Bug 1422248 - Don't reject promises of dispatch cancel events when canceling idle animations; r=hiro

MozReview-Commit-ID: EGg6Ntj0loq

--HG--
extra : rebase_source : 8edd6fdd73440fe55fd8be1d524767fdaf55b872

dom/animation/Animation.cpp

@@ -786,14 +786,16 @@ Animation::SilentlySetPlaybackRate(double aPlaybackRate)
 void
 Animation::CancelNoUpdate()
 {
-  ResetPendingTasks();
+  if (PlayState() != AnimationPlayState::Idle) {
+    ResetPendingTasks();
 
-  if (mFinished) {
-    mFinished->MaybeReject(NS_ERROR_DOM_ABORT_ERR);
+    if (mFinished) {
+      mFinished->MaybeReject(NS_ERROR_DOM_ABORT_ERR);
+    }
+    ResetFinishedPromise();
+
+    DispatchPlaybackEvent(NS_LITERAL_STRING("cancel"));
   }
-  ResetFinishedPromise();
-
-  DispatchPlaybackEvent(NS_LITERAL_STRING("cancel"));
 
   StickyTimeDuration activeTime = mEffect
                                   ? mEffect->GetComputedTiming().mActiveTime

testing/web-platform/meta/web-animations/timing-model/animations/canceling-an-animation.html.ini

@@ -1,7 +0,0 @@
-[canceling-an-animation.html]
-  type: testharness
-  [The finished promise should NOT be rejected if the animation is already idle]
-    expected: FAIL
-
-  [The cancel event should NOT be fired if the animation is already idle]
-    expected: FAIL