The ComboBox widget, introduced in Motif 2.0, combines direct TextField editing with the convenience of a popup List. There are three kinds of ComboBox: a normal ComboBox, a Drop-Down ComboBox, and a Drop-Down List.
The normal ComboBox presents itself as a TextField with a permanently visible List directly underneath. The user can either type directly into the TextField, or select a List item.
A Drop-Down ComboBox displays a TextField, but this time does not place a permanently visible List directly underneath. Instead, it adds an ArrowButton which can be used to display the List of items on demand.
A Drop-Down List looks identical to the Drop-Down ComboBox, except that the user is not allowed to directly edit the TextField: she must specify a new value for the widget by explicitly selecting an item from the popup List.
Note that the default Motif ComboBox type is the ComboBox with the permanently visible List, rather than a popup List. This is considered to be a mistake, since it is not the usual requirement. X-Designer therefore overrides the toolkit behavior, and presents the Drop-Down ComboBox as the default.
For more details of the ComboBox widget, see XmComboBox(3X) in the OSF/Motif Programmer's Reference.
See also: