[scroll-animations-1] it is not clear when to update stale timelines

[graouts]

The HTML Processing Model: Event loop section of the Scroll-driven Animations spec discusses how scroll and view timelines may become stale in the process of updating the rendering since timeline sources or subjects may change bounds during that process.

However, the spec does not reference the HTML spec with hard links and rather references sections which are by now outdated. For instance (at the time of writing this issue), the step to update animations and send events is step 11 and the current spec references step 10. As such it is not clear exactly when in the process of updating the page rendering stale timelines should be updated. Using informative notes, it appears it is after resize observers have been evaluated and styles have been recalculated and layout updated.

We should probably update the HTML spec itself to add a step that clearly references the process of updating stale timelines.

Here are related spec issues and spec change:

• [scroll-animations] Behavior of scroll-linked animations in the first frame
• [scroll-animations-1] Handling changed size/position in current frame
• additional spec change

Cc @flackr who made the spec change as well as @kevers-google and @andruud who reviewed it.

[BorisChiou]

Agree. At least we have to update the words in scroll-timelines to make it clear to link the steps in HTML spec. I also noticed this issue when updating the sampling recently on Gecko and it is still unclear for me about the timing to update the stale timelines.

[BorisChiou]

It seems this is similar to this spec issue: #10769