[Webkit-unassigned] [Bug 227635] New: Drag-and-drop gradually degrades transition performance on iOS/iPadOS Safari

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=227635

            Bug ID: 227635
           Summary: Drag-and-drop gradually degrades transition
                    performance on iOS/iPadOS Safari
           Product: WebKit
           Version: Safari 14
          Hardware: iPhone / iPad
                OS: iOS 14
            Status: NEW
          Severity: Major
          Priority: P2
         Component: UI Events
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: darin.senneff at gmail.com

Created attachment 432813

  --> https://bugs.webkit.org/attachment.cgi?id=432813&action=review

Safari 15 showing transition degradation upon drop events

Using the HTML drag-and-drop API, I created draggable items that can be swapped with one another. Upon a drop action occurring, I temporarily add a class to the items which transitions them into their new locations. However, each time an item is dropped, the performance of the transition becomes worse and worse. You can see that the transitions are smooth to start, then after each drop they get more and more abrupt, and then finally no transition occurs at all.

Reloading the page does not correct the problem. However, force-quitting Safari and re-opening it does temporarily fix it, as you can see in the attached screen capture.

This problem is only present in Safari on iPadOS 14 and iOS 15. Performance is great in Safari on MacOS.

Screen captures attached from an iPhone 12 Pro running iOS 15beta, which shows the gradual degradation of transition performance. Second screen capture shows the same demo page on Safari 14 on MacOS, which works perfectly with no reduction in quality.

Demo page: https://codepen.io/dsenneff/pen/35f4af2b0b94e72cbc0ba441ec8cf550

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