[css-view-transitions-2] [scoped] What is the paint order of the ::v-t pseudo in relation to the scope?

[skobes-chromium]

This issue tracks an open question about the behavior of Scoped View Transitions. Further context appears in the document Self-Participating Scopes.

Q: What is the paint order of the ::v-t pseudo in relation to the scope?

A child of the scope would normally be stacked with other children according to its z-index, but the ::view-transition pseudo-element should probably be handled specially so that it sits on top of everything in the scope.

This is not what Chromium currently does (crbug.com/421927605).

(Because participants have their render output redirected, this question only matters for children that are not participants and whose z-index is larger than that of the ::view-transition.)

cc @noamr @vmpstr @flackr @bramus @jakearchibald