IMO, Control B should always scroll and Control A never.
The two main reasons are:
- Many users do not have the complete understanding of what an
activewindow means. - Users expect a reaction from the window behind the cursor.
And this is getting more and more true with the significant use of smartphone/tablet operating systems in which the concept of plural windows disappears completely.
By the way, it is the case on OSX since Snow Leopard. (maybe even before)