Bug 250919

Created attachment 464584 [details]
Minimal reproduction of the bug

Overlaid elements with CSS rotate CSS animation transforms break through the UI in unexpected ways resulting in a glitchy rendering.

The attached file index.html showcases the bug. On the page is a loading spinner with CSS animations that should be hidden behind an overlay/modal with purple background and text. The expected behaviour would be that the spinner would be hidden behind the overlay. On tested WebKit browsers the spinner is unexpectedly partially rendered on top. Renders as expected in Chrome & Firefox.

Live demo of index.html:
https://webkit-bug-2023-01-20.vercel.app

Reproduction repo with video & screenshots: 
https://github.com/jonrh/webkit-bug-2023-01-20

Browsers tested that exhibit the bug:
+ Safari running on iOS 16.1, iPhone 13 Mini
+ Safari Version 16.2 (18614.3.7.1.5) (WebKit 605.1.15 ?), M2 MacBook Air, macOS 13.1
+ Orion Version 0.99.122-beta (WebKit 615.1.11.7), M2 MacBook Air, macOS 13.1

Browsers tested that render as expected:
+ Chrome Version 108.0.5359.124 (Official Build) (arm64)
+ Firefox Version 109.0 (64-bit)

Possible duplicates:
+ https://bugs.webkit.org/show_bug.cgi?id=236019
+ https://bugs.webkit.org/show_bug.cgi?id=212130
+ https://bugs.webkit.org/show_bug.cgi?id=181709

Background:
Originally observed when implementing a Paddle Checkout [0], an iframe overlay for collecting credit card payments. On a successful completion the underlying website behind the iframe modal would render the same square loading spinner. This resulted in jarring glitches where the UI that should be hidden (not just the spinner) became partially overlaid over the modal.

[0]: https://developer.paddle.com/guides/b4f911a991bd7-overlay-checkout