This behavior is by design. What you can do to make sure which of the states you are in is to check if the RemovedItems collection is empty or not (whether this collection has no items):
private void comboSelectionChanged(object sender, Telerik.Windows.Controls.SelectionChangedEventArgs e)
if (e.RemovedItems.Count != 0)
//this ensures a selection from the drop down is performed.
I hope that helps.
All the best,
the Telerik team
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