[scroll-animations?] Proposal: Add overscroll gestures with ability to reveal elements

[vmpstr]

This is an early proposal to get the conversations started.

Explainer

https://github.com/vmpstr/htmldemos/blob/master/gestures/README.md

Summary of the proposal

We're proposing to add primitives to create "overscroll" areas on an element and be able to position elements into the overscroll areas. This means that an element in the overscroll will be reachable after the regular scroll finishes and a new one begins. This is similar to scroll chaining into an ancestor scroll container

Please see the explainer above for more information and some details

/cc @flackr @noamr

[fantasai]

(Marking Agenda+ for the F2F since I think it would make a good presentation/discussion topic there.)

[schenney-chromium]

Getting the a11y right for this is extremely important. As the explainer notes, assistive technology will need to know there is content that can be accessed and there must be some reasonable way to activate and engage with the overscroll content. We don't want to find ourselves in the same mess we have for carousels, which are similar in that they involve off-screen content that can be brought into view with actions.

Would this entire proposal be better placed in OpenUI?

[vmpstr]

The behaviors as it relates to scrolling and scroll chaining (the internal model) would remain similar independently of how the functionality is exposed. I think it's reasonable to start with CSSWG for this reason.

Once we get into the details of how the functionality is exposed, we can also propose this at either OpenUI or WHATWG as necessary