Make Iris layout dependencies explicit

core/src/ui/active.rs

@@ -11,12 +11,16 @@ pub struct ActiveData {
     pub textures: Vec<TextureHandle>,
     pub primitives: Vec<PrimitiveHandle>,
     pub children: Vec<WidgetId>,
+    /// Direct children whose reported size this widget used during its
+    /// latest draw. Dirtiness propagates across these edges before layout
+    /// starts, so the resulting draw still travels only parent to child.
+    pub size_dependencies: Vec<WidgetId>,
     /// The inherited mask, not `own_mask`.
     pub mask: MaskIdx,
     /// The widget's retained mask slot, or `MaskIdx::NONE`.
     pub own_mask: MaskIdx,
     pub layer: LayerId,
-    /// The last `Widget::draw` result.
+    /// The size recorded by the last `Widget::draw` through its painter.
     pub size: Size,
     /// Retained so descendants' parent links stay valid across redraws.
     pub move_slot: MoveIdx,