[Webkit-unassigned] [Bug 226195] New: Mobile Safari: Scrolling stops when a child is added to or removed from an overflow:auto element with a shadow root

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=226195

            Bug ID: 226195
           Summary: Mobile Safari: Scrolling stops when a child is added
                    to or removed from an overflow:auto element with a
                    shadow root
           Product: WebKit
           Version: Safari 14
          Hardware: iPhone / iPad
                OS: iOS 14
            Status: NEW
          Severity: Major
          Priority: P2
         Component: DOM
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: graynorton at google.com

Created attachment 429583

  --> https://bugs.webkit.org/attachment.cgi?id=429583&action=review

Screen recording of bug

1. Using iOS 14.4.2, open https://codepen.io/graynorton/full/yLMbxXE in Safari or Chrome.
2. Flick or drag within the blue-bordered element to scroll vertically.

Expected result: Scrolls smoothly, with velocity
Actual result: For each flick or drag gesture, scrolls only ~10 pixels and stops

Works as expected in Safari, Chrome, Firefox desktop (OS X).
Works as expected in Chrome Android.

In Mobile Safari, scrolling stops whenever a child element is added to or removed from the scrolling element, which has a shadow root with a single <slot> element inside it. In this repro case, the first child is removed and immediately re-added whenever the element's scrollTop changes by > 10 pixels, but the same problem occurs with any addition or removal of child elements, regardless of whether the mutation is triggered by a scroll event.

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