This function hides or shows the tracking rectangle.

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

HWND    hwnd;   /*  Window handle. */
BOOL    fShow;  /*  Show indicator. */
BOOL    rc;     /*  Success indicator. */

rc = WinShowTrackRect(hwnd, fShow);