2 files changed, 33 insertions, 0 deletions
diff --git a/dom/animation/Animation.cpp b/dom/animation/Animation.cpp
index 6dd583ed1..cefdbb76d 100644
--- a/dom/animation/Animation.cpp
+++ b/dom/animation/Animation.cpp
@@ -230,6 +230,10 @@ Animation::SetTimelineNoUpdate(AnimationTimeline* aTimeline)
     return;
   }
 
+  StickyTimeDuration activeTime = mEffect
+                                  ? mEffect->GetComputedTiming().mActiveTime
+                                  : StickyTimeDuration();
+
   RefPtr<AnimationTimeline> oldTimeline = mTimeline;
   if (oldTimeline) {
     oldTimeline->RemoveAnimation(this);
@@ -240,6 +244,9 @@ Animation::SetTimelineNoUpdate(AnimationTimeline* aTimeline)
     mHoldTime.SetNull();
   }
 
+  if (!aTimeline) {
+    MaybeQueueCancelEvent(activeTime);
+  }
   UpdateTiming(SeekFlag::NoSeek, SyncNotifyFlag::Async);
 }
 
@@ -771,6 +778,10 @@ Animation::CancelNoUpdate()
 
   DispatchPlaybackEvent(NS_LITERAL_STRING("cancel"));
 
+  StickyTimeDuration activeTime = mEffect
+                                  ? mEffect->GetComputedTiming().mActiveTime
+                                  : StickyTimeDuration();
+
   mHoldTime.SetNull();
   mStartTime.SetNull();
 
@@ -779,6 +790,7 @@ Animation::CancelNoUpdate()
   if (mTimeline) {
     mTimeline->RemoveAnimation(this);
   }
+  MaybeQueueCancelEvent(activeTime);
 }
 
 void
@@ -819,6 +831,17 @@ Animation::HasLowerCompositeOrderThan(const Animation& aOther) const
       return thisTransition->HasLowerCompositeOrderThan(*otherTransition);
     }
     if (thisTransition || otherTransition) {
+      // Cancelled transitions no longer have an owning element. To be strictly
+      // correct we should store a strong reference to the owning element
+      // so that if we arrive here while sorting cancel events, we can sort
+      // them in the correct order.
+      //
+      // However, given that cancel events are almost always queued
+      // synchronously in some deterministic manner, we can be fairly sure
+      // that cancel events will be dispatched in a deterministic order
+      // (which is our only hard requirement until specs say otherwise).
+      // Furthermore, we only reach here when we have events with equal
+      // timestamps so this is an edge case we can probably ignore for now.
       return thisTransition;
     }
   }
diff --git a/dom/animation/Animation.h b/dom/animation/Animation.h
index c59d7d6ce..3263b30c4 100644
--- a/dom/animation/Animation.h
+++ b/dom/animation/Animation.h
@@ -326,6 +326,16 @@ public:
 
   void NotifyEffectTimingUpdated();
 
+  /**
+   * Used by subclasses to synchronously queue a cancel event in situations
+   * where the Animation may have been cancelled.
+   *
+   * We need to do this synchronously because after a CSS animation/transition
+   * is canceled, it will be released by its owning element and may not still
+   * exist when we would normally go to queue events on the next tick.
+   */
+  virtual void MaybeQueueCancelEvent(StickyTimeDuration aActiveTime) {};
+
 protected:
   void SilentlySetCurrentTime(const TimeDuration& aNewCurrentTime);
   void SilentlySetPlaybackRate(double aPlaybackRate);