Bug 300837 – [css-anchor-position-1] Enable scrollable overflow of scrollable containing blocks by default

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

[fantasai]

The default alignment safety for absolutely positioned boxes pulls them away from overflow the containing block edge. This is good in general, but when the containing block is a scroll container, it's safe to overflow into the scrollable overflow region.

Per CSSWG resolution, we should allow overflow into these regions rather than trying to strictly contain the abspos:
    https://github.com/w3c/csswg-drafts/issues/12106

[Radar WebKit Bug Importer]

<rdar://problem/162722820>

[Ehren]

Is this a duplicate of the following? https://bugs.webkit.org/show_bug.cgi?id=299950

If so, this has been fixed in Safari TP 230. If I've misunderstood your bug report, then please ignore this!

[fantasai]

That was a quick fix just for the initial containing block to handle the most common broken case, this is a more general bug for a proper fix for the general case. :)

[fantasai]

Pull request: https://github.com/WebKit/WebKit/pull/52970

[fantasai]

There's also two things interacting here:
- What's the size of the containing block? This is https://bugs.webkit.org/show_bug.cgi?id=291864 which also needs to be fixed.
- What happens when we overflow the containing block. That's this bug.

Both of them mitigate situations where the box can overlap the anchor, but they are slightly different in their effects... this one is the easier one to fix. :)

[EWS]

Committed 302259@main (c16b70c96c64): <https://commits.webkit.org/302259@main>

Reviewed commits have been landed. Closing PR #52970 and removing active labels.