CB_GETDROPPEDSTATE
wParam = 0; /* not used, must be zero */
lParam = 0; /* not used, must be zero */
An application sends a CB_GETDROPPEDSTATE message to determine whether the list box of a combo box is visible (dropped down).
This message has no parameters.
The return value is TRUE if the list box is visible; otherwise, it is FALSE.
CB_SHOWDROPDOWN