Bug 307171 – Anchor positioning: abspos elements can anchor to viewport-fixedpos elements

• Status: RESOLVED
• Resolution: FIXED
• Component: Layout and Rendering

[David Shin]

Created attachment 478269 [details]
Minimum Reproducible Case

(Discovered from Firefox bug: https://bugzilla.mozilla.org/show_bug.cgi?id=2014997)

Given that:

1. The initial fixed containing block is the parent of the initial containing block in the containing block chain [1],
2. Absolute containing block can have at most the initial containing block as its absolute containing block,
3. And an acceptable anchor should be strictly laid out before the positioned element [2],

Strictly spec-wise, fixed elements (Using the initial fixed containing block) should not be a valid anchor for an absolute positioned element in the testcase.

Perhaps relatedly - the positioned element doesn't seem to be properly scroll (un?)compensated.

[1] https://drafts.csswg.org/css-position-3/#def-cb
[2] https://drafts.csswg.org/css-anchor-position-1/#acceptable-anchor-element

[Radar WebKit Bug Importer]

<rdar://problem/170323196>

[[e-mail]]

Pull request: https://github.com/WebKit/WebKit/pull/60480

[David Shin]

There's a proposed spec change to in fact get rid of the special fixed ICB, which would allow this behaviour: https://github.com/w3c/csswg-drafts/issues/13468

[fantasai]

Pull request: https://github.com/WebKit/WebKit/pull/66208

[EWS]

Committed 314706@main (fe7bcd9ee8c2): <https://commits.webkit.org/314706@main>

Reviewed commits have been landed. Closing PR #66208 and removing active labels.