To have better control over which part of the RadAutoCompleteBox
should trigger your custom logic, you can handle the MouseLeftButtonDownEvent of the control in a similar fashion:
sender, MouseButtonEventArgs e)
var source = e.OriginalSource
// if (source.ParentOfType<RadWatermarkTextBox>() != null)
The above two examples will show a message box if the actual text box of the control or anything except an autocomplete box item is clicked. You're free, of course, to further modify this to suit your needs.
Please let me know whether such an approach would work for you.
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