[web-animations-1] Use cases for `timeline` being writable

[stephenmcgruer]

Currently, an Animation's timeline member is writable: https://drafts.csswg.org/web-animations-1/#the-animation-interface

I was wondering what the use cases were for replacing a timeline on an Animation. This came up in the context of https://github.com/WICG/scroll-animations/issues/44 - I'm not committing to any particular thought on that issue at the current time, but it would be a simpler world if timeline were readonly.

[stephenmcgruer]

The example given in the scroll-animations issue was "using a ScrollTimeline until you pass some threshold and then swap in a DocumentTimeline so that the animation runs to completion".

This isn't quite a use case (it describes steps instead of describing a goal), and I'm not clear in this description why you wouldn't just cancel the Animation and start a new one at that point. (Also, is it using the same effect, or does one swap that out as well?).

[birtles]

There are a number of hidey bar effects where after passing a certain threshold, the animation proceeds to the end ignoring the scroll progress--a kind of snap-to-end effect. Starting a new animation (especially one where all the units need to be rewritten) at that point seems sub-optimal.

[bradkemper]

https://github.com/WICG/scroll-animations/issues/44#issuecomment-479983091

[bradkemper]

Having two animations based on length of scroll in existing properties seems simpler than all the scroll timeline values.