1989059 - Implement `position-try-order` for anchor fallbacks.

Status: RESOLVED FIXED

(Core :: CSS Parsing and Computation, enhancement)

Description

[David Shin[:dshin]]

This property introduces a step where we compute inset-modified containing block of all fallbacks and sort fallbacks based on that. This implies resolution of all inset properties and position-area. Note that the grid area computed by position-area is considered IMCB, as per position-try-order-position-area.html.

InterOp 2025 WPT:

• position-try-order-basic.html
• position-try-order-position-area.html

Comment 1

[David Shin[:dshin]]

Naive implementation of this shouldn't be hard - We have everything we need to resolve these. Since @position-try can change the default anchor, this could get expensive, though.

Main work will be to factor out function for computation of IMCB, resolving inset and position-area properties (& associated anchor references, plus position-anchor), which then can be used as a comparator to sort the fallbacks.

Comment 2

[Jonathan Watt [:jwatt]]

Although this is 5 points of effort, per the interop-2025 bug, this would get us quite a few points if it fixes all of these:

• parsing/ (3 tests worth of points)
• property-interpolations.html
• position-try-order-basic.html
• position-try-order-position-area.html

Comment 3

[Jonathan Kew [:jfkthame]]

Attachment: Bug 1989059 - Account for position-try-order when choosing position fallback. r=#anchor-pos

Comment 4

[Jonathan Kew [:jfkthame]]

Attachment: Bug 1989059 - Fix up position-try-order-logical testcase to force overflow & fallback to happen. r=#anchor-pos

This test seems questionable as written, because the anchored box fits at its "base" position
without overflowing the IMCB, and therefore there doesn't seem to be any justification for
applying fallbacks at all (see also https://github.com/w3c/csswg-drafts/issues/13268).

This patch forces fallbacks to be considered in all cases, by ensuring the base position always
overflows and then providing a fallback that matches the "original" (non-overflowing) base position;
then the browser will choose between this and the flipped fallback according to the specified
position-try-order and writing modes involved.

Comment 5

[Jonathan Kew [:jfkthame]]

Attachment: Bug 1989059 - Enable the position-try-order property. r=#anchor-pos

Comment 6

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/f9538b019e72
https://hg.mozilla.org/integration/autoland/rev/fa41b29265c4
Account for position-try-order when choosing position fallback. r=layout-anchor-positioning-reviewers,layout-reviewers,emilio
https://github.com/mozilla-firefox/firefox/commit/a9184ac28d5d
https://hg.mozilla.org/integration/autoland/rev/39886ca6f78a
Fix up position-try-order-logical testcase to force overflow & fallback to happen. r=layout-anchor-positioning-reviewers,emilio
https://github.com/mozilla-firefox/firefox/commit/b3c5c336322f
https://hg.mozilla.org/integration/autoland/rev/b8e959d3f3c8
Enable the position-try-order property. r=layout-anchor-positioning-reviewers,emilio

Comment 7

[Web Platform Test Sync Bot [:wpt-sync] (Matrix: #interop:mozilla.org)]

Created web-platform-tests PR https://github.com/web-platform-tests/wpt/pull/56931 for changes under testing/web-platform/tests

Comment 8

[amarc]

https://hg.mozilla.org/mozilla-central/rev/fa41b29265c4
https://hg.mozilla.org/mozilla-central/rev/39886ca6f78a
https://hg.mozilla.org/mozilla-central/rev/b8e959d3f3c8

Comment 9

[Sebastian Zartner [:sebo]]

[Why is this notable]: Part of CSS Anchor Positioning
[Affects Firefox for Android]: yes
[Suggested wording]: Introduced the position-try-order property, which is part of CSS Anchor Positioning and allows to change the order the position fallback options are tried.
[Links (documentation, blog post, etc)]:
https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/position-try-order
https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Anchor_positioning
https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Anchor_positioning/Using

Comment 10

[Donal Meehan [:dmeehan]]

Thanks, added to the Fx148 nightly release notes, please allow 30 minutes for the site to update.
Keeping the relnote-firefox flag as ? to keep it on the radar for inclusion in the final Fx148 release notes.

Comment 11

[Web Platform Test Sync Bot [:wpt-sync] (Matrix: #interop:mozilla.org)]

Upstream PR merged by moz-wptsync-bot

Comment 12

[Emilio Cobos Álvarez [:emilio]]

Attachment: Bug 1989059 - Account for position-try-order when choosing position fallback.

Original Revision: https://phabricator.services.mozilla.com/D277556

Comment 13

[Emilio Cobos Álvarez [:emilio]]

Attachment: Bug 1989059 - Fix up position-try-order-logical testcase to force overflow & fallback to happen.

This test seems questionable as written, because the anchored box fits at its "base" position
without overflowing the IMCB, and therefore there doesn't seem to be any justification for
applying fallbacks at all (see also https://github.com/w3c/csswg-drafts/issues/13268).

This patch forces fallbacks to be considered in all cases, by ensuring the base position always
overflows and then providing a fallback that matches the "original" (non-overflowing) base position;
then the browser will choose between this and the flipped fallback according to the specified
position-try-order and writing modes involved.

Original Revision: https://phabricator.services.mozilla.com/D277557

Comment 14

[Emilio Cobos Álvarez [:emilio]]

Attachment: Bug 1989059 - Enable the position-try-order property.

Original Revision: https://phabricator.services.mozilla.com/D277558

Comment 15

[Pulsebot]

https://github.com/mozilla-firefox/firefox/commit/623115197d7f
https://hg.mozilla.org/releases/mozilla-release/rev/a8823e4f7fb0
https://github.com/mozilla-firefox/firefox/commit/21dd34699dcc
https://hg.mozilla.org/releases/mozilla-release/rev/c019c9cd39d1
https://github.com/mozilla-firefox/firefox/commit/db7310a1d12c
https://hg.mozilla.org/releases/mozilla-release/rev/40d1443932e4

Comment 16

[Donal Meehan [:dmeehan]]

Added to the final Fx148 release notes

Comment 17

[Dave Letorey]

Associate Pull Requests

• Firefox Release
• BCD