Re: [csswg-drafts] [css-position-3] Containing block of dialog fixed position children (#8040)

If I get it right, https://fullscreen.spec.whatwg.org/#new-stacking-layer says

> It is rendered as an atomic unit as if it were a sibling of its [root](https://dom.spec.whatwg.org/#concept-tree-root).

which seems to imply the dialog is reparented in the box tree. Then the transformed div is not an ancestor of the positioned div in the box tree. And this is a requirement https://drafts.csswg.org/css-position/#fixed-cb

> The [containing block](https://www.w3.org/TR/css-display-3/#containing-block) is established by the **nearest ancestor box** that establishes an fixed positioning containing block

So the transformed div should not be the containing block.

Instead, I guess the containing block should be the viewport, since the dialog doesn't have a transform or something that makes it establish a fixed positioning containing block.

Alternatively, `<dialog>` could automagically establish a fixed positioning containing block if that's considered to be better behavior.

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Received on Monday, 7 November 2022 22:26:52 UTC