Bug 300089 – REGRESSION (Safari 26): Container queries length unit wrong values

• Status: RESOLVED
• Resolution: FIXED
• Component: Layout and Rendering

Created attachment 476948 [details]
The same element is attached to two different parents, with one defining a shadow root with only one <slot> element inside

This is almost exactly the same issue reported in https://bugs.webkit.org/show_bug.cgi?id=260462, which has been fixed, but now happens when the element containing the container-type property is placed in the light DOM of another element with a shadow ROOT.

It is a regression because I couldn't notice this problem in Safari 18.6, which is the version I am using in my desktop, but can be verified in the Safari 26 (I could only test the iOS version, so it needs to be verified in the other versions too). 

Attached, you can see the reproduction of the issue, that was taken from the one previously linked, with the change to add the element defining the container context to a parent with a shadow root.

[Alexey Proskuryakov]

Thank you for the report! Confirmed that this reproduces on macOS.

[Radar WebKit Bug Importer]

<rdar://problem/161897768>

[Antti Koivisto]

Pull request: https://github.com/WebKit/WebKit/pull/52125

[EWS]

Committed 301324@main (3286a2880158): <https://commits.webkit.org/301324@main>

Reviewed commits have been landed. Closing PR #52125 and removing active labels.

[Antti Koivisto]

Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/55368