I caught a bug with this minimal replication code. ``` <div style="width:100%;height: 100%; position: fixed; overflow-x:hidden; background-color: rgba(0,0,0,0.5);"> <div style="overflow-x: hidden;height: 18000px;width: 100px; background-color: white"> <div style="height: 20000px;"> </div> </div> </div> ``` When the inner div is bigger than the middle div, the background opacity of the top div is wrong. (20000px > 18000px) When I change 20000 to 10000, the opacity is correct. I have tried this in Chrome and Firefox and it does not happen there, so it must be a webkit bug. I don't see Big Sur in build archives so I cannot try this there; I use Safari Technology Preview, Release 117 (Safari 14.1, WebKit 16611.1.7.2)
Created attachment 416894 [details] Simple testcase Simple testcase
I have added an even simpler testcase. Left and right column should be the same, but they are not.
Comment on attachment 416894 [details] Simple testcase It seems the *top* `overflow-x:hidden` is not actually needed to replicate the bug.
Ignore comment 3. The top `overflow-x:hidden` is needed to replicate the bug.
Created attachment 417064 [details] Clearer testcase This is about "simple container" layers. We set the alpha color both as layer background color, and also paint it into a sublayer. Adding border, box-shadow or contents to the outer element makes the bug not happen.
RenderLayerBacking::updateDrawsContent() calls m_graphicsLayer->setDrawsContent(hasNonScrollingPaintedContent) but that ignores that fact that we've already determined that the content is a simple background color.
<rdar://problem/72961068>