fix: preserve visible declaration names under scope overlays

src/tui.rs

@@ -4166,6 +4166,31 @@ fn draw_diff_scope_labels(
                 let Some(y) = y.checked_sub(1).filter(|&y| y >= content.y) else {
                     continue;
                 };
+                let previous = &viewport.rows[screen - 1];
+                let previous_focus = if unified {
+                    diff.display_focus(previous.position.display)
+                        .unwrap_or(focus)
+                } else {
+                    focus
+                };
+                let previous_side = usize::from(previous_focus == Focus::After);
+                if previous.scope == [false; 2]
+                    && let Some(previous_scope) =
+                        syntax.scopes(diff, previous.position.display)[previous_side]
+                    && diff.display_source_row(previous_focus, previous.position.display)
+                        == Some(previous_scope.name.row)
+                {
+                    let source = if previous_side == 0 {
+                        &file.before
+                    } else {
+                        &file.after
+                    };
+                    let prefix = &source[previous_scope.name.row][..previous_scope.name.column];
+                    let column = display_column(prefix, usize::MAX);
+                    if lefts[pane] <= column && column < lefts[pane].saturating_add(width) {
+                        continue;
+                    }
+                }
                 y
             } else {
                 if !row.scope[side] {
@@ -8349,6 +8374,34 @@ mod tests {
         }
     }
 
+    #[test]
+    fn diff_scope_overlay_preserves_visible_declaration_above() {
+        let before = "pub fn old_name() {\n    shared_call_with_a_long_argument_name();\n}\n";
+        let after = "fn new_name() {\n    shared_call_with_a_long_argument_name();\n}\n";
+        for focus in [Focus::Before, Focus::After] {
+            let mut file = syntax_diff_file("scope.rs", before, after);
+            file.hunks = crate::diff::line_hunks(&file.before, false, &file.after, false);
+            let mut diff = diff_comparison(vec![file]);
+            diff.files_visible = false;
+            diff.set_view_mode(ViewMode::Unified);
+            diff.focus = focus;
+            diff.top = diff.source_display_row(Focus::Before, 0).unwrap();
+            diff.left_before = 4;
+            diff.left_after = 4;
+            let mut terminal = Terminal::new(ratatui::backend::TestBackend::new(100, 20)).unwrap();
+            let mut panels = WheelPanels::default();
+            render_diff(&mut terminal, &diff, &mut panels);
+            let pane = usize::from(focus == Focus::After);
+            let area = diff_inner(panels.diff.as_ref().unwrap().regions[pane + 1].0);
+            let first = pane_screen(&terminal, Rect::new(area.x, area.y, area.width, 1));
+            assert!(
+                first.contains("fn old_name()"),
+                "Visible declaration was covered: {first}"
+            );
+            assert!(!first.contains("+ new_name"));
+        }
+    }
+
     #[test]
     fn diff_scope_labels_follow_declaration_visibility() {
         let before = "@decorate()\nclass PolicyActiveStatusEnum(BaseEnum):\n    ACTIVE = 1\n    EXPIRED = 2\n    FUTURE = 3\n    UNKNOWN = 4\n";