Applications need to be able to disable top-level windows, so these windows don't receive input events, don't change z-order, etc. Some native platforms also provide visual effects for such windows, e.g. flash windows' titlebar, when a disabled window is clicked.

On Windows, corresponding native API is ::EnableWindow(HWND, BOOL). On Mac OS X, it can be simulated using -[NSWindow canBecomeKeyWindow:] and -[NSWindow canBecomeMainWindow:].

AWT/Swing has this API: java.awt.Window.setEnabled(boolean).