In its current version RadComboBox cannot autocomplete when the filtering is enabled. This is something that we will change in the near future, but unfortunately I cannot promise a specific date for this feature.
By the way, the CanAutocompleteSelectItems property affects the selection of the control, not the autocomplete, e.g. when the property is true and the autocomplete completes an item, the SelectedItem/Value properties are updated. If the property is false, the values are not updated. The default value (null) switches the behavior depending on the value of the IsEditable property.
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