Animation::UpdateTiming takes a SyncNotifyFlag parameter. This is passed to
UpdateFinishedState where it determines how we handle finish actions.
If it is async we queue a microtask where we re-evaluate if the animation is
finished or not before queuing events / resolving promises.
That allows code like the following to _not_ trigger finish events:
```
const animation = elem.animate({...}, 1000);
animation.currentTime += 1000;
animation.effect.updateTiming({ duration: 2000 });
```
(Since the check that the animation is finished will run in a microtask _after_
the call to updateTiming.)
When the flag is "sync" we still don't _actually_ run the finish actions
entirely synchronously: the finished promise is resolved synchronously, but
resolving a promise actually queues a microtask for each callback. Likewise, the
finish event is queued synchronously, but not dispatched.
Since there should be no opportunity for script to run between when we call
Animation::Tick and when we run the next microtask checkpoint (currently at the
end of DocumentTimeline::WillRefresh but that will change slightly in the next
patch in this series) there is no need to introduce the extra "async" microtask
for re-evaluating an animation's finished state. Instead it should be possible
to use the "sync" finishing behavior. Such a change should be unobservable to
Web content but will reduce indirection somewhat.
Differential Revision: https://phabricator.services.mozilla.com/D30318
--HG--
extra : moz-landing-system : lando
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