I used your snippet to create a project to investigate the reported layout and the result can be seen in the attached picture. As you have set the RadPanelBarItem.HorizontalContentAlignment to Center the text "Search" is centered. I cannot see any part of the RadPanelBarItem that is aligned to the right. This is why I would like to ask you to provide us with more information on your requirement. You can edit the image and send it over to help us understand.
On the other hand, if you are going to have only one RadPanelBarItem in your application you can consider using our RadExpander
control instead the RadPanelBar. You do not need an ItemsControl with only one item when you can achieve the same layout with a content control.
I hope this information is helpful.
Pavel R. Pavlov
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