[csswg-drafts] [css-align] Default safety for abspos/fixpos (#10859)

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

== [css-align] Default safety for abspos/fixpos ==
@fantasai and I recently committed a new [default safety behavior](https://drafts.csswg.org/css-align/#auto-safety-position) for abspos elements, largely to make elements using Anchor Positioning and `position-area` work better. In short, the element is first aligned within its IMCB as normal, but if that would overflow, it shifts to stay within its "original containing block" (before reduction from `position-area`, `inset`, etc) until it would overflow *that* too, at which point it aligns unsafely within the larger block. (Technically, it uses the bounding box of its IMCB and its original CB; this makes it act better when it's being positioned well outside of the original CB.)

@bfgeek has been experimenting with implementing this, and has ran into a few situations where we think the definitions need tweaking.

This behavior currently applies in situations like, say, `position-area: right`. If the anchor is too close to the right edge of the screen, so that the `right` area is too narrow, the element overflows *over* the anchor. That's probably not what the author intended; we believe in this situation it's better to stick to unsafe alignment and just overflow off to the right.

In terms of edits, I think the relevant text is that *if* you are using `position-area`, you only do this medium-safety alignment in an axis if your area covers the center track in that axis. Otherwise, the default alignment from `position-area` should be unsafe in that axis.

That is, `position-area: bottom span-right` would allow the element to shift leftwards to avoid overflow (as the area is already covering the horizontal center track), but not shift upwards to avoid overflow (as the area does not cover the vertical-center track).

This implies that `bottom right` would be unsafe in both axises, even tho it could in theory shift up *or* left and avoid overlapping the anchor. Perhaps in this case we should default it to unsafe in *one* axis and medium-safety in the other? I don't know if it's possible to really predict which is better for a given situation, so maybe just let it be medium-safety in the inline axis and unsafe in the block axis, to reduce the chances of popping a horizontal scrollbar.

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/10859 using your GitHub account


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Received on Monday, 9 September 2024 22:24:20 UTC