&self.buffer, cx);
+        let (snapshot, edits) = self.inlay_map.sync(snapshot, edits);
+        let (snapshot, edits) = self.fold_map.read(snapshot, edits);
+        let (snapshot, edits) = self.tab_map.sync(snapshot, edits, tab_size);
+        let (snapshot, edits) = self
+            .wrap_map
+            .update(cx, |map, cx| map.sync(snapshot, edits, cx));
+        let mut block_map = self.block_map.write(snapshot, edits);
+        block_map.unfold_buffer(buffer_id, self.buffer.read(cx), cx)
+    }
+
+    pub(crate) fn buffer_folded(&self, buffer_id: language::BufferId) -> bool {
+        self.block_map.folded_buffers.contains(&buffer_id)
+    }
+
     pub fn insert_creases(
         &mut self,
         creases: impl IntoIterator<Item = Crease<Anchor>>,
@@ -712,7 +744,11 @@ impl DisplaySnapshot {
         }
     }
 
-    pub fn next_line_boundary(&self, mut point: MultiBufferPoint) -> (Point, DisplayPoint) {
+    pub fn next_line_boundary(
+        &self,
+        mut point: MultiBufferPoint,
+    ) -> (MultiBufferPoint, DisplayPoint) {
+        let original_point = point;
         loop {
             let mut inlay_point = self.inlay_snapshot.to_inlay_point(point);
             let mut fold_point = self.fold_snapshot.to_fold_point(inlay_point, Bias::Right);
@@ -723,7 +759,7 @@ impl DisplaySnapshot {
             let mut display_point = self.point_to_display_point(point, Bias::Right);
             *display_point.column_mut() = self.line_len(display_point.row());
             let next_point = self.display_point_to_point(display_point, Bias::Right);
-            if next_point == point {
+            if next_point == point || original_point == point || original_point == next_point {
                 return (point, display_point);
             }
             point = next_point;
@@ -1081,10 +1117,6 @@ impl DisplaySnapshot {
             || self.fold_snapshot.is_line_folded(buffer_row)
     }
 
-    pub fn is_line_replaced(&self, buffer_row: MultiBufferRow) -> bool {
-        self.block_snapshot.is_line_replaced(buffer_row)
-    }
-
     pub fn is_block_line(&self, display_row: DisplayRow) -> bool {
         self.block_snapshot.is_block_line(BlockRow(display_row.0))
     }
@@ -2231,7 +2263,7 @@ pub mod tests {
                 [BlockProperties {
                     placement: BlockPlacement::Replace(
                         buffer_snapshot.anchor_before(Point::new(1, 2))
-                            ..buffer_snapshot.anchor_after(Point::new(2, 3)),
+                            ..=buffer_snapshot.anchor_after(Point::new(2, 3)),
                     ),
                     height: 4,
                     style: BlockStyle::Fixed,