FWIW in specs I write I tend to just use the first box for things like this. Of course for existing features what browsers do today is more the question.
It's not even immediately obvious what the "first" box is in some cases....
And also, the set of actual boxes is not specified in detail by the CSS spec and may differ between different browsers, esp. where bidi is concerned. So the "first" box could mean different things in different browsers, because nothing actually specifies it.
reassign to myself; apparently i am not the default assignee for this component
Fixed for scrollIntoView.
(More work needed to address this for the rest of the spec.)