Introduce internal overlay::Nested for UserInterface

core/src/overlay.rs

@@ -91,9 +91,23 @@ where
     ///
     /// By default, it returns true if the bounds of the `layout` contain
     /// the `cursor_position`.
-    fn is_over(&self, layout: Layout<'_>, cursor_position: Point) -> bool {
+    fn is_over(
+        &self,
+        layout: Layout<'_>,
+        _renderer: &Renderer,
+        cursor_position: Point,
+    ) -> bool {
         layout.bounds().contains(cursor_position)
     }
+
+    /// Returns the nested overlay of the [`Overlay`], if there is any.
+    fn overlay<'a>(
+        &'a mut self,
+        _layout: Layout<'_>,
+        _renderer: &Renderer,
+    ) -> Option<Element<'a, Message, Renderer>> {
+        None
+    }
 }
 
 /// Returns a [`Group`] of overlay [`Element`] children.