Certain notifications and messages apply only to combo boxes containing drop-down lists. When a drop-down list is open or closed, the parent window of a combo box receives a notification in the form of a WM_COMMAND message. If the list is being opened, the high-order word of wParam is CBN_DROPDOWN. If the list is being closed, it is CBN_CLOSEUP.
An application can open the list of a drop-down combo box or drop-down list box by using the CB_SHOWDROPDOWN message. It can determine whether the list is open by using the CB_GETDROPPEDSTATE message and can determine the coordinates of a drop-down list by using the CB_GETDROPPEDCONTROLRECT message. An application can also increase the width of a drop-down list by using the CB_SETDROPPEDWIDTH message.