In all releases of the RadDropDownButton
for WPF, the control has always used the MS Popup
control in its template and this is why we aren't aware of a version where the DropDownContent will display behind another window.
However, as you said that it worked in a previous version, if you can send us a sample project where it works we'll gladly further investigate the case.
All the best,
the Telerik team
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