Window manager (xfwm4)
The Xfce 4 window manager. It manages the placement of application windows on the screen, provides window decorations and manages workspaces or virtual desktops. Xfwm4 natively supports multihead mode. It provides its own compositing manager (a manager for the Xorg Composite extension, which brings true transparency to the Xfce desktop). The themes support image composition with PNG images as layers, which means that one can do gradients or anything else, and that applies to any pixmap that make the window frame. Xfwm4 includes a keyboard shorcuts editor. Moreover, users can shade windows and switch desktops using the mouse wheel.
I've seen other people asking about the same thing as you, in 2004, but haven't seen a solution posted.
Luck!
Cheers,
Scott.