Track retained layout validity explicitly

src/widget/rect.rs

@@ -41,11 +41,6 @@ impl Widget for Rect {
     fn size_hint(&self, _: Axis) -> Option<Len> {
         Some(Len::LEFTOVER)
     }
-
-    /// Its box is its primitive's own region, so a new one is written there.
-    fn on_resize(&self, _: Axis) -> OnResize {
-        OnResize::Scale
-    }
 }
 
 pub fn rect(color: UiColor) -> Rect {