Bug 220225

Summary:

Wrong background opacity with overflowing divs and fixed position

Product:

WebKit

Reporter:

Karel Bilek <kb>

Component:

Compositing

Assignee:

Simon Fraser (smfr) <simon.fraser>

Status:

NEW

Severity:

Normal

CC:

simon.fraser, webkit-bug-importer, zalan

Priority:

Keywords:

InRadar

Version:

Safari Technology Preview

Hardware:

Mac (Intel)

OS:

macOS 11

Attachments:

Description	Flags
Simple testcase	none
Clearer testcase	none

Karel Bilek

Reported 2021-01-02 02:52:52 PST

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)

Attachments
Simple testcase (579 bytes, text/html) 2021-01-02 02:57 PST, Karel Bilek	no flags	Details
Clearer testcase (973 bytes, text/html) 2021-01-05 19:52 PST, Simon Fraser (smfr)	no flags	Details
View All Add attachment proposed patch, testcase, etc.

Karel Bilek

Comment 1 2021-01-02 02:57:16 PST

Created attachment 416894 [details] Simple testcase Simple testcase

Karel Bilek

Comment 2 2021-01-02 02:58:17 PST

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

Karel Bilek

Comment 3 2021-01-02 03:02:37 PST

Comment on attachment 416894 [details] Simple testcase It seems the *top* `overflow-x:hidden` is not actually needed to replicate the bug.

Karel Bilek

Comment 4 2021-01-02 03:20:06 PST

Ignore comment 3. The top `overflow-x:hidden` is needed to replicate the bug.

Simon Fraser (smfr)

Comment 5 2021-01-05 19:52:36 PST

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)

Comment 6 2021-01-05 20:51:50 PST

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

Comment 7 2021-01-09 02:53:11 PST

<rdar://problem/72961068>

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