[Webkit-unassigned] [Bug 279013] New: Transitioning Registered Custom Properties with discretely animatable syntaxes cause a transition loop

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=279013

            Bug ID: 279013
           Summary: Transitioning Registered Custom Properties with
                    discretely animatable syntaxes 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 Registered Custom Properties with discretely animatable syntaxes cause a transition loop

- Demo: https://allow-discrete-bugs.netlify.app/safari/registered-discrete-loop/ – the custom properties `--variable1` and `--variable2` are registered with @property. The former as a `<number>` and the latter as a `<custom-ident>`
- Expected behavior: When clicking the `--variable2` button, 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`. This does not happen when clicking the `--variable1` button.
- Noteworthy: When changing `--variable2` to a syntax of `<string>` it also doesn’t work, which leads me to believe that this behavior shows for all syntaxes that cannot be interpolated.

Tested in Safari Technology Preview 202

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