This function allows an application to request that a given window receive notifications every time that its visible region gets altered.

#define INCL_WINMESSAGEMGR /* Or use INCL_WIN, INCL_PM, */
#include <os2.h>

HWND    hwnd;      /*  The window handle. */
BOOL    fEnable;   /*  Enable flag. */
BOOL    fSuccess;  /*  Success indicator. */

fSuccess = WinSetVisibleRegionNotify(hwnd,
             fEnable);