Created attachment 439303 [details] Reduced test case showcasing the issue Safari 15. Though I believe it still occurs on 14 and lower. Issue: When toggling display on a parent container of a video there is a visual glitch: none -> block: The red background will render, then the video will appear. Happens less frequently block -> none: The video will disappear immediately but the red background will remain for a frame before being removed. This case is much more frequent than the former and can be repeatedly triggered without refreshing. This can be reproduced in the minimal test case attached. Expected: That the video and the red background appear and disappear simultaneously. Same as other browsers.
Created attachment 439304 [details] Video demo of issue Video demo of both issues occurring. Progress frame by frame to see the visual glitch clearly at both toggle points
Created attachment 439305 [details] Video demo of second issue, repeatedly triggered
Something compositing related, maybe?
<rdar://problem/83674133>