[Webkit-unassigned] [Bug 211505] New: Element styles incorrect after media query evaluation changes

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=211505

            Bug ID: 211505
           Summary: Element styles incorrect after media query evaluation
                    changes
           Product: WebKit
           Version: Safari 13
          Hardware: Macintosh
                OS: macOS 10.15
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: james.brooks at starlingbank.com

Created attachment 398603

  --> https://bugs.webkit.org/attachment.cgi?id=398603&action=review

A video demonstrating the bug in Safari 13.1

Media queries don't always yield the expected results when resizing my browser window. I have a media query that repositions two divs side-by-side on screen sizes wider the 900px, and on mobile they stack on top of each other. When resizing up from mobile the styles don't update until the page is reloaded. The media queries behave as expected on both Safari 13.0 and Safari 12.1, this issue only seems to be present in Safari 13.1.

The release notes for Safari 13.1 contained the extract:
> Made media query updates more efficient.

I traced that back to the equivalent Safari Technical Preview release notes and found:
> Improved performance by invalidating only affected elements after media query evaluation changes

I think the issues I am seeing are linked to the changes made in r253875 as a result of bug 205392.

I have also been able to validate that the issue is still present on the most recent WebKit build archive at the time of writing this r261218.

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