Interaction with scroll timeline

[vmpstr]

This was raised at a partner sync, but since we support web animations API and from my understanding one can specify a scroll timeline instead of an animation time, we should figure out what we want to happen when the pseudo elements are tied to the scroll timeline in such a way.

Right now we destroy the pseudo element tree when there are no active animations. Also, I believe the intent is to not do hit testing in the pseudo trees (or elsewhere?) while the animation is taking place. All this leads me to believe that we don't want to support a scroll timeline for these animations. However, maybe there is an elegant way to do it.

/cc @flackr

[flackr]

Reading between the lines, it sounds like you want to have a guarantee the animation is ephemeral since the state it presents the user with is one in which you can't interact with the page.

A scroll timeline animation would not be guaranteed to finish in any given time. However, the web animations api also supports specifying infinite iterations on an animation or infinite or virtually infinite durations. What do you do in these cases?

On the flip side, if a scroll linked animation were tied to a scroll snap in the right way, it would in most cases have a finite end time (unless the user held the scroll partway through in which case they couldn't interact with the page anyways).

[absurdprofit]

Also what about gesture linked navigation? For example on iOS safari you can swipe to navigate between pages. In native contexts apps have the ability to link their shared element transitions to the navigation gestures. Afik web animations don't support any other timelines than ScrollTineline and DocumentTimeline. I've had to do pretty nasty hacks to get this behaviour (for gesture linked animations).

[bramus]

@bokand brought this up within the CSSWG for discussion: w3c/csswg-drafts#9901