This feature is called window ghosting. Basically what happens is if the window manager decides the window is hung, it will capture the contents of it's client area, and create a window on the screen with the class 'ghost', with the contents it was able to capture, and the window styles of the original window(in most cases, iirc). So the window you see on screen when the app is not responding is not actually the app's window, just a replacement that will hang around until the thread responsible for that window starts responding again. The menubar is gone because the fake window does not have a menu, just extra space for it which is painted white. A disabled menubar would probably cause support problems.