Shadow and <dialog>

[annevk]

As far as I can tell most of this is relatively straightforward. Should alignment work across boundaries? @hayatoito @smaug---- @upsuper @rniwa?

[tabatkins]

Alignment is a feature of CSS layout model, which works on the flattened tree, and has no knowledge of shadows or boundaries or anything like that. So yes, it should work across boundaries unless we specifically define the behavior otherwise.

[domenic]

This got fixed at some point; in particular we define the inertness in terms of the full tree including shadow-including descendants these days.

#763 is somewhat related and broken, but we can track it there.