1779332 - Implement proposed behavior for `overflow:visible` on replaced elements

Status: ASSIGNED

(Core :: Layout: Images, Video, and HTML Frames, enhancement)

Description

[Daniel Holbert [:dholbert]]

Per CSSWG discussion in https://github.com/w3c/csswg-drafts/issues/7144#issuecomment-1090933632 , it looks like overflow is changing so that now replaced elements will all have overflow:clip by default, and they'll be allowed to over-draw if you manually specify overflow:visible on them.

There's one WPT test for this already:
https://wpt.fyi/results/css/css-overflow/overflow-img-border-radius.html
test: https://wpt.live/css/css-overflow/overflow-img-border-radius.html
ref: https://wpt.live/css/css-overflow/overflow-img-ref.html

This part of the Chromium team's "Shared Element Transitions" proposal, but also implementable and potentially useful on its own.

Comment 1

[Daniel Holbert [:dholbert]]

[CC'ing some folks who might be interested in following and/or working on this]

Comment 2

[Daniel Holbert [:dholbert]]

Note that this is potentially a breaking change, which TabAtkins called out in https://github.com/w3c/csswg-drafts/issues/7144#issuecomment-1099606286 ; but it appears they added some use counters that suggest it's not risky:
https://github.com/w3c/csswg-drafts/issues/7144#issuecomment-1163724792

Note that our bug 1661582 is related, as well (to define where clipping happens, when overflow:clip is applied). We may want to ship both of these changes at the same time (i.e. put them behind a common pref).

Comment 3

[Daniel Holbert [:dholbert]]

Also, Tab posted some new proposed UA stylesheet rules that would be required as part of this change, here:
https://github.com/whatwg/html/pull/8028
https://github.com/tabatkins/html/commit/16943c6e9231a4f5fab89c65ae7ddbe49f40f01b