An application sends the CB_GETTOPINDEX message to retrieve the zero-based index of the first visible item in the list box portion of a combo box. Initially the item with index 0 is at the top of the list box, but if the list box contents have been scrolled, another item may be at the top.
CB_GETTOPINDEX wParam = 0; // not used, must be zero lParam = 0; // not used, must be zero
This message has no parameters.
If the message is successful, the return value is the index of the first visible item in the list box of the combo box.
If the message fails, the return value is CB_ERR.