git: Render hunk gutter highlights for the side-by-side diff (#48433)

Relanding #48321 

Release Notes:

- N/A

crates/editor/src/editor.rs

@@ -27095,12 +27095,20 @@ impl EditorSnapshot {
         self.buffer_snapshot()
             .diff_hunks_in_range(buffer_start..buffer_end)
             .filter_map(|hunk| {
-                if folded_buffers.contains(&hunk.buffer_id) {
+                if folded_buffers.contains(&hunk.buffer_id)
+                    || (hunk.row_range.is_empty() && self.buffer.all_diff_hunks_expanded())
+                {
                     return None;
                 }
 
                 let hunk_start_point = Point::new(hunk.row_range.start.0, 0);
-                let hunk_end_point = Point::new(hunk.row_range.end.0, 0);
+                let hunk_end_point = if hunk.row_range.end > hunk.row_range.start {
+                    let last_row = MultiBufferRow(hunk.row_range.end.0 - 1);
+                    let line_len = self.buffer_snapshot().line_len(last_row);
+                    Point::new(last_row.0, line_len)
+                } else {
+                    Point::new(hunk.row_range.end.0, 0)
+                };
 
                 let hunk_display_start = self.point_to_display_point(hunk_start_point, Bias::Left);
                 let hunk_display_end = self.point_to_display_point(hunk_end_point, Bias::Right);
@@ -27111,7 +27119,7 @@ impl EditorSnapshot {
                     }
                 } else {
                     let mut end_row = hunk_display_end.row();
-                    if hunk_display_end.column() > 0 {
+                    if hunk.row_range.end > hunk.row_range.start || hunk_display_end.column() > 0 {
                         end_row.0 += 1;
                     }
                     let is_created_file = hunk.is_created_file();

crates/editor/src/element.rs

@@ -6077,7 +6077,12 @@ impl EditorElement {
         }
     }
 
-    fn paint_gutter_diff_hunks(layout: &mut EditorLayout, window: &mut Window, cx: &mut App) {
+    fn paint_gutter_diff_hunks(
+        layout: &mut EditorLayout,
+        split_side: Option<SplitSide>,
+        window: &mut Window,
+        cx: &mut App,
+    ) {
         if layout.display_hunks.is_empty() {
             return;
         }
@@ -6104,37 +6109,37 @@ impl EditorElement {
                         status,
                         display_row_range,
                         ..
-                    } => hitbox.as_ref().map(|hunk_hitbox| match status.kind {
-                        DiffHunkStatusKind::Added => (
-                            hunk_hitbox.bounds,
-                            cx.theme().colors().version_control_added,
-                            Corners::all(px(0.)),
-                            *status,
-                        ),
-                        DiffHunkStatusKind::Modified => (
-                            hunk_hitbox.bounds,
-                            cx.theme().colors().version_control_modified,
-                            Corners::all(px(0.)),
-                            *status,
-                        ),
-                        DiffHunkStatusKind::Deleted if !display_row_range.is_empty() => (
-                            hunk_hitbox.bounds,
-                            cx.theme().colors().version_control_deleted,
-                            Corners::all(px(0.)),
-                            *status,
-                        ),
-                        DiffHunkStatusKind::Deleted => (
-                            Bounds::new(
-                                point(
-                                    hunk_hitbox.origin.x - hunk_hitbox.size.width,
-                                    hunk_hitbox.origin.y,
+                    } => hitbox.as_ref().map(|hunk_hitbox| {
+                        let color = match split_side {
+                            Some(SplitSide::Left) => cx.theme().colors().version_control_deleted,
+                            Some(SplitSide::Right) => cx.theme().colors().version_control_added,
+                            None => match status.kind {
+                                DiffHunkStatusKind::Added => {
+                                    cx.theme().colors().version_control_added
+                                }
+                                DiffHunkStatusKind::Modified => {
+                                    cx.theme().colors().version_control_modified
+                                }
+                                DiffHunkStatusKind::Deleted => {
+                                    cx.theme().colors().version_control_deleted
+                                }
+                            },
+                        };
+                        match status.kind {
+                            DiffHunkStatusKind::Deleted if display_row_range.is_empty() => (
+                                Bounds::new(
+                                    point(
+                                        hunk_hitbox.origin.x - hunk_hitbox.size.width,
+                                        hunk_hitbox.origin.y,
+                                    ),
+                                    size(hunk_hitbox.size.width * 2., hunk_hitbox.size.height),
                                 ),
-                                size(hunk_hitbox.size.width * 2., hunk_hitbox.size.height),
+                                color,
+                                Corners::all(1. * line_height),
+                                *status,
                             ),
-                            cx.theme().colors().version_control_deleted,
-                            Corners::all(1. * line_height),
-                            *status,
-                        ),
+                            _ => (hunk_hitbox.bounds, color, Corners::all(px(0.)), *status),
+                        }
                     }),
                 };
 
@@ -6320,8 +6325,8 @@ impl EditorElement {
                     GitGutterSetting::TrackedFiles
                 )
             });
-        if show_git_gutter && self.split_side.is_none() {
-            Self::paint_gutter_diff_hunks(layout, window, cx)
+        if show_git_gutter {
+            Self::paint_gutter_diff_hunks(layout, self.split_side, window, cx)
         }
 
         let highlight_width = 0.275 * layout.position_map.line_height;

crates/multi_buffer/src/multi_buffer.rs

@@ -6807,6 +6807,10 @@ impl MultiBufferSnapshot {
         self.diffs.get(&buffer_id).map(|diff| &diff.diff)
     }
 
+    pub fn all_diff_hunks_expanded(&self) -> bool {
+        self.all_diff_hunks_expanded
+    }
+
     /// Visually annotates a position or range with the `Debug` representation of a value. The
     /// callsite of this function is used as a key - previous annotations will be removed.
     #[cfg(debug_assertions)]