[css-view-transitions] Rendering consolidation section should special-case root element

[emilio]

https://drafts.csswg.org/css-view-transitions-1/#named-and-transitioning has:

Elements captured in a view transition during a view transition or whose view-transition-name computed value is not none (at any time):

• Form a stacking context.
• Are flattened in 3D transforms.
• Form a backdrop root.

Interpreted literally, it means that the root element box would become a stacking context always (via :root { view-transition-name: root } in the UA sheet). That seems unintended and doesn't match implementations.

My understanding is that the root element is "special" and gets propagated to the whole snapshot-containing-block rect / doesn't need its own stacking context.

The spec should reflect that, unless I'm missing something?

cc @chrishtr @noamr @nt1m @vmpstr