1962598 - Implement custom `@position-try` fallback support for position-try-fallbacks

Status: RESOLVED FIXED

(Core :: CSS Parsing and Computation, enhancement)

Description

[Jari Jalkanen [:jari]]

Position-try fallbacks accept a dashed identifier referring to a position-try option containing position property alternatives. https://drafts.csswg.org/css-anchor-position-1/#position-try-fallbacks The feature is not exclusive to anchor positioned elements.

For a basic implementation, the supported position-try properties may include inset properties, margin properties, sizing properties and self-alignment properties, and not include position-anchor and position-area properties, which require anchor positioning.

Position-try order and try-tactics features are excluded from the scope of this bug. Invalidation (e.g. when fallbacks get scrolled out of view) is also out of scope.

After this bug is complete, the test position-try-fallbacks-limit.html
should pass.

Comment 1

[Jonathan Watt [:jwatt]]

Jari is going to be working on bug 1924210 / bug 1928128.

Comment 2

[David Shin[:dshin]]

Breakdown, drawing a parallel with @keyframe:

• Save @position-try data in CacadeData: 1, 2
• Implement lookup for @position-try declarations by name in Stylist: 1
• Implement declaration -> computed value conversion 1
• Expose to layout: 1
• Trigger reflow using overridden values (Something smiliar to this)
• Ensure that getComptuedStyle reflects the used value based on the selected fallback.

Comment 3

[Emilio Cobos Álvarez [:emilio]]

Attachment: Bug 1962598 - Implement full @position-try fallback for anchor positioning. r=layout-anchor-positioning-reviewers,layout-reviewers,dshin

Remaining failures are due to position-try-order, afaict.

Comment 4

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/9903a171b5ef
https://hg.mozilla.org/integration/autoland/rev/dd81e48aa0f0
Implement full @position-try fallback for anchor positioning. r=layout-anchor-positioning-reviewers,layout-reviewers,dshin
https://github.com/mozilla-firefox/firefox/commit/13f52674383b
https://hg.mozilla.org/integration/autoland/rev/a6531711d24c
1992124: apply code formatting via Lando

Comment 5

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/c7e03c999971
https://hg.mozilla.org/integration/autoland/rev/f3a916c5fec7
Add some missing expectations that I forgot to git add.

Comment 6

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/bc30450405ea
https://hg.mozilla.org/integration/autoland/rev/2bfe875c5d66
Revert (Bug 1962598, Bug 1992124): for causing wpt failures.

Comment 7

[agoloman]

Backed out for causing wpt failures.

• Backout link
• Push with failures
• Failure Log

Comment 8

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/f9637192c040
https://hg.mozilla.org/integration/autoland/rev/f08b02209552
Implement full @position-try fallback for anchor positioning. r=layout-anchor-positioning-reviewers,layout-reviewers,dshin
https://github.com/mozilla-firefox/firefox/commit/f425f91079b9
https://hg.mozilla.org/integration/autoland/rev/9332758579d0
1992124: apply code formatting via Lando

Comment 9

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/37d30808aee2
https://hg.mozilla.org/integration/autoland/rev/4e5b5627e3ee
Undo fuzzy expectation change.

Comment 10

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/b383d13135c0
https://hg.mozilla.org/integration/autoland/rev/dad455aa07a4
Annotate some tests on android.

Comment 11

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

Created web-platform-tests PR https://github.com/web-platform-tests/wpt/pull/55227 for changes under testing/web-platform/tests

Comment 12

[Serban Stanca [:SerbanS]]

https://hg.mozilla.org/mozilla-central/rev/f08b02209552
https://hg.mozilla.org/mozilla-central/rev/9332758579d0
https://hg.mozilla.org/mozilla-central/rev/4e5b5627e3ee
https://hg.mozilla.org/mozilla-central/rev/dad455aa07a4

Comment 13

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

Upstream PR merged by moz-wptsync-bot

Comment 14

[Chris Mills [:cmills]]

Hi all. I'm currently documenting this on MDN. I'm testing it with the examples available at:

• https://developer.mozilla.org/en-US/docs/Web/CSS/@position-try
• https://developer.mozilla.org/en-US/docs/Web/CSS/position-try-fallbacks
• https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_anchor_positioning/Try_options_hiding

The fallbacks stuff seems really quite broken in Firefox (I'm using Nightly). Is there a pref I need to turn on for this?

They work better in Chrome, but the behavior is still a bit strange. I'm wondering if the behavior has changed since I first documented these? Maybe I need to rewrite the examples so that they work better in general.

Comment 15

[Chris Mills [:cmills]]

See https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/145#experimental_web_features.