1916203 - Registered Custom Properties rarely trigger a transitionstart

Status: UNCONFIRMED

(Core :: CSS Transitions and Animations, defect)

Description

[Bramus]

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/128.0.0.0 Safari/537.36

Steps to reproduce:

(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.)

• Navigate to https://allow-discrete-bugs.netlify.app/firefox/registered-no-transition/
• Make note that the custom properties --variable1 and --variable2 are registered with @property. The former as a <number> and the latter as a <custom-ident>
• Click the --variable1 button

Tested in Firefox Nightly 131.0a1 (2024-09-01) (aarch64)

Actual results:

Clicking the --variable1 button does not update the output, or only very rarely does when rage clicking the button.

Expected results:

Clicking the --variable1 button should update the output

Noteworthy:

Triggering the --variable2 button (after a first miss, see https://bugzilla.mozilla.org/show_bug.cgi?id=1916202) does always update trigger a transitionstart. Is this bug only related to values/syntaxes that can be interpolated?

Comment 1

[BugBot [:suhaib / :marco/ :calixte]]

The Bugbug bot thinks this bug should belong to the 'Core::CSS Transitions and Animations' component, and is moving the bug to that component. Please correct in case you think the bot is wrong.