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css.properties.transition-behavior - allow-discrete does not transition display in Firefox #26155

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rviscomi opened this issue Mar 9, 2025 · 4 comments · May be fixed by #26507
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css.properties.transition-behavior - allow-discrete does not transition display in Firefox #26155

rviscomi opened this issue Mar 9, 2025 · 4 comments · May be fixed by #26507
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@caugner
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data:css Compat data for CSS features. https://developer.mozilla.org/docs/Web/CSS meeting agenda Issues or pull requests in need of discussion in a project meeting. p2 High priority – Community PRs highly encouraged.

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@rviscomi
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rviscomi commented Mar 9, 2025 •
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What type of issue is this?

Incorrect support data (example: BrowserX says "86" but support was added in "40")

What information was incorrect, unhelpful, or incomplete?

MDN is showing that Firefox has full support for transition-behavior:

Image

However, the display compat table shows that Firefox does not support transitioning the property when transition-behavior: allow-discrete is set:

Image

It's also broken out on caniuse under the "CSS property: display: Transitionable when setting transition-behavior: allow-discrete" feature:

Image

What browsers does this problem apply to, if applicable?

Firefox

What did you expect to see?

I'd expect to see Firefox have partial support for css.properties.transition-behavior, with a note about the transitionable display issue.

I'd also expect this to affect transition-bahavior's Baseline status to limited availability, because it seems like it doesn't fully work in Firefox.

Image

It probably makes sense to keep the compatibility note on display because the issue affects Firefox's ability to transition that property, but I would have also expected to see a corresponding note on transition-behavior.

Did you test this? If so, how?

No

Can you link to any release notes, bugs, pull requests, or MDN pages related to this?

https://developer.mozilla.org/en-US/docs/Web/CSS/transition-behavior#browser_compatibility
https://developer.mozilla.org/en-US/docs/Web/CSS/display#browser_compatibility
https://caniuse.com/mdn-css_properties_display_is_transitionable

Do you have anything more you want to share?

I found out about the support issue in Firefox through https://youtu.be/VA975GOUFmM?si=WvtQxcksDbT_H6m0&t=844. I'm impressed that they knew about it despite the transition-behavior docs suggesting that the feature is Baseline and fully supported in Firefox.

MDN URL

https://developer.mozilla.org/en-US/docs/Web/CSS/transition-behavior

MDN metadata

MDN page report details
  • Query: css.properties.transition-behavior
  • Report started: 2025-03-09T14:41:23.194Z
@rviscomi rviscomi added the needs triage This issue needs to be confirmed label Mar 9, 2025
@github-actions github-actions bot added the data:css Compat data for CSS features. https://developer.mozilla.org/docs/Web/CSS label Mar 9, 2025
@caugner caugner added the meeting agenda Issues or pull requests in need of discussion in a project meeting. label Apr 14, 2025
@caugner caugner linked a pull request Apr 14, 2025 that will close this issue
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@caugner
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caugner commented Apr 14, 2025

Given the number of upvotes, I think it would make sense to move these subfeatures under transition-behavior (see #26507), but we will discuss this in one of our next BCD meetings.

@caugner caugner added p2 High priority – Community PRs highly encouraged. and removed needs triage This issue needs to be confirmed labels Apr 14, 2025
@caugner caugner self-assigned this Apr 14, 2025
@caugner
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caugner commented Apr 16, 2025

We discussed this in the BCD meeting on 2025-04-15, and decided to add corresponding subfeatures under transition-behavior, rather than moving the is_transitionable subfeatures.

@ddbeck Can you share the rationale for why we want to add separate subfeatures?

@dphlin
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dphlin commented Apr 22, 2025

Bugzilla defect cross-reference bugzilla:1834877

@ddbeck
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ddbeck commented Apr 24, 2025

Can you share the rationale for why we want to add separate subfeatures?

If I remember correctly, the discussion went something like this: if we understand the feature properly, transition-behavior changes the way other properties transition (i.e., it's a backwards-compatible way to opt-in to new behavior on existing properties). So we end up recording a few distinct pieces of information:

  • Does the browser parse, handle, cascade, etc. the transition-behavior property?
  • If transition-behavior: allow-discrete has been opted into, then does it apply to…
    • display?
    • content-visibility?

The first one is given by css.properties.transition-behavior, while the rest are given by is_transitionable features. I think the original issue is strictly correct that compatibility here is closely related to transition-behavior. But we've run into a problem (primarily for MDN), where if you're a developer and you want to know why a given property is transitioning in a certain way, then you're unlikely to start by looking at the docs for the property you're trying to transition, not the transition-behavior property.

In an ideal world, MDN would do something like consume a web-features feature and show all the relevant features (no matter where they are in the BCD tree). In a non-ideal world, we have the unhappy compromise options of:

  • Duplicating is_transitionable data in specific transitionable properties and css.properties.transition-behavior
  • Centralizing the data in css.properties.transition-behavior.*
  • The status quo

Duplicating is probably the nicest for MDN readers. 🤷

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Move is_transitionable features under transition-behavior mdn/browser-compat-data
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@ddbeck @caugner @rviscomi @dphlin