Bug 220225 – Wrong background opacity with overflowing divs and fixed position

• Component: Compositing
• Status: NEW

[Karel Bilek]

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)

[Karel Bilek]

Created attachment 416894 [details]
Simple testcase

Simple testcase

[Karel Bilek]

I have added an even simpler testcase. Left and right column should be the same, but they are not.

[Karel Bilek]

Comment on attachment 416894 [details]
Simple testcase

It seems the *top* `overflow-x:hidden` is not actually needed to replicate the bug.

[Karel Bilek]

Ignore comment 3.

The top `overflow-x:hidden` is needed to replicate the bug.

[Simon Fraser (smfr)]

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.

[Simon Fraser (smfr)]

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.

[Radar WebKit Bug Importer]

<rdar://problem/72961068>