[csswg-drafts] [css-anchor-position-1] Compensating for scroll & "as if affected by a transform" (#12877)

dshin-moz has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-anchor-position-1] Compensating for scroll & "as if affected by a transform" ==
[Spec says](https://drafts.csswg.org/css-anchor-position-1/#compensate-for-scroll):

> After layout has been performed for abspos, it is additionally shifted by the [default scroll shift](https://drafts.csswg.org/css-anchor-position-1/#default-scroll-shift), as if affected by a [transform](https://streams.spec.whatwg.org/#generictransformstream-transform) (before any other transforms).

What's the extent of "as if affected by a transform?"

**Does it establish a containing block for all descendants?**

e.g. If a scroll compensated positioned element has a fixedpos frame inside it, does it get trapped inside the positioned element? 
Both Blink and WebKit does not seem to do this - The fixedpos frame's containing block seems to be the viewport.  [asfia.html](https://github.com/user-attachments/files/22604710/asfia.html)
 
**Does it have the same overflow behaviour?**

e.g. Does the ink overflow caused by the positioned element affect its containing block's overflow?  
If the abspos containing block has `overflow: scroll`, and a positioned element compensating for scroll overflows it, but then the anchor's scroller moves so that the positioned element no longer overflows the abspos cb, does its scrollbar shrink?

Both Blink and WebKit seems to keep the overflow constant [positioned-overflows-abscb.html](https://github.com/user-attachments/files/22604807/positioned-overflows-abscb.html)
OTOH, when this is emulated with JS + `transform: translateY()`, the overflow changes [positioned-scroll-emulate-transform.html](https://github.com/user-attachments/files/22604816/positioned-scroll-emulate-transform.html)

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/12877 using your GitHub account


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Received on Monday, 29 September 2025 19:25:35 UTC