layout: Preserve focused window in column when window above is closed

Might be the longest standing bug in niri?

src/layout/scrolling.rs

@@ -984,7 +984,13 @@ impl<W: LayoutElement> ScrollingSpace<W> {
             is_floating: false,
         };
 
-        column.active_tile_idx = min(column.active_tile_idx, column.tiles.len() - 1);
+        if tile_idx < column.active_tile_idx {
+            // A tile above was removed; preserve the current position.
+            column.active_tile_idx -= 1;
+        } else {
+            column.active_tile_idx = min(column.active_tile_idx, column.tiles.len() - 1);
+        }
+
         column.update_tile_sizes_with_transaction(true, transaction);
         self.data[column_idx].update(column);
         let offset = prev_width - column.width();

src/layout/tests.rs

@@ -3012,6 +3012,31 @@ fn move_workspace_to_same_monitor_doesnt_reorder() {
     assert_eq!(counts, &[1, 2, 0]);
 }
 
+#[test]
+fn removing_window_above_preserves_focused_window() {
+    let ops = [
+        Op::AddOutput(0),
+        Op::AddWindow {
+            params: TestWindowParams::new(0),
+        },
+        Op::AddWindow {
+            params: TestWindowParams::new(1),
+        },
+        Op::AddWindow {
+            params: TestWindowParams::new(2),
+        },
+        Op::FocusColumnFirst,
+        Op::ConsumeWindowIntoColumn,
+        Op::ConsumeWindowIntoColumn,
+        Op::FocusWindowDown,
+        Op::CloseWindow(0),
+    ];
+
+    let layout = check_ops(&ops);
+    let win = layout.focus().unwrap();
+    assert_eq!(win.0.id, 1);
+}
+
 #[test]
 fn preset_column_width_fixed_correct_with_border() {
     let ops = [