animation.updatePlaybackRate(0) does not set holdTime

[johannesodland]

Problem

Setting the playback rate to 0 should probably freeze the animation, similar to pausing it.

Expected result

In Chromes implementation the animation freezes at the current time in the timeline.

Actual result

In the polyfill, the animation is reset

Probable cause

I think the cause is in commitPendingPlay().

Playing an animation (step 17) says:

If animation’s playback rate is zero, let animation’s hold time be current time to match.

But: the polyfill implementation sets holdTime to null:

scroll-timeline/src/proxy-animation.js

Lines 152 to 155 in daef204

	if (playbackRate == 0) {
	details.holdTime = null;
	details.startTime = timelineTime;
	} else {

Setting the holdTime to currentTimeToMatch alone does not solve the issue, as startTime is set. If startTime is set to unresolved however, the animation freezes as expected. Am I reading the standard correct here, to set both startTime and holdTime?

Screenshots

Result of scrolling into view and setting playbackRate to 0:

Chrome:

Safari with polyfill:

[johannesodland]

@flackr @bramus What do you think should be the correct behavior here?
To me it makes more sense that hold-time is set, and start time is unset.