A good window manager makes the window-in-window concept unnecessary. However if someone wanted to develop such an approach for all the poor users that are forced to use a not so advanced window manager, then it should probably be addressed at the toolkit level.
What Sven is saying, is that it is basically impossible to get Window in Window support working on linux.
Just a small precision - it is impossible to do WiW with gtk+. QT does it OK. And in fact, there is a project (I don't recall the name) which does just that - creates a window, and then embeds GIMP windows in it, using QT.
Cheers, Dave.
-- Dave Neary bolsh@xxxxxxxx