Track retained layout validity explicitly

src/widget/mask.rs

@@ -9,9 +9,4 @@ impl Widget for Masked {
         painter.set_mask(painter.region());
         painter.widget(&self.inner).size()
     }
-
-    /// It clips to the box it was given, not to the part its child used.
-    fn on_resize(&self, _: Axis) -> OnResize {
-        OnResize::Redraw
-    }
 }