This function disables or enables output to a window and its descendants.

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

HWND    hwndDeskTop;     /*  Desktop handle of the screen containing the window to be locked. */
HWND    hwndLockUpdate;  /*  Handle of window in which output is to be prevented. */
BOOL    rc;              /*  Success indicator. */

rc = WinLockWindowUpdate(hwndDeskTop, hwndLockUpdate);