[scroll-animations] WPT test scroll-animations/view-timelines/animation-events.html is a flaky failure

https://bugs.webkit.org/show_bug.cgi?id=284299

Reviewed by Anne van Kesteren.

The Web Animations spec says that pending animation events should be enqueued per document, and we
made the choice years ago to host the event queue on the `DocumentTimeline` created for each document.

When document timelines were the only timeline types, running the "update animation and send events"
procedure [0] would only ever run if the document timeline needed an update. When we started supporting
multiple types of timelines, this meant we could run that procedure when a scroll or view timeline
needed an update but the document timeline did not.

As such, we change the step where we process events in the "update animation and send events" procedure
to not only look for a document timeline among the timelines needing an update, but always looking at
the document timeline created for the document.

A better fix would be to stop enqueuing events on `DocumentTimeline` and instead on
`AnimationTimelinesController` but the given the scope of that potential change and how simple
this fix is, it seems like a better solution for the time being.

[0] https://drafts.csswg.org/web-animations-1/#update-animations-and-send-events

* LayoutTests/TestExpectations:
* Source/WebCore/animation/AnimationTimelinesController.cpp:
(WebCore::AnimationTimelinesController::updateAnimationsAndSendEvents):

Canonical link: https://commits.webkit.org/294068@main

Author: graouts

LayoutTests/TestExpectations

@@ -7055,8 +7055,6 @@ webkit.org/b/283701 imported/w3c/web-platform-tests/scroll-animations/css/deferr
 webkit.org/b/283702 imported/w3c/web-platform-tests/scroll-animations/scroll-timelines/constructor-no-document.html [ Skip ]
 webkit.org/b/283107 imported/w3c/web-platform-tests/scroll-animations/scroll-timelines/updating-the-finished-state.html [ Skip ]
 
-webkit.org/b/284299 imported/w3c/web-platform-tests/scroll-animations/view-timelines/animation-events.html [ Pass Failure ]
-
 # webkit.org/b/263872 [scroll-animations] some WPT tests are ImageOnlyFailure
 imported/w3c/web-platform-tests/scroll-animations/scroll-timelines/layout-changes-on-percentage-based-timeline.html [ ImageOnlyFailure ]
 imported/w3c/web-platform-tests/scroll-animations/view-timelines/range-boundary.html [ ImageOnlyFailure ]

Source/WebCore/animation/AnimationTimelinesController.cpp

@@ -202,22 +202,23 @@ void AnimationTimelinesController::updateAnimationsAndSendEvents(ReducedResoluti
     // 3. Perform a microtask checkpoint.
     protectedDocument()->eventLoop().performMicrotaskCheckpoint();
 
-    // 4. Let events to dispatch be a copy of doc's pending animation event queue.
-    // 5. Clear doc's pending animation event queue.
-    AnimationEvents events;
-    for (auto& timeline : timelinesToUpdate) {
-        if (RefPtr documentTimeline = dynamicDowncast<DocumentTimeline>(timeline))
-            events.appendVector(documentTimeline->prepareForPendingAnimationEventsDispatch());
-    }
+    if (RefPtr documentTimeline = m_document->existingTimeline()) {
+        // FIXME: pending animation events should be owned by this controller rather
+        // than the document timeline.
 
-    // 6. Perform a stable sort of the animation events in events to dispatch as follows.
-    std::stable_sort(events.begin(), events.end(), [] (const Ref<AnimationEventBase>& lhs, const Ref<AnimationEventBase>& rhs) {
-        return compareAnimationEventsByCompositeOrder(lhs.get(), rhs.get());
-    });
+        // 4. Let events to dispatch be a copy of doc's pending animation event queue.
+        // 5. Clear doc's pending animation event queue.
+        auto events = documentTimeline->prepareForPendingAnimationEventsDispatch();
 
-    // 7. Dispatch each of the events in events to dispatch at their corresponding target using the order established in the previous step.
-    for (auto& event : events)
-        event->target()->dispatchEvent(event);
+        // 6. Perform a stable sort of the animation events in events to dispatch as follows.
+        std::stable_sort(events.begin(), events.end(), [] (const Ref<AnimationEventBase>& lhs, const Ref<AnimationEventBase>& rhs) {
+            return compareAnimationEventsByCompositeOrder(lhs.get(), rhs.get());
+        });
+
+        // 7. Dispatch each of the events in events to dispatch at their corresponding target using the order established in the previous step.
+        for (auto& event : events)
+            event->target()->dispatchEvent(event);
+    }
 
     // This will cancel any scheduled invalidation if we end up removing all animations.
     for (auto& animation : animationsToRemove) {