[Webkit-unassigned] [Bug 212439] New: Horizontally scrolling elements are broken under specific conditions

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=212439

            Bug ID: 212439
           Summary: Horizontally scrolling elements are broken under
                    specific conditions
           Product: WebKit
           Version: Safari 13
          Hardware: iPhone / iPad
                OS: iOS 13
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: New Bugs
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: cesium at google.com

Created attachment 400388

  --> https://bugs.webkit.org/attachment.cgi?id=400388&action=review

minimal test case

The expected behavior for scrolling is that when beginning a swipe in a horizontal direction on an element that can scroll horizontally, the element scrolls and vertical page scrolling is locked. (Sorry, I'm not sure if there's a technical term for this.) The buggy behavior is that vertical page scrolling is not locked, so that swiping diagonally or in a circle (for example) will scroll both the carousel and the page simultaneously, which isn't disastrous but is quite annoying.

To reproduce, open the attached HTML file in iOS Safari. There are three horizontally scrolling elements on the page, two of which are initially hidden. The first, unhidden scrollable displays the expected behavior. The second and third scrollables, after being unhidden, display the buggy behavior. There appear to be three requirements:

1. The scrollable must be initially hidden. visibility: hidden triggers the bug; I think display: none does not.
2. The scrollable must have certain styles set. position: relative and -webkit-overflow-scrolling: touch both seem to trigger the bug.
3. The page must have a touchmove event handler set.

If the page does not have a touchmove handler, I see different buggy behavior where the second and third scrollables don't scroll at all. (The first is still unaffected.) Also, the scrollable does not have momentum even if -webkit-overflow-scrolling is set.

This bug appeared in iOS Safari 13.4.1 and was not present on 13.3.1.

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