[Webkit-unassigned] [Bug 176454] New: Implement CSS scroll-boundary-behavior

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=176454

            Bug ID: 176454
           Summary: Implement CSS scroll-boundary-behavior
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: majidvp at chromium.org

Consider implementing the new CSS scroll-boundary-behavior property.

## Specification
This feature specification is being incubated in WICG: 
https://wicg.github.io/scroll-boundary-behavior/


Note that this is first CSSWG spec which is going through this new process). 

Additional background on this feature may be found here: 
https://github.com/w3c/csswg-drafts/issues/769

## Summary
CSS scroll-boundary-behavior standardizes a supercharged version of "-ms-scroll-chaining". It
allows developers to determine the browser's behavior once a scroller has reached its full extent.
The unused delta can be propagated to the parent causing scroll chaining, create a glow/bounce
effect without chaining, or just get consumed silently. In particular when used on viewport
defining element, it controls if overscroll can be used for navigation actions such as pull-to-
refresh or swipe navigation.


scroll-boundary-behavior: auto | contain | none

  * `auto`: propagate scroll to the parent scroller. If there is no parent scroller (e.g.,
    viewport) user-agent may perform a default action (e.g. navigation) or show any appropriate
    overscroll UI affordance. This is the default value.
  * `contain`: do not propagate. The user agent may show an appropriate overscroll UI affordance
    such as bounce or glow.
  * `none`: same as contain but also prevents any overscroll UI affordance e.g. bounce or glow.

There will be scroll-boundary-behavior-{x,y} long-hands to control each axis individually.


## Current State
The specification and API has received positive feedback and support from EdgeHTML [1], Mozilla
[2], Chromium[3], and web developers [4,5]. We at Chromium have an implementation landed in
Canary (behind a feature flag**). If we don't receive any strong negative signal we plan to ship
in M63 per intent-to-ship. [3]


[1] https://github.com/w3c/csswg-drafts/issues/769#issuecomment-270228948
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=951793#c11
[3] https://groups.google.com/a/chromium.org/d/msg/blink-dev/OqBNF2efmFA/3ByBUKyaCgAJ
[4] https://github.com/w3c/csswg-drafts/issues/769#issuecomment-279832555
[5] https://github.com/w3c/csswg-drafts/issues/769#issuecomment-288878908

** --enable-web-platform-experimental-features or --enable-blink-features=CSSScrollBoundaryBehavior

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