Stephane,

Just briefly:

> Does a graphics or window guru can assess if these changes 
> would be worth to be integrated?

I share Jim's opinion that a bit more radical approach would actually
have been useful. The changes add quite a bit of complexity to
SystemWindow in such that they duplicate various responsibilities and
(at least by my glancing over it) it looks like those shouldn't even be
present in SystemWindow at all if a window frame handles them.

So, Jim, are you up to a little more agressive refactoring of those?!
Basically, taking out all the stuff that should be handled by the window
frame into the window frame and only leave a few high-level protocols in
SystemWindow itself?!

Cheers,
  - Andreas