Use "user attention" of top-level traversable

See: 
https://github.com/whatwg/html/commit/2db564c6902ace8c2539560e508ce22c2948c4d8

Handles a bunch of stuff we do in the spec "for free"™️. 

The definition to ref is [[=Document/fully active descendant of a top-level traversible with user attention=]](https://html.spec.whatwg.org/multipage/interaction.html#fully-active-descendant-of-a-top-level-traversible-with-user-attention)