Emit TransitionEnd events in the layout thread and process it in the script thread

components/script/dom/bindings/js.rs

@@ -444,6 +444,14 @@ impl<T: Reflectable> LayoutJS<T> {
         debug_assert!(thread_state::get().is_layout());
         *self.ptr
     }
+
+    /// Returns a reference to the interior of this JS object. This method is
+    /// safe to call because it originates from the layout thread, and it cannot
+    /// mutate DOM nodes.
+    pub fn get_for_script(&self) -> &T {
+        debug_assert!(thread_state::get().is_script());
+        unsafe { &**self.ptr }
+    }
 }
 
 /// Get an `&T` out of a `Rc<T>`

components/script/dom/macros.rs

@@ -472,6 +472,7 @@ macro_rules! global_event_handlers(
         event_handler!(suspend, GetOnsuspend, SetOnsuspend);
         event_handler!(timeupdate, GetOntimeupdate, SetOntimeupdate);
         event_handler!(toggle, GetOntoggle, SetOntoggle);
+        event_handler!(transitionend, GetOntransitionend, SetOntransitionend);
         event_handler!(volumechange, GetOnvolumechange, SetOnvolumechange);
         event_handler!(waiting, GetOnwaiting, SetOnwaiting);
     )

components/script/dom/webidls/EventHandler.webidl

@@ -89,6 +89,11 @@ interface GlobalEventHandlers {
            attribute EventHandler onwaiting;
 };
 
+// https://drafts.csswg.org/css-transitions/#interface-globaleventhandlers-idl
+partial interface GlobalEventHandlers {
+           attribute EventHandler ontransitionend;
+};
+
 // https://html.spec.whatwg.org/multipage/#windoweventhandlers
 [NoInterfaceObject, Exposed=(Window,Worker)]
 interface WindowEventHandlers {

components/script/dom/webidls/TransitionEvent.webidl

@@ -6,7 +6,8 @@
  * https://dom.spec.whatwg.org/#event
  */
 
-[Constructor(DOMString type, optional TransitionEventInit transitionEventInitDict)]
+[Constructor(DOMString type, optional TransitionEventInit transitionEventInitDict),
+ Exposed=Window]
 interface TransitionEvent : Event {
   readonly attribute DOMString          propertyName;
   readonly attribute float              elapsedTime;