Created attachment 220059 [details]
In the attached example there is a small example where a fixed positioned element is clipped with the clip-path property. It seems like the fixed positioned element is drawn on a graphics buffer and this buffer is not cleared. This leads to artifacts on areas that are clipped away.
Reload the example multiple times to see the issue. In Safari stable the background is always black. In nightlies the background is not stable. It even happened that a video playing in another application show through.
This just happens with "position: fixed".
Hmm, we probably think it's opaque when it isn't.
Created attachment 224397 [details]