1916202 - Custom Properties with discretely animatable syntaxes / Unregistered Custom Properties don’t trigger transitionstart upon the first change

Status: RESOLVED INVALID

(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/string-first-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 --variable2 button.

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

Actual results:

The first click on the --variable2 button does nothing. All successive clicks on that button do update the output.

Expected results:

Clicking the --variable2 button should update the output from the first click on.

Noteworthy: This applies to both Unregistered Custom Properties a Registered Custom Properties that have a syntax that can only be animated discretely.

Comment 1

[Bramus]

Turns out the test for this was wrong. Closing.