Vincent,
My apologies for not replying sooner. I was on multiple contracts and very short on time. I can confirm that this an issue, though it is not a Gorm issue (i.e. not in the Gorm application) but a GNUstep issue.
This issue is somewhat complicated, explanation follows:
The content view in a window is of a specific size and this can't really change. The issue is that the Window, even though it is specified to be of a particular size on Mac OS X sometimes needs to be adjusted to accommodate the view or vice-verse. Also, on Linux, when GNUstep is not managing it's own windows it relies on the window manager to do this and, in spite of FreeDesktop conventions, some window managers report information about the window border dimensions inconsistently. In those cases we are forced to take our "best guess" to get the best result we can.
It's important to realize that this inconsistent information coming back from the window manager is NOT an GNUstep issue. There is no way short of some pretty ugly hacks to determine this information. The fact of the matter is that many window manager just don't accurately report their information and that is their problem, not ours.
I can't promise that this will be fixed given the factors above, but I can say we will continue to improve it.
I will take a look at your patch and see if it makes things better under Windows.
Thank you. :)