[web-animations-1] Should setting the startTime of a play-pending animation to null have side effects?

[birtles]

Typically setting the start time of an animation should cancel any pending tasks. This provides applications with a way to synchronously seek/sync animations. Furthermore, by setting the start time to null applications can perform a synchronous pause.

However, if an animation is play-pending its start time will already be unresolved (null).

If the animation is play-pending when script sets the start time to null what should happen?

a. The change is redundant and should be ignored. (Seems intuitive from a JS point of view, redundant changes have no side effects.), or
b. The pending play task is canceled. (Currently speced behavior. Allows applications to perform a synchronous pause even in the play-pending state.)

i.e.

const anim = elem.animate(...);
// anim.pending === true
// anim.startTime === null
// anim.currentTime === 0

anim.startTime = null;
// Is the animation still play-pending here?

(Firefox actually does (a) as an optimization but the spec says we should do (b) and I'm wondering which to fix.)

/cc @stephenmcgruer @flackr @graouts @dadaa

[stephenmcgruer]

Apologies for the delay on this. @flackr and I talked it over this morning and option (a) - 'has no effect' makes most sense to us. We don't have a strong opinion on this however. Happy to discuss at the next monthly sync too.

[birtles]

Interesting, I was actually leaning towards (b) since it gives author consistent behavior -- setting startTime to null always does a synchronous pause whether you're pending or not.

[stephenmcgruer]

In which case something to discuss at our next sync I think! @graouts - any opinion?

[birtles]

Discussed this yesterday with @graouts @stephenmcgruer @flackr and agreed that we should go with (b), i.e. no change to the spec.

Part of the reasoning is that this not only makes it easy to synchronously pause without having to check the pending state, but it is also consistent with the fact that setting the startTime always finishes a pending-play, regardless of whether or not the value you set is null.

Firefox will be updated in Mozilla bug 1478213 where I will also add a test for this.

[birtles]

A test for this is added in: web-platform-tests/wpt#12195