[Webkit-unassigned] [Bug 284945] New: Query container breaks position: fixed

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=284945

            Bug ID: 284945
           Summary: Query container breaks position: fixed
           Product: WebKit
           Version: Safari 18
          Hardware: Unspecified
                OS: macOS 15
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: New Bugs
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: sebastianzartner at gmail.com

Created attachment 473603

  --> https://bugs.webkit.org/attachment.cgi?id=473603&action=review

Test case

If an element is turned into a query container via container-type, elements within it can not be fixed positioned.
See the attached test case. If the container-type property is removed, the nested element is positioned at the top of the viewport as expected.

For a real test case, see https://www.gamestar.de/artikel/drohnen-new-jersey-usa-ostkueste-lage-deutschland,3424742.html. The video embedded in the article content has a picture-in-picture mode when scrolling away from it, which should be fixed positioned. Though in Safari 18.2 it is stuck at the top and positioned behind the header even with a extremely big z-index set.

I've tested this in Safari 18 on macOS 15 and an iPhone 14. In Chrome 131.0 and Firefox 133.0 the test case works as expected.

Sebastian

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