treecombobox AutoComplete property
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Applies to
acTreeComboBox component.  

property AutoComplete: Boolean;  

The AutoComplete property specifies whether the combo box automatically completes words that user typed by selecting the first item that begins with the currently typed string.  
When AutoComplete is True, the combo box responds to user keystrokes by searching the Items property array for the first item that matches the string entered so far. If it finds an item that begins with the prefix typed so far, the combo box selects that item, "completing" the user's typing. If the user continues to type, selection may move to another, later, item, as the currently typed prefix no longer matches the initial AutoComplete selection. If the user types a string that is not the prefix of a string in the combo box, the string is taken as a unique entry and none of the items in the list are selected.  
When AutoComplete is False, this feature is disabled. Strings that the user types are taken literally, with no attempt to match them to a string in the combo box. Setting AutoComplete to False is necessary if you want to allow the user to enter a value that is not in the Items list but that is a prefix on one of the items.  

See also
AutoCompleteDelay, AutoCompleteIgnoreCase, Items, ShowEditor and SelectedNode properties.