This call causes a notification message to be sent to the specified window every time its visible area is modified on the screen. In addition, the window is notified when an application issues a WinLockWindowUpdate call that affects it, so that any asynchronous drawing activity can be suspended.

Applications that blit to the screen memory directly can use the notification messages WM_VRNENABLED and WM_VRNDISABLED to determine when to suspend blitting and also to determine when the clipping areas for the visible region have been modified.

Use WinQueryVisibleRegion to obtain the current visible region when a window is notified that its visible region has changed.