[web-animations-2] Playing an animation might return too early when auto-aligning start time

[johannesodland]

It seems to me that playing an animation is returning too early when auto-aligning start time due to a finite timeline.

Given the following code, playing an animation seems to return on step 10.

const viewTimeline = new ViewTimeline({ subject, axis});
const effect = new KeyframeEffect(subject, [{ opacity: 1}, { opacity: 0}], { duration: 'auto', fill: 'both' });
const animation = new Animation(effect, viewTimeline);
animation.play();

Step 10 in Playing an animation states:

10. If the following three conditions are all satisfied:

• animation’s hold time is unresolved, and
• aborted pause is false, and
• animation does not have a pending playback rate,

abort this procedure.

At this point the procedure aborts as the hold time is not resolved, aborted pause is false and animation does not have a pending playback rate.

The animation does not have a ready promise nor a pending play task as this is first created on step 11 and 12.

As a result, the play state is still idle due to having an unresolved current time, having an unresolved start time and no pending play task.

This again causes Auto-aligning the start time to abort on step 3

3. If play state is idle, abort this procedure.

I'm not sure what the correct solution is here, and I might be reading the spec wrong. Maybe step 10 should check that the auto align start time flag is not set?

[johannesodland]

@flackr Do you know what the correct behavior here should be?

It seems to me like sted 10 is designed to abort the procedure if the animation is already playing, hasn't been auto-rewinded and did not have a pending pause or playback rate.

Should it also check for the auto align start time flag, or that start time is resolved?

[johannesodland]

This seems to be the same issue as #10965.

Closing as duplicate.