如何制作出现在全屏窗口顶部的Cocoa窗口？

I need to make a "full screen overlay window" in a program for OS X. Precisely, the window must at all times be on top of all other windows, including on top of the menu bar in non-maximized-window spaces, and on top of maximized windows in their own spaces, and on top during Exposé. It must not participate in "change space" animations; i.e., its position must be fixed relative to the physical screen at all times. (The window will be semi-transparent and will ignore mouse events, but I know how to do this already.)I need to make a "full screen overlay window" i