[Webkit-unassigned] [Bug 279012] New: Transitioning Unregistered Custom Properties cause a transition loop

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=279012

            Bug ID: 279012
           Summary: Transitioning Unregistered Custom Properties cause a
                    transition loop
           Product: WebKit
           Version: Other
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Animations
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: bramus at google.com
                CC: graouts at apple.com

(This bug was discovered while building `@bramus/style-observer`, a library that responds to changes in computed styles. For this it relies on CSS Transitions and `transition-behavior: allow-discrete`. See https://brm.us/style-observer for details about this library.)

Transitioning Unregistered Custom Properties cause a transition loop

- Demo: https://allow-discrete-bugs.netlify.app/safari/unregistered-loop/ – the custom properties `--variable1` and `--variable2` are not registered.
- Expected behavior: When clicking the `--variable1` or `--variable2` buttons, the output should change and the transition should have triggered only once.
- Actual Behavior: While the output does change, the transition is repeatedly triggered. You can verify this by checking the console, which logs `"_handleUpdate"` on `transitionstart`

Tested in Safari Technology Preview 202

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