Move LazySpan rows through one retained offset

core/src/ui/mod.rs

@@ -18,11 +18,12 @@ pub struct UiData {
     pub textures: Textures,
     pub text: TextData,
     pub masks: TrackedArena<Mask, u32>,
-    /// One entry per widget ever drawn, forming the parent-linked chain
-    /// `resolve_move` walks in both shader stages. Allocated once on a
-    /// widget's first draw and reused for every later redraw of the same
-    /// id (never reallocated), so a retained descendant's `parent` index
-    /// never goes stale -- see LAYOUT.md section 2.
+    /// One entry per widget ever drawn, plus optional child-coordinate
+    /// boundaries owned by containers. Together they form the parent-linked
+    /// chain `resolve_move` walks in both shader stages. A widget's ordinary
+    /// entry is allocated once on its first draw and reused for every later
+    /// redraw of the same id, so a retained descendant's `parent` index never
+    /// goes stale -- see LAYOUT.md section 2.
     pub move_offsets: TrackedArena<MoveOffset, u32>,
     /// Every widget whose [`crate::Widget::tick`] should run before the
     /// next frame -- today, a `LazySpan` coasting through a fling. Added by