In order to achieve the desired behavior you will need to create a custom FilteringBehavior that returns the entire ItemsSource collection when the typed in SearchText is empty.
You can take a look at our OpenWithDropDownButton example in our online SDK repository here. The example is listed as AutoCompleteBox / OpenWithDropDownButton and demonstrates how to create a button that will populate the DropDown portion of the control with the entire ItemsSource collection when the text in the control is empty and a button is pressed.
Hope this is helpful.
Regards,
Vladi
Telerik
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