Add Response::contains_pointer (#3859)

* Part of #3841

crates/egui/src/response.rs

@@ -212,7 +212,9 @@ impl Response {
 
     /// The pointer is hovering above this widget or the widget was clicked/tapped this frame.
     ///
-    /// Note that this is slightly different from checking `response.rect.contains(pointer_pos)`.
+    /// This will be `false` whenever some other widget is being dragged.
+    ///
+    /// Note that this is slightly different from [`Self::contains_pointer`].
     /// For one, the hover rectangle is slightly larger, by half of the current item spacing
     /// (to make it easier to click things). But `hovered` also checks that no other area
     /// is covering this response rectangle.
@@ -221,6 +223,17 @@ impl Response {
         self.hovered
     }
 
+    /// Returns true if the pointer is contained by the response rect.
+    ///
+    /// In contrast to [`Self::hovered`], this can be `true` even if some other widget is being dragged.
+    /// This means it is useful for styling things like drag-and-drop targets.
+    ///
+    /// In contrast to [`Self::hovered`], this is true even when dragging some other widget
+    /// onto this one.
+    pub fn contains_pointer(&self) -> bool {
+        self.ctx.rect_contains_pointer(self.layer_id, self.rect)
+    }
+
     /// The widget is highlighted via a call to [`Self::highlight`] or [`Context::highlight_widget`].
     #[doc(hidden)]
     pub fn highlighted(&self) -> bool {

crates/egui_demo_lib/src/demo/drag_and_drop.rs

@@ -49,11 +49,14 @@ pub fn drop_target<R>(
     let outer_rect = Rect::from_min_max(outer_rect_bounds.min, content_ui.min_rect().max + margin);
     let (rect, response) = ui.allocate_at_least(outer_rect.size(), Sense::hover());
 
-    let style = if is_being_dragged && can_accept_what_is_being_dragged && response.hovered() {
-        ui.visuals().widgets.active
-    } else {
-        ui.visuals().widgets.inactive
-    };
+    // NOTE: we use `response.contains_pointer` here instead of `hovered`, because
+    // `hovered` is always false when another widget is being dragged.
+    let style =
+        if is_being_dragged && can_accept_what_is_being_dragged && response.contains_pointer() {
+            ui.visuals().widgets.active
+        } else {
+            ui.visuals().widgets.inactive
+        };
 
     let mut fill = style.bg_fill;
     let mut stroke = style.bg_stroke;
@@ -149,7 +152,12 @@ impl super::View for DragAndDropDemo {
                 });
 
                 let is_being_dragged = ui.memory(|mem| mem.is_anything_being_dragged());
-                if is_being_dragged && can_accept_what_is_being_dragged && response.hovered() {
+                // NOTE: we use `response.contains_pointer` here instead of `hovered`, because
+                // `hovered` is always false when another widget is being dragged.
+                if is_being_dragged
+                    && can_accept_what_is_being_dragged
+                    && response.contains_pointer()
+                {
                     drop_col = Some(col_idx);
                 }
             }