[csswg-drafts] [css-view-transitions-1] Enforce ::view-transition to be fixed position (#8505)

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

== [css-view-transitions-1] Enforce ::view-transition to be fixed position ==
The `::view-transition` pseudo-element is positioned relative to the snapshot root outlined in the spec [here](https://drafts.csswg.org/css-view-transitions-1/#pseudo:~:text=Its%20containing%20block%20is%20the%20snapshot%20root.) irrespective of the value of its `position` property. This was resolved in #7859.

It is also meant to be the containing block for its child pseudo-elements, the `::view-transition-group`, clarified in the spec [here](https://drafts.csswg.org/css-view-transitions-1/#pseudo:~:text=This%20element%20provides%20a%20containing%20block%20for%20all%20%3A%3Aview%2Dtransition%2Dgroup()%20pseudo%2Delements.). This is done by UA CSS setting its `position` property to `fixed`.

It's unclear what should happen if developer CSS modifies the `position` value for this element. We don't have any developer use-case where that's needed. Also the transform value computed by the UA and set on `::view-transition-group` pseudo-elements is in the snapshot root's coordinate space. This relies on these elements being positioned relative to `::view-transition` which itself uses snapshot root as its containing block.

The proposal is to state that the `position` value for `::view-transition` always computes to `fixed`. I'd also be open to using `absolute` since its containing block is not the ICB and the desired behaviour is to make it a containing block for descendants. This is similar to what's done for top layer elements, spec'd [here](https://fullscreen.spec.whatwg.org/#new-stacking-layer:~:text=If%20its%20specified%20position%20property%20is%20not%20absolute%20or%20fixed%2C%20it%20computes%20to%20absolute.).

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/8505 using your GitHub account


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Received on Tuesday, 28 February 2023 13:21:00 UTC