[css-position-3] Should there be a special fixedpos ICB? #13468
Description
https://drafts.csswg.org/css-position-3/#fixed-positioning-containing-block
"The initial fixed containing block is the parent of the initial containing block in the containing block chain."
From:
https://bugzilla.mozilla.org/show_bug.cgi?id=2014997
https://issues.chromium.org/u/1/issues/482079659
Effectively Blink/WebKit don't have this special fixedpos ICB structure, instead just have a single ICB shared between fixpos & abspos elements.
IMO I suspect this language was in the spec so that we could explain how fixedpos element don't move upon scroll, e.g. "boxes do not move when the document is scrolled", and don't contribute to scrollable overflow.
However we are going to soon be expanding this capability to attach to the scrollport on all scrollers as part of:
#12607 (comment)
E.g. the "position-attachment" will explain how fixedpos attach to any scrollers (and we won't be adding 2 nested containing blocks for this purpose).
cc/ @dshin-moz
IMO We should remove this nested containing-block explanation, and instead explain the side-effects a different way.
(and more generally not being able to attach an abspos to a fixedpos at the ICB level seems like a mistake - e.g. the Firefox issue seems like a reasonable things for a web-developer to do).
Ian