There's a method called something like "constrainToScreen" in NSWindow. return NO from that (or return the rect without constraining it, or something) and it should work. I usually use NSBorderlessWindowMask NSWindows, and I've never even heard of the "constrainToScreen" method ;-) Could it be that using borderless windows liberates you from having to implement constrainToScreen? (Come to think of it, NSWindow's standard reply for constrainToScreen might even depend on the window's style mask...)
There's a method called something like "constrainToScreen" in NSWindow. return NO from that (or return the rect without constraining it, or something) and it should work.