[Webkit-unassigned] [Bug 228917] New: iOS 15 Shadow DOM CSS OM styles don't work when class names change

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=228917

            Bug ID: 228917
           Summary: iOS 15 Shadow DOM CSS OM styles don't work when class
                    names change
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: iPhone / iPad
                OS: Other
            Status: NEW
          Severity: Major
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: thomas.rye.ladd at gmail.com

Created attachment 435189

  --> https://bugs.webkit.org/attachment.cgi?id=435189&action=review

The grow.me widget losing its styles on the ios 15 beta as its positioning changes

On the iOS 15 beta, an app that renders its styles to a shadow DOM and uses styled-components and has SC_DISABLE_SPEEDY set to false (so how it behaves in production by default), any class names that change because of a prop change ends up losing their styles. The new class name is applied to the element but the styles associated with that class name aren't found. When styled-components speedy mode is enabled, it injects styles using the CSS OM instead of putting them into a style tag in the DOM.  When the styles get injected into a style tag, the styles work. Styles also work if using CSS OM and not using shadow DOM. So it seems like the regression deals with the combination of shadow DOM and CSS OM together.

I created a small reproduction repository here https://github.com/TLadd/styled-components-shadow-dom-ios15-beta-bug

Issue filed on styled-components: https://github.com/styled-components/styled-components/issues/3544
Feedback Assistant issue: https://feedbackassistant.apple.com/feedback/9449844

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