An application sends a CB_GETEXTENDEDUI message to determine whether a combo box has the default user interface or the extended user interface.
CB_GETEXTENDEDUI wParam = 0; // not used; must be zero lParam = 0; // not used; must be zero
This message has no parameters.
If the combo box has the extended user interface, the return value is TRUE; otherwise, it is FALSE.
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, the F4 key is disabled and pressing the DOWN ARROW key opens the drop-down list.