2050613 - position-area block-axis center not scroll-compensated in position-try fit check (copy-paste in GetScrollCompensatedSidesFor)

Status: RESOLVED FIXED

(Core :: Layout: Positioned, defect)

Description

[Sylvestre Ledru [:Sylvestre]]

GetScrollCompensatedSidesFor (layout/generic/AbsoluteContainingBlock.cpp) has a copy-paste error: the block-axis Center branch tests aPositionArea.first instead of aPositionArea.second (the two branches above it correctly test .second).

So when a position-area uses center on the block axis with a non-center inline keyword (e.g. left center), eTopBottom is never added to mScrollCompensatedSides. That field feeds FitsInContainingBlock (the position-try fit check): after the anchor scrolls, the element is measured against the full containing-block height instead of the center band, so an oversized anchored element is wrongly judged to fit and the position-try fallback that should apply is skipped.

Fix: test aPositionArea.second in the Center branch. Added a WPT regression test (css/css-anchor-position/anchor-position-area-center-scroll-fallback.html); verified on local builds that it fails before the fix and passes after, and matches Chrome's behavior.

Found by Coverity (CID 1678404).

Comment 1

[Sylvestre Ledru [:Sylvestre]]

Attachment: fx nightly

Comment 2

[Sylvestre Ledru [:Sylvestre]]

Attachment: chromium

Comment 3

[Sylvestre Ledru [:Sylvestre]]

Attachment: test case

Comment 4

[Sylvestre Ledru [:Sylvestre]]

Attachment: Bug 2050613 - Fix axis mixup in GetScrollCompensatedSidesFor (Coverity CID 1678404) r?#layout-reviewers

The vertical-axis Center branch tested aPositionArea.first (copy-pasted
from the horizontal block) instead of aPositionArea.second, so a vertical
center was never detected for anchor-positioned elements.
+add a test

Comment 5

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/f78bb5507e15
https://hg.mozilla.org/integration/autoland/rev/d6d3de961526
Fix axis mixup in GetScrollCompensatedSidesFor (Coverity CID 1678404) r=layout-reviewers,dshin

Comment 6

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

Created web-platform-tests PR https://github.com/web-platform-tests/wpt/pull/60935 for changes under testing/web-platform/tests

Comment 7

[Cosmin Sabou [:CosminS]]

https://hg.mozilla.org/mozilla-central/rev/d6d3de961526

Comment 8

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

Upstream PR merged by moz-wptsync-bot