[Webkit-unassigned] [Bug 219960] New: Scroll-chaining not triggering before complete end of overscroll

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=219960

            Bug ID: 219960
           Summary: Scroll-chaining not triggering before complete end of
                    overscroll
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: Mac (Intel)
                OS: macOS 10.15
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Scrolling
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: augus.dupin at gmail.com

Created attachment 416361

  --> https://bugs.webkit.org/attachment.cgi?id=416361&action=review

Test case

Safari Technology Preview 117, on macOS 10.15.7


When scrolling inside a scroll container, after reaching the end, the scroll does not propagate to the parent scroll container (whether it is another element or the page) as long as the overscroll is ongoing. The current scroll has to be perfectly still for scroll-chaining to happen.

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

Steps to reproduce:
1. scroll down to the end of the outer scroll (to reach the inner scroll)
2. scroll down inside the inner scroll, then up until reaching the top edge
3. scroll up again BEFORE the end of the overscroll

Expected behaviour: the scroll should propagate to the outer scroll container as soon as a new "up" scroll happens, and the outer scroll container should scroll up as a consequence.
Result: the inner scroll container get scrolled again, persisting in its overscroll. You have to wait for the overscroll to end completely to be able to scroll out of it.

Scroll-chaining is thus made pretty unusable, as you have to wait for the overscroll to end in order to escape the current scroll context.


This didn't happen in previous macOS Safari version (at least before STP 116, which I didn't test).
To be noted: this problem has been present on iOS for a while. Not sure if I should create another bug for this?

This bug might be related to this other one, on nested scrolling: https://bugs.webkit.org/show_bug.cgi?id=219923
Maybe it can even be considered as the same? As this bug (scroll-chaining) is present on iOS but not the other (nested scrolling) one I considered them to be different.

As mentioned in the other bug, this might be a regression due to the recent changes on scrolling.

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