[Webkit-unassigned] [Bug 213456] New: Box with border radius sometimes gets rendered with crossed lines from corners

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=213456

            Bug ID: 213456
           Summary: Box with border radius sometimes gets rendered with
                    crossed lines from corners
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Macintosh
                OS: macOS 10.15
            Status: NEW
          Severity: Minor
          Priority: P2
         Component: New Bugs
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: paulgerarts at me.com

Created attachment 402461

  --> https://bugs.webkit.org/attachment.cgi?id=402461&action=review

Block with painting issue

When a block is painted with a border radius it sometimes causes a faint cross (x) to appear from corner to corner. It seems to happen more often on non-retina displays (although it may just be more visible with the reduced density). I can reliably reproduce this in the rendering of this YouTube page on the annotation bubble (the modal that opens on the top-right corner of the video). https://youtu.be/KRGb2s5Ko0Y?t=92 Modal triggers at 01:35 timestamp.

It's not a YouTube issue. This just makes for an example that seems to trigger the issue reliably. I've also had it happen on me while writing CSS. It seems to be a combination of a small border-radius and transparency or opacity.

See the attached image for an example.

I'm running 10.15.5 (19F101) but I've seen this happening on older versions (on a MacBook Pro (Retina, 15-inch, Late 2013) NVIDIA GeForce GT 750M 2 GB, Intel Iris Pro 1536 MB).
Also seen it on a 2019 Touch Bar 4 thunderbolt port 13 inch MacBook Pro without dedicated graphics.
I see it both in Safari Version 13.1.1 (15609.2.9.1.2) and the (at this moment) latest WebKit build (263344).
Again, it's more prevalent on an external monitor (1920 x 1080 in this case).

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