[Webkit-unassigned] [Bug 206024] New: Thin line sometimes shown above heading on twitter.com

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=206024

            Bug ID: 206024
           Summary: Thin line sometimes shown above heading on twitter.com
           Product: WebKit
           Version: Safari 13
          Hardware: Unspecified
                OS: iOS 13
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: kenneth at kufluk.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Created attachment 387254

  --> https://bugs.webkit.org/attachment.cgi?id=387254&action=review

Screenshot of Twitter on my iPad showing the line at the top of the site.

Hi. I'm a web engineer on twitter.com.

I have found an infrequent rendering bug when using iPhone or iPad with mobile.twitter.com.
I use dark mode on the OS and on twitter.
I scroll my home timeline. When scrolling stops, I sometimes see one pixel of rendering from the timeline under the top header, appearing above the header.
This is most apparent when there is a bright image in the timeline.

This bug either does not appear, or is much less apparent, when the phone is not in dark mode. I suspect it is less apparent.

I have made a reduced test case here:
https://clumsy-biplane.glitch.me (make sure your phone is in dark mode)
The bug appears far less frequently in the reduced test case. Perhaps 20-30 scrolls up and down might be needed. I find the line only appears after the scroll moment has finished.

By comparing screenshots, I believe the line is an additional line, because I measured an extra 1px between the top of the screen and the text in the header.

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