1988225 - Enable anchor positioning pref (layout.css.anchor-positioning.enabled) by default on all channels

Status: RESOLVED FIXED

(Core :: CSS Parsing and Computation, enhancement)

Description

[Daniel Holbert [:dholbert]]

Followup to bug 1988224 - this bug tracks letting the anchor positioning pref ride the trains, removing any nightly/early-beta-only guard that we might initially impose in a tentative-enabling in bug 1988224.

(Hypothetically if we decide to go straight to fully-enabled without any sort of initial nightly-only guard, then we can dupe bug 1988224 together with this bug here.)

(We can use the dependency field to track [some of] the work that must happen before we can do this. Though also I think this work is tracked via [anchorpositioning:m whiteboard-field-contents, and maybe it's not worth trying to get those milestones to be perfectly reflected/mirrored in the dependency field of this bug.)

Comment 1

[Sebastian Zartner [:sebo]]

Once the pref is flipped, this the related BCD needs to be updated and it should be mentioned in the release notes on MDN and the general release notes.

[Why is this notable]: Feature CSS authors were requesting for a long time and part of Interop 2025.
[Affects Firefox for Android]: yes
[Suggested wording]: Support for CSS anchor positioning was added, which allows to tether elements together. The anchor-positioned elements can then have their size and position set relative to the size and location of the anchor elements to which they are bound.
[Links (documentation, blog post, etc)]:
https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_anchor_positioning
https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_anchor_positioning/Using

Comment 2

[一丝]

Currently, the following two scenarios do not function correctly in Firefox Nightly(145.0a1 (2025-10-08) ). I hope to fix them before release.

• https://codepen.io/yisi/pen/eYaJopE
• https://codepen.io/yisi/pen/yLrjKEO

Comment 3

[David Shin[:dshin]]

(In reply to 一丝 from comment #2)

Currently, the following two scenarios do not function correctly in Firefox Nightly(145.0a1 (2025-10-08) ). I hope to fix them before release.

• https://codepen.io/yisi/pen/eYaJopE

Bug 1993692 filed

• https://codepen.io/yisi/pen/yLrjKEO

::-webkit-slider-thumb is non-standard - We have ::-moz-range-thumb, but that doesn't solve it.

Bug 1993695 filed re: potentially aliasing them, bug 1993699 filed re: allowing it to be anchored against

Comment 4

[Sebastian Zartner [:sebo]]

Bug 1979593 is responsible for a bunch of WPT failures and is one of the main things pushing down the Interop 2025 score. So I wonder if that should be fixed before shipping.

Sebastian

Comment 5

[Emilio Cobos Álvarez [:emilio]]

I think we should fix the test. The code works, but the test relied on some interleaving behavior that we decided not to include in interop. I sent a fix.

Comment 6

[Jonathan Watt [:jwatt]]

Attachment: Bug 1988225. Enable anchor positioning pref on all channels. r=emilio

Comment 7

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/e9854d00f8c7
https://hg.mozilla.org/integration/autoland/rev/cd4251648976
Enable anchor positioning pref on all channels. r=layout-anchor-positioning-reviewers,webidl,emilio

Comment 8

[Jonathan Watt [:jwatt]]

Intent to ship: https://groups.google.com/a/mozilla.org/g/dev-platform/c/B71NFNrZ8Lo/m/e1ZMg8fjCAAJ

Comment 9

[Norisz Fay [:noriszfay]]

https://hg.mozilla.org/mozilla-central/rev/cd4251648976

Comment 10

[Ryan VanderMeulen [:RyanVM]]

Added to the Fx147 relnotes.