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.

core/src/overlay/element.rs

@@ -112,8 +112,22 @@ where
     }
 
     /// Returns true if the cursor is over the [`Element`].
-    pub fn is_over(&self, layout: Layout<'_>, cursor_position: Point) -> bool {
-        self.overlay.is_over(layout, cursor_position)
+    pub fn is_over(
+        &self,
+        layout: Layout<'_>,
+        renderer: &Renderer,
+        cursor_position: Point,
+    ) -> bool {
+        self.overlay.is_over(layout, renderer, cursor_position)
+    }
+
+    /// Returns the nested overlay of the [`Element`], if there is any.
+    pub fn overlay<'b>(
+        &'b mut self,
+        layout: Layout<'_>,
+        renderer: &Renderer,
+    ) -> Option<Element<'b, Message, Renderer>> {
+        self.overlay.overlay(layout, renderer)
     }
 }
 
@@ -248,7 +262,12 @@ where
         self.content.draw(renderer, theme, style, layout, cursor)
     }
 
-    fn is_over(&self, layout: Layout<'_>, cursor_position: Point) -> bool {
-        self.content.is_over(layout, cursor_position)
+    fn is_over(
+        &self,
+        layout: Layout<'_>,
+        renderer: &Renderer,
+        cursor_position: Point,
+    ) -> bool {
+        self.content.is_over(layout, renderer, cursor_position)
     }
 }

core/src/overlay/group.rs

@@ -147,11 +147,18 @@ where
         });
     }
 
-    fn is_over(&self, layout: Layout<'_>, cursor_position: Point) -> bool {
+    fn is_over(
+        &self,
+        layout: Layout<'_>,
+        renderer: &Renderer,
+        cursor_position: Point,
+    ) -> bool {
         self.children
             .iter()
             .zip(layout.children())
-            .any(|(child, layout)| child.is_over(layout, cursor_position))
+            .any(|(child, layout)| {
+                child.is_over(layout, renderer, cursor_position)
+            })
     }
 }