Bug 304551

Summary:

CSS anchor positioning with a fixed-positioned anchored element works incorrectly in the top layer

Product:

WebKit

Reporter:

Pavel Pomerantsev <pomerantsevp>

Component:

Layout and Rendering

Assignee:

Nobody <webkit-unassigned>

Status:

NEW

Severity:

Normal

CC:

bfulgham, fantasai.bugs, karlcow, kiet.ho, simon.fraser, webkit-bug-importer, zalan

Priority:

Keywords:

InRadar

Version:

WebKit Nightly Build

Hardware:

Mac (Apple Silicon)

OS:

macOS 26

Attachments:

Description	Flags
Chrome (correct) and Safari (incorrect) layouts side by side	none

Pavel Pomerantsev

Reported 2025-12-20 11:26:42 PST

Created attachment 477806 [details] Chrome (correct) and Safari (incorrect) layouts side by side An element is positioned incorrectly if all of the following are true: * It's anchor-positioned (has a `position-anchor` set to an existing element). * It has `position: fixed`. * Both the element and its anchor are in the top layer. See example with a modal dialog and a fullscreen element (both examples of top layer usage): https://codepen.io/pavelp/pen/ByzBKrE Compare layout in latest Chrome (appears to be correct) vs latest Safari TP (appears incorrect). Note: this may or may not be a duplicate of https://bugs.webkit.org/show_bug.cgi?id=289743. Top layer is mentioned there, but it looks like currently all the top-layer tests in https://wpt.fyi/results/css/css-anchor-position?label=experimental&label=master&aligned are passing in Safari TP, so this might be a case for which a web platform test doesn't exist yet.

Attachments
Chrome (correct) and Safari (incorrect) layouts side by side (410.18 KB, image/png) 2025-12-20 11:26 PST, Pavel Pomerantsev	no flags	Details
View All Add attachment proposed patch, testcase, etc.

Radar WebKit Bug Importer

Comment 1 2025-12-27 11:27:12 PST

<rdar://problem/167239379>

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