Move LazySpan rows through one retained offset

core/src/ui/active.rs

@@ -20,6 +20,10 @@ pub struct ActiveData {
     pub size: Size,
     /// Retained so descendants' parent links stay valid across redraws.
     pub move_slot: MoveIdx,
+    /// The optional coordinate boundary between this widget and its direct
+    /// children. Descendants retain links to it across redraws, just as they
+    /// do to `move_slot`.
+    pub child_move_slot: Option<MoveIdx>,
     /// The part of this widget's move delta already folded into `region`.
     pub move_applied: Vec2,
 }