fix: target caret excerpts and retain diff viewport offsets

src/editor.rs

@@ -1462,6 +1462,22 @@ impl Editor {
         }
     }
 
+    pub(crate) fn set_diff_viewport(
+        &mut self,
+        comparison_id: u64,
+        file: usize,
+        top: usize,
+        left_after: usize,
+    ) {
+        if let Some(diff) = &mut self.diff
+            && diff.comparison_id == Some(comparison_id)
+            && diff.file == file
+        {
+            diff.top = top;
+            diff.left_after = left_after;
+        }
+    }
+
     fn handle_diff(&mut self, input: Input) -> Option<Invocation> {
         if input == Input::Escape
             && (self.count.is_some() || self.g_pending || self.line_pending.is_some())
@@ -1565,6 +1581,14 @@ impl Editor {
         if live && !matches!(input, Input::Char('\t' | '[' | ']') | Input::BackTab) {
             return self.handle_input(input);
         }
+        if live && matches!(input, Input::Char('[' | ']')) {
+            let row = self.cursor().row;
+            self.diff
+                .as_mut()
+                .expect("active diff")
+                .expand_after_excerpt(input == Input::Char(']'), row);
+            return None;
+        }
         if !files_focused
             && self
                 .diff
@@ -7050,6 +7074,66 @@ mod tests {
         buffers
     }
 
+    #[test]
+    fn editable_diff_brackets_expand_caret_excerpt_without_moving_source_anchor() {
+        for selected in [30, 50] {
+            let directory = Directory::new();
+            let path = directory.path("file.txt");
+            let before: Vec<String> = (0..80).map(|row| format!("line {row}")).collect();
+            fs::write(&path, before.join("\n")).unwrap();
+            let mut editor = Editor::open(Some(path), vec![]).unwrap();
+            let mut after = before;
+            for row in [10, 30, 50] {
+                after[row] = format!("changed {row}");
+            }
+            editor
+                .apply(vec![Effect::Replace(after.join("\n"))])
+                .unwrap();
+            editor.set_diff_context_options(1, 4);
+            working_diff(&mut editor, &directory.0, &["file.txt"]);
+            editor.handle(Input::BackTab);
+            assert!(editor.diff_editing());
+            editor.active.targets = vec![Target::caret(Cursor {
+                row: selected,
+                column: 2,
+            })];
+            let top = editor.diff().unwrap().after_display_row(9).unwrap();
+            editor.diff.as_mut().unwrap().top = top;
+            let cursor = editor.cursor();
+            let visible = |editor: &Editor| editor.diff().unwrap().after_rows().collect::<Vec<_>>();
+            assert_eq!(
+                visible(&editor),
+                (9..12).chain(29..32).chain(49..52).collect::<Vec<_>>()
+            );
+            editor.handle(Input::Char('['));
+            editor.handle(Input::Char(']'));
+            let expected = if selected == 30 {
+                (9..12).chain(25..36).chain(49..52).collect::<Vec<_>>()
+            } else {
+                (9..12).chain(29..32).chain(45..56).collect::<Vec<_>>()
+            };
+            assert_eq!(visible(&editor), expected);
+            assert_eq!(editor.cursor(), cursor);
+            assert_eq!(
+                editor.diff().unwrap().after_display_row(9),
+                Some(editor.diff().unwrap().top)
+            );
+            if selected == 30 {
+                // The caret keeps targeting the second excerpt after it merges
+                // with the third, rather than expanding the first excerpt.
+                for _ in 0..5 {
+                    editor.handle(Input::Char(']'));
+                }
+                assert_eq!(visible(&editor), (9..12).chain(25..56).collect::<Vec<_>>());
+                assert_eq!(editor.cursor(), cursor);
+                assert_eq!(
+                    editor.diff().unwrap().after_display_row(9),
+                    Some(editor.diff().unwrap().top)
+                );
+            }
+        }
+    }
+
     #[test]
     fn working_diff_reuses_dirty_buffers_and_normal_edit_undo_save_paths() {
         let directory = Directory::new();