[scroll-animations-1] Triggering scroll-based animations once

[fantasai]

In #4354 (comment) @JayGeorge wrote:

Would it be possible only to trigger scroll-based animations once?

This is an option in the scroll-trigger library 'AOS' (or 'Animate on Scroll') – https://github.com/michalsnik/aos#1-initialize-aos
– See the 'once' option.

The use case would be: Sometimes, I would like to scroll back up a page and re-read content without having it 'introduced' a second time with animation. For me, this is like a halfway house between some animation but not too much.

Wanted to file this into its own issue for consideration.

[ydaniv]

From what I can see, the library linked above is not for scroll-linked animations, but for scroll/viewport-triggered animations, so probably not relevant at this stage.

[flackr]

We used to have a note in the spec (removed here) that said scroll triggered animations were currently out of scope, it might be worth bringing this back so that it's clear it's currently out of scope. Note this was also previously discussed in #4339. I certainly welcome ideas for how to do this but the mechanism is very different in that it likely will still involve the animation being attached to the document timeline rather than a new timeline.

[ydaniv]

Well, it's simply a whole different family. *-linked effects have their timeline attached to a continuous event that sets effect's progress accordingly, while *-triggered ones have a usual time-based timeline.
So this would normally be achieved today via IntersectionObserver, which currently hasn't a CSS counterpart, but I guess maybe in the future with Toggles via new "viewport" trigger? 🤔

[fantasai]

@flackr There's a short note in the Introduction currently that says

Note: Scroll-driven animations, whose progress is linked to the scroll position, are distinct from scroll-triggered animations, which are triggered by a scroll position, but whose progress is driven by time.

The removed text you link to has a bit more information, do you want to update the note a bit further or do you think it's fine as-is?

[bramus]

Closing this one in favor of #8942. Some good discussion about this going on in there.

[bramus]

Reopening as #8942 is more concerned scroll-triggered animations (which could also run just once). Both topics can be resolved on independently.