By spec [1], if the current time of an animation becomes newly unresolved, if the previous current time is resolved, we should use the previous current time to run the animation. However our implementation for scroll timeline [2] may feed an unresolved current time to the animation.

In fact by this time the wording in spec may need an update. i.e. unresolved current time doesn't mean inactive timeline. We should use the last current time if the timeline becomes inactive but disable the animation when the new current time becomes unresolved.

[1] https://www.w3.org/TR/web-animations-1/#responding-to-a-newly-inactive-timeline
[2] https://cs.chromium.org/chromium/src/cc/animation/worklet_animation.cc?dr&g=0&l=110