Drop-down combo boxes and drop-down list boxes support an alternative keyboard interface called the extended user interface. By default, the F4 key opens or closes the list, and the DOWN ARROW changes the current selection. In a combo box with the extended user interface, however, the F4 key is disabled and pressing the DOWN ARROW key opens the drop-down list.
To select the user interface for a combo box, an application can send the CB_SETEXTENDEDUI message to the combo box. A TRUE value for the wParam parameter enables the extended user interface; a FALSE value sets the default user interface. To determine whether a combo box uses the extended user interface, an application can send the CB_GETEXTENDEDUI message to the combo box.