REGRESSION: Scroll snapping triggers body scroll

[Bruno Stasse]

Created attachment 416602 [details]
Test case

Safari Technology Preview 117, on macOS 10.15.7

In a scroll container where scroll-snapping is enabled, a scroll gesture (followed by the snapping mechanism) causes the body to scroll as well in the same direction.

See the attachment for a test case, also available here: https://output.jsbin.com/qewoyuraya

Steps to reproduce:
1. Scroll vertically inside the yellow box with a trackpad

You will see the body scroll vertically as well as soon as the snapping takes place.

This is quite unexpected and problematic in some cases.
I know this could be prevented by setting the body to overflow: hidden, but it is not always possible or desired.
Note besides that scrolling programmatically the body in this case does not stop the scroll, as it should considering the revisions passed with STP 117.

This problem was not present before STP 116, so this might be a regression due to the changes made on scrolling and scroll-snapping in STP 117.
Possibly related to:
- https://bugs.webkit.org/show_bug.cgi?id=219923
- https://bugs.webkit.org/show_bug.cgi?id=219960
which also appeared in STP 117.

[Smoley]

Thanks for filing. I'm not sure about the regression status as I can reproduce this on Safari 13.1.3 as well as STP 117 (14.1).

[Radar WebKit Bug Importer]

<rdar://problem/72595482>

[Bruno Stasse]

Yes, that's right I can too. However, I'm pretty sure it was not the case in STP 115 (I don't know about 116, I didn't test it).

[Simon Fraser (smfr)]

Latching isn't working correctly; with scroll snap we fail to tell ScrollingTreeLatchingController that we've received events, so the latch times out.

[Simon Fraser (smfr)]

This might be:
    if (processWheelEventForScrollSnap(wheelEvent))
        return false; // FIXME: Why don't we report that we handled it?

[Simon Fraser (smfr)]

Created attachment 416694 [details]
Patch

[Sam Weinig]

Comment on attachment 416694 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=416694&action=review

> Source/WebCore/platform/cocoa/ScrollController.h:172
>      bool processWheelEventForScrollSnap(const PlatformWheelEvent&);

Not for this change obviously, but maybe it's time to replace these boolean return values with something a bit more expressive.

[Simon Fraser (smfr)]

Yeah, I spent 5 minutes trying to think of an enum name and failed. We need this handled/unhandled enum more extensively.

[Simon Fraser (smfr)]

https://trac.webkit.org/r271072