This function is used by threads that need to draw on an area of the screen over which they have no control. For example, the user interface sizing and moving calls use this call when drawing the shadow box, as a window is sized or moved.

All threads continue to run while the window is disabled; only output is prevented.

If one thread disables the window, other threads using this function are blocked until the first enables the window, although they can still receive messages.

This function does not prevent screen group switches, because these may be necessary to handle "hard errors" in other screen groups.