Intent to implement: scroll-boundary-behavior

[Botond Ballo]

Summary:
A CSS property that allows web developers to control what scroll
boundary actions performed by the browser (such as scroll handoff
to a parent scrollable element, visual overscroll effect, swipe
navigation, etc.) should apply to a scrollable element.

Tracking bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=951793

Draft spec:
https://wicg.github.io/scroll-boundary-behavior/

Platform coverage:
The CSS property will be supported on all platforms. The browser's
default boundary actions may vary from platform to platform (for
example, Firefox currently shows a visual overscroll effect on
Android but not on desktop platforms).

Estimated target release:
Behind a pref: Firefox 58 (estimated)
Enabled by default: TBD

Preference behind which this will be implemented:
layout.css.scroll-boundary-behavior.enabled

DevTools support:
I don't believe any special support is required beyond adding the
property to the list of properties surfaced by the inspector.

Support by other browser engines:
Blink: Yes, will ship in Chrome 63 [1]
Edge: Public support [2]
WebKit: No public signals

Tests:
The Blink implementers have upstreamed a web-platform-test [3].
I plan to look into getting that to run on Firefox. Additional test
cases can be added to it if necessary.

Cheers,
Botond

[1] https://www.chromestatus.com/features/5734614437986304
[2] https://discourse.wicg.io/t/generic-scroll-chaining-prevention-mechanism-or-expand-standardize-ms-scroll-chaining/1811/5?u=majidvp
[3] http://searchfox.org/mozilla-central/rev/d30462037ffea383e74c42542c820cf65b2b144e/testing/web-platform/tests/cssom-view/scrollBoundaryBehavior-manual.html