Hmmm, what’s required?

You subclass NSWindow and override - (BOOL)canBecomeKeyWindow to return YES.

Then you instantiate it, passing NSBorderlessWindowMask as the styleMask parameter of a call to NSWindow’s -initWithContentRect:styleMask:backing:defer:screen: initializer.

Then you probably want to pass something like CGShieldingWindowLevel() to NSWindow’s -setLevel: method before setting the window’s frame (CORRECTLY handling the case where -[NSScreen screens] returns an array of more than one item (i.e. you’re on a dual display box)) and putting it on-screen.

Oh, and don’t forget to capture some keystroke so people can get out of your fullscreen world!