Bug 1826603 - Don't dispatch transition events if there are no listeners, r=masayuki

Differential Revision: https://phabricator.services.mozilla.com/D174829

dom/animation/AnimationEventDispatcher.h

@@ -280,6 +280,18 @@ struct AnimationEventInfo {
     MOZ_ASSERT(mEvent.is<InternalTransitionEvent>() ||
                mEvent.is<InternalAnimationEvent>());
 
+    if (mEvent.is<InternalTransitionEvent>() && target->IsNode()) {
+      nsPIDOMWindowInner* inner =
+          target->AsNode()->OwnerDoc()->GetInnerWindow();
+      if (inner && !inner->HasTransitionEventListeners()) {
+        MOZ_ASSERT(AsWidgetEvent()->mMessage == eTransitionStart ||
+                   AsWidgetEvent()->mMessage == eTransitionRun ||
+                   AsWidgetEvent()->mMessage == eTransitionEnd ||
+                   AsWidgetEvent()->mMessage == eTransitionCancel);
+        return;
+      }
+    }
+
     EventDispatcher::Dispatch(target, aPresContext, AsWidgetEvent());
   }
 };

dom/base/nsGlobalWindowInner.cpp

@@ -7931,6 +7931,7 @@ nsPIDOMWindowInner::nsPIDOMWindowInner(nsPIDOMWindowOuter* aOuterWindow,
       mMayHaveFormSelectEventListener(false),
       mMayHaveMouseEnterLeaveEventListener(false),
       mMayHavePointerEnterLeaveEventListener(false),
+      mMayHaveTransitionEventListener(false),
       mMayHaveBeforeInputEventListenerForTelemetry(false),
       mMutationObserverHasObservedNodeForTelemetry(false),
       mOuterWindow(aOuterWindow),

dom/base/nsINode.cpp

@@ -3388,6 +3388,9 @@ already_AddRefed<nsINode> nsINode::CloneAndAdopt(
         if (elm->MayHaveFormSelectEventListener()) {
           window->SetHasFormSelectEventListeners();
         }
+        if (elm->MayHaveTransitionEventListener()) {
+          window->SetHasTransitionEventListeners();
+        }
       }
     }
     if (wasRegistered) {

dom/base/nsPIDOMWindow.h

@@ -236,6 +236,20 @@ class nsPIDOMWindowInner : public mozIDOMWindow {
     mMayHavePointerEnterLeaveEventListener = true;
   }
 
+  /**
+   * Call this to check whether some node (this window, its document,
+   * or content in that document) has a transition* event listeners.
+   */
+  bool HasTransitionEventListeners() { return mMayHaveTransitionEventListener; }
+
+  /**
+   * Call this to indicate that some node (this window, its document,
+   * or content in that document) has a transition* event listener.
+   */
+  void SetHasTransitionEventListeners() {
+    mMayHaveTransitionEventListener = true;
+  }
+
   /**
    * Call this to check whether some node (this window, its document,
    * or content in that document) has a beforeinput event listener.
@@ -667,6 +681,7 @@ class nsPIDOMWindowInner : public mozIDOMWindow {
   bool mMayHaveFormSelectEventListener;
   bool mMayHaveMouseEnterLeaveEventListener;
   bool mMayHavePointerEnterLeaveEventListener;
+  bool mMayHaveTransitionEventListener;
   // Only used for telemetry probes.  This may be wrong if some nodes have
   // come from another document with `Document.adoptNode`.
   bool mMayHaveBeforeInputEventListenerForTelemetry;

dom/events/EventListenerManager.cpp

@@ -114,6 +114,7 @@ EventListenerManagerBase::EventListenerManagerBase()
       mMayHaveInputOrCompositionEventListener(false),
       mMayHaveSelectionChangeEventListener(false),
       mMayHaveFormSelectEventListener(false),
+      mMayHaveTransitionEventListener(false),
       mClearingListeners(false),
       mIsMainThreadELM(NS_IsMainThread()),
       mHasNonPrivilegedClickListeners(false),
@@ -461,6 +462,16 @@ void EventListenerManager::AddEventListenerInternal(
           }
         }
         break;
+      case eTransitionStart:
+      case eTransitionRun:
+      case eTransitionEnd:
+      case eTransitionCancel:
+      case eWebkitTransitionEnd:
+        mMayHaveTransitionEventListener = true;
+        if (nsPIDOMWindowInner* window = GetInnerWindowForTarget()) {
+          window->SetHasTransitionEventListeners();
+        }
+        break;
       default:
         // XXX Use NS_ASSERTION here to print resolvedEventMessage since
         //     MOZ_ASSERT can take only string literal, not pointer to

dom/events/EventListenerManager.h

@@ -157,11 +157,11 @@ class EventListenerManagerBase {
   uint16_t mMayHaveInputOrCompositionEventListener : 1;
   uint16_t mMayHaveSelectionChangeEventListener : 1;
   uint16_t mMayHaveFormSelectEventListener : 1;
+  uint16_t mMayHaveTransitionEventListener : 1;
   uint16_t mClearingListeners : 1;
   uint16_t mIsMainThreadELM : 1;
   uint16_t mHasNonPrivilegedClickListeners : 1;
   uint16_t mUnknownNonPrivilegedClickListeners : 1;
-  // uint16_t mUnused : 1;
 };
 
 /*
@@ -490,6 +490,9 @@ class EventListenerManager final : public EventListenerManagerBase {
   bool MayHaveFormSelectEventListener() const {
     return mMayHaveFormSelectEventListener;
   }
+  bool MayHaveTransitionEventListener() {
+    return mMayHaveTransitionEventListener;
+  }
 
   bool HasNonPrivilegedClickListeners();