Fix step 10 of eventdispatcher::dispatch_event (fixes #11609)

We consolidate steps 10-12 in a single function to expose less general-purpose
mutating methods on Event.

components/script/dom/event.rs

@@ -153,11 +153,6 @@ impl Event {
         self.cancelable.set(cancelable);
     }
 
-    #[inline]
-    pub fn clear_current_target(&self) {
-        self.current_target.set(None);
-    }
-
     #[inline]
     pub fn set_current_target(&self, val: &EventTarget) {
         self.current_target.set(Some(val));
@@ -199,8 +194,22 @@ impl Event {
     }
 
     #[inline]
-    pub fn set_dispatching(&self, val: bool) {
-        self.dispatching.set(val)
+    // https://dom.spec.whatwg.org/#concept-event-dispatch Step 1.
+    pub fn mark_as_dispatching(&self) {
+        assert!(!self.dispatching.get());
+        self.dispatching.set(true);
+    }
+
+    #[inline]
+    // https://dom.spec.whatwg.org/#concept-event-dispatch Steps 10-12.
+    pub fn clear_dispatching_flags(&self) {
+        assert!(self.dispatching.get());
+
+        self.dispatching.set(false);
+        self.stop_propagation.set(false);
+        self.stop_immediate.set(false);
+        self.set_phase(EventPhase::None);
+        self.current_target.set(None);
     }
 
     #[inline]

components/script/dom/eventdispatcher.rs

@@ -123,7 +123,7 @@ pub fn dispatch_event(target: &EventTarget,
     }
 
     // Step 1. Postponed here for the reason stated above.
-    event.set_dispatching(true);
+    event.mark_as_dispatching();
 
     // Step 3. The "invoke" algorithm is only used on `target` separately,
     // so we don't put it in the path.
@@ -158,14 +158,8 @@ pub fn dispatch_event(target: &EventTarget,
         None => {}
     }
 
-    // Step 10.
-    event.set_dispatching(false);
-
-    // Step 11.
-    event.set_phase(EventPhase::None);
-
-    // Step 12.
-    event.clear_current_target();
+    // Step 10-12.
+    event.clear_dispatching_flags();
 
     // Step 13.
     !event.DefaultPrevented()

tests/wpt/metadata/dom/events/Event-dispatch-multiple-stopPropagation.html.ini

@@ -1,5 +0,0 @@
-[Event-dispatch-multiple-stopPropagation.html]
-  type: testharness
-  [ Multiple dispatchEvent() and stopPropagation() ]
-    expected: FAIL
-