In the upper right corner is a switch to toggle light and dark mode.
Click on it.
While animating to light mode the bands that make up the background don't get masked by the shape of the switch.
Not a recent regression, this was already broken in Safari 14.1.2.
This uses border-radius to achieve the clipping of the toggle, not a mask or a clip.
Created attachment 438472 [details]
First stab at reducing this and slowing things down to see what's going on. Will reduce further later.
I think this is about changing the compositing layer tree and failing to reconfigure the clip path/mask.