I have a bit of data on the window managers on my own (Red Hat) system. There are three window managers available: Gnome, KDE, and "failsafe".
"Failsafe" seems to be an extremely minimalist windowing system. It brings up windows without any decorations, and when I ask for something at position (20,20) I get the drawable area at (20,20). This is what "glutGet" returns as well. With no decorations, there really isn't any alternative.
KDE and Gnome both behave in a similar fashion. When I ask for a window at position (20,20), I get a window whose outside decorations start at (20,20) and whose drawable area starts at (26,40). When I call "glutGet" to ask for the window position I get (26,40).
KDE and Gnome differ in the size of the initial window. I asked for a window of 30x30 pixels. Gnome gave me what I asked for; KDE gave me a window 78 pixels wide and 30 pixels high. This was the case for three different windows, with three different titles. I find that the width for KDE is 78 pixels if I vary the height and desired width as well. Gnome gave me the desired size every time.
I have a fourth entry in the Window Manager list called "Default." It looks and behaves just like KDE, so it probably is.
What happens with other people's systems?
John F. Fay
"It is a poverty to decide that an unborn child must die in order that you may live as you like." - Mother Teresa