[Webkit-unassigned] [Bug 219923] New: Scroll stutter/blocked with nested scrolling

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=219923

            Bug ID: 219923
           Summary: Scroll stutter/blocked with nested scrolling
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: Unspecified
                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 416298

  --> https://bugs.webkit.org/attachment.cgi?id=416298&action=review

Test case

Safari Technology Preview 117, on macOS 10.15.7

With one scroll container containing another scroll container. The outer one is set to overflow-x: auto, and the inner one to overflow-y: auto.

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

Steps to reproduce:
1. scroll vertically (inner) with a trackpad
2. scroll horizontally (outer) with a trackpad BEFORE the end of the momentum, OR BEFORE the end of overscroll if you reach the end of the scroll container

Expected behaviour: the outer container should scroll horizontally right away.
Result: the scroll stutters and gets blocked.

This happens in all directions, with the inner or the outer scroll. It didn't happen in the previous version of Safari (at least not until 116 which I didn't test).
This makes nested scrolling pretty unusable, because you have to wait for the end of the momentum and overscroll to scroll in another direction.

I guess this is a regression due to the recent changes on scrolling.

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