Eh, right of course... crap.
I'm trying to figure a way to make this work while keeping the logic distributed in every process but to be honest this is the kind of thing that would be much more nicely solved with code in wineserver.
And I don't think delegating it to explorer, as a unique proxy for active window tracking, is strictly equivalent, especially when we will want to request host active window changes.
Would keeping a hwnd / host window mapping in wineserver be acceptable?