Re: [csswg-drafts] [css-align][css-anchor-position] Default safety and fixpos (#10860)

The CSS Working Group just discussed `[css-align][css-anchor-position] Default safety and fixpos`, and agreed to the following:

* `RESOLVED: change the way medium-safety alignment is handled to instead invoke the scroll adjustment machinery when handling an anchorpos fixpos element`

<details><summary>The full IRC log of that discussion</summary>
&lt;noamr> TabAtkins: a corner case in anchor positioning, common in certain situations. original containing block, plus "safety". If something overflows the containing block it can still overflow until its original containing block<br>
&lt;noamr> TabAtkins: the problem is that for fixed positining you'd get some bad behavior, because the original containing block is the viewport. if you scroll a bit you'd get bad overflowing behavior<br>
&lt;noamr> TabAtkins: proposing to always to default a fixed pos element to unsafe element, rather than default to the medium safety<br>
&lt;noamr> TabAtkins: this shouldn't have real effects to users of the feature, the element should remain on screen when possible, but it's a bit of a tweak<br>
&lt;noamr> astearns: is this modifiable by the author?<br>
&lt;noamr> TabAtkins: this is just the default behavior if you don't say safe/unsafe on one of the alignment keywords<br>
&lt;noamr> TabAtkins: we shouldn't have the default do the wrong thing for common scenarios. We just propose to change the default safety<br>
&lt;noamr> fantasai: re-adjust it to be as if it was safe?<br>
&lt;noamr> TabAtkins: the medium safety is correct but can't be implemented. current definition of medium safety rely on layout information but not on scroll information<br>
&lt;noamr> TabAtkins: anchor position can respond to scrolling. in order to do the medium safety, it can't work in the way defined in the space. We need to define it as unsafe in the layout and then correct it using the scroll adjustment behavior<br>
&lt;noamr> TabAtkins: rather than layout time behavior creating the medium safety, it needs to be created by scroll adjustment, in order to be compositable<br>
&lt;fantasai> https://github.com/w3c/csswg-drafts/issues/10999#issuecomment-2430366312<br>
&lt;fantasai> This never got resolved either<br>
&lt;noamr> fantasai: I am glad that we're not changing the user observable behavior; no opinion about how we get there<br>
&lt;TabAtkins> proposed: change the way medium-safety alignment is handled to instead invoke the scroll adjustment machinery when handling an anchorpos fixpos element<br>
&lt;TabAtkins> (note to the above IRC-only comment; the resolution was in the preceding comment, the edits were for that)<br>
&lt;TabAtkins> RESOLVED: change the way medium-safety alignment is handled to instead invoke the scroll adjustment machinery when handling an anchorpos fixpos element<br>
</details>


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Received on Wednesday, 12 March 2025 16:18:31 UTC