script: Ensure that leaving the WebView sets the cursor back to the default cursor (#38759)

This changes makes a variety of changes to ensure that the cursor is set
back to the default cursor when it leaves the `WebView`:

1. Display list updates can come after a mouse leaves the `WebView`, so
   when refreshing the cursor after the update, base the updated cursor
   on the last hovered location in the `DocumentEventHandler`, rather
   than the compositor. This allows us to catch when the last hovered
   position is `None` (ie the cursor has left the `WebView`).
2. When handling `MouseLeftViewport` events for the cursor leaving the
   entire WebView, properly set the
   MouseLeftViewport::focus_moving_to_another_iframe` on the input event
   passed to the script thread.
3. When moving out of the `WebView` entirely, explicitly ask the
   embedder to set the cursor back to the default.

Testing: This change adds a unit test verifying this behavior.
Fixes: #38710.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/document_event_handler.rs

@@ -79,7 +79,7 @@ pub(crate) struct DocumentEventHandler {
     current_hover_target: MutNullableDom<Element>,
     /// The most recent mouse movement point, used for processing `mouseleave` events.
     #[no_trace]
-    most_recent_mousemove_point: Point2D<f32, CSSPixel>,
+    most_recent_mousemove_point: Cell<Option<Point2D<f32, CSSPixel>>>,
     /// The currently set [`Cursor`] or `None` if the `Document` isn't being hovered
     /// by the cursor.
     #[no_trace]
@@ -212,13 +212,15 @@ impl DocumentEventHandler {
             }));
     }
 
-    pub(crate) fn set_cursor(&self, cursor: Cursor) {
-        if Some(cursor) == self.current_cursor.get() {
+    pub(crate) fn set_cursor(&self, cursor: Option<Cursor>) {
+        if cursor == self.current_cursor.get() {
             return;
         }
-        self.current_cursor.set(Some(cursor));
-        self.window
-            .send_to_embedder(EmbedderMsg::SetCursor(self.window.webview_id(), cursor));
+        self.current_cursor.set(cursor);
+        self.window.send_to_embedder(EmbedderMsg::SetCursor(
+            self.window.webview_id(),
+            cursor.unwrap_or_default(),
+        ));
     }
 
     fn handle_mouse_left_viewport_event(
@@ -238,8 +240,9 @@ impl DocumentEventHandler {
             }
 
             if let Some(hit_test_result) = self
-                .window
-                .hit_test_from_point_in_viewport(self.most_recent_mousemove_point)
+                .most_recent_mousemove_point
+                .get()
+                .and_then(|point| self.window.hit_test_from_point_in_viewport(point))
             {
                 MouseEvent::new_simple(
                     &self.window,
@@ -263,15 +266,23 @@ impl DocumentEventHandler {
             }
         }
 
-        self.current_cursor.set(None);
-        self.current_hover_target.set(None);
-
-        // If focus is moving to another frame, it will decide what the new status text is, but if
-        // this mouse leave event is leaving the WebView entirely, then clear it.
+        // We do not want to always inform the embedder that cursor has been set to the
+        // default cursor, in order to avoid a timing issue when moving between `<iframe>`
+        // elements. There is currently no way to control which `SetCursor` message will
+        // reach the embedder first. This is safer when leaving the `WebView` entirely.
         if !mouse_leave_event.focus_moving_to_another_iframe {
+            // If focus is moving to another frame, it will decide what the new status
+            // text is, but if this mouse leave event is leaving the WebView entirely,
+            // then clear it.
             self.window
                 .send_to_embedder(EmbedderMsg::Status(self.window.webview_id(), None));
+            self.set_cursor(None);
+        } else {
+            self.current_cursor.set(None);
         }
+
+        self.current_hover_target.set(None);
+        self.most_recent_mousemove_point.set(None);
     }
 
     fn handle_mouse_enter_leave_event(
@@ -336,7 +347,7 @@ impl DocumentEventHandler {
         };
 
         // Update the cursor when the mouse moves, if it has changed.
-        self.set_cursor(hit_test_result.cursor);
+        self.set_cursor(Some(hit_test_result.cursor));
 
         let Some(new_target) = hit_test_result
             .node
@@ -451,6 +462,8 @@ impl DocumentEventHandler {
         .fire(new_target.upcast(), can_gc);
 
         self.update_current_hover_target_and_status(Some(new_target));
+        self.most_recent_mousemove_point
+            .set(Some(hit_test_result.point_in_frame));
     }
 
     fn update_current_hover_target_and_status(&self, new_hover_target: Option<DomRoot<Element>>) {
@@ -502,6 +515,21 @@ impl DocumentEventHandler {
         }
     }
 
+    pub(crate) fn handle_refresh_cursor(&self) {
+        let Some(most_recent_mousemove_point) = self.most_recent_mousemove_point.get() else {
+            return;
+        };
+
+        let Some(hit_test_result) = self
+            .window
+            .hit_test_from_point_in_viewport(most_recent_mousemove_point)
+        else {
+            return;
+        };
+
+        self.set_cursor(Some(hit_test_result.cursor));
+    }
+
     /// <https://w3c.github.io/uievents/#mouseevent-algorithms>
     /// Handles native mouse down, mouse up, mouse click.
     fn handle_native_mouse_button_event(