[Webkit-unassigned] [Bug 220225] New: Wrong background opacity with overflowing divs and fixed position

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=220225

            Bug ID: 220225
           Summary: Wrong background opacity with overflowing divs and
                    fixed position
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: Mac (Intel)
                OS: macOS 11
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: kb at karelbilek.com

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)

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