This function sets the window visibility state for subsequent drawing.

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

HWND    hwnd;     /*  Window handle. */
BOOL    fEnable;  /*  New visibility state. */
BOOL    rc;       /*  Visibility-changed indicator. */

rc = WinEnableWindowUpdate(hwnd, fEnable);