The container control sends a WM_CONTROL (in Container Controls) message with the CN_SCROLL notification code to its owner whenever the container window scrolls.

param1
     USHORT         id             /*  Container control ID. */
     USHORT         CN_SCROLL      /*  Notification code. */

param2
     PNOTIFYSCROLL  pNotifyScroll  /*  Pointer to the NOTIFYSCROLL structure. */