[Webkit-unassigned] [Bug 218489] New: 1px line following the contour of border radii

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=218489

            Bug ID: 218489
           Summary: 1px line following the contour of border radii
           Product: WebKit
           Version: Safari 14
          Hardware: Macintosh
                OS: macOS 10.15
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Compositing
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: luke at channings.me

Created attachment 413023

  --> https://bugs.webkit.org/attachment.cgi?id=413023&action=review

Reduced example

A 1px line is drawn along the contours of a border radius, assuming the background colour of the element it is rendered within.

The steps needed to reproduce this behaviour are:

- An element with overflow: hidden and border-radius
- A child element (or pseudo, or box-shadow, etc) positioned along the edges of the border radius

In the example below I reproduce this rendering issue with as little CSS as possible, whilst making it easy to see the issue.
When a background colour other than white is used the bug is harder to see, but can still be seen.

I have been able to reproduce this in Safari, Chrome, and FireFox.

There's a possibility it is deliberate, but am unaware of a reason why, or a possible workaround.

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