Created attachment 426856 [details] Safari 14 (Bad) A recent bug was introduced by Safari 14 where elements positioned via negative margin to be hidden "behind" the padding of a parent are no longer hidden if they are contained within a Scrollable area. I'm not a layout expert, so this is my best interpretation of what is happening. However, this JSFiddle reproduces the issue in a roughly minimal way: https://jsfiddle.net/whelchel/veL7dkyj/18/. I've also attached one screenshot - one from Safari 14 (bad). I will try and see if I can attach a second with the good behavior. In Safari 13, both times the SVG positioned with negative margin is hidden. This is the correct, expected behavior. In Safari 14, when contained by a scrollable, the SVG is visible, despite the padding that should "cover" it. You can compare the top and bottom instances to see the difference, with and without the scrollable.
Created attachment 426857 [details] Safari 13 (Good)
This is about accelerated overflow scroll having to make self-painting layers.
Cool, that's definitely beyond me to identify :) Happy to update the title if that's helpful
<rdar://problem/77346751>
Created attachment 465487 [details] WebKit ToT Broken vs Chrome Canary 113 and Firefox Nightly 113 It is still an issue in WebKit ToT as shown in the picture.