[csswg-drafts] [web-animations-2] [scroll-animations] "at progress timeline boundary" for view timelines (#13509)

graouts has just created a new issue for https://github.com/w3c/csswg-drafts:

== [web-animations-2] [scroll-animations] "at progress timeline boundary" for view timelines ==
I think that the notion of the [at progress timeline boundary](https://drafts.csswg.org/web-animations-2/#at-progress-timeline-boundary) is only well defined for scroll timelines but not view timelines.

Indeed, with a scroll timeline, computing the _timeline progress_ as _timeline time_ divided by _timeline duration_ will yield 0 when the scroll container is scrolled to its minimum value and 1 when scrolled to its maximum value because a scroll timelines's current time will go from 0% to 100% in that same range.

However, in the case of a view timeline, a 0% current time will most likely not match the mimimim scroll value, throwing that logic off.

The question then is to figure out the purpose of "at progress timeline boundary" in the context of a view timeline. An inline [issue in the spec](https://drafts.csswg.org/web-animations-2/#issue-c8759f87) says the following:

> The surprising behavior that this is trying to prevent is an animation becoming inactive precisely at the scroll limit, alleviating the need for set a fill-mode with a ScrollTimeline.

With that in mind, I guess the idea is that the _at progress timeline boundary_ would prevent an animation associated with a view timeline to enter the after phase precisely as the scrolling container reaches it maximum scroll value. In that case, I think we'll have to come up with a new definition for _at progress timeline boundary_ that work in the sole context of a conceptual scroll timeline and disregards the animations current time. This is actually in part what that [same inline issue](https://drafts.csswg.org/web-animations-2/#issue-c8759f87) discusses:

> Checking if timeline [current time](https://www.w3.org/TR/web-animations-1/#timeline-current-time) is 0 or timeline duration may be sufficient.

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/13509 using your GitHub account


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Received on Tuesday, 17 February 2026 16:49:52 UTC