As the root element of the Backstage ControlTemplate is a ScrollViewer, the content of the backstage items is measured with infinity. And if your scenario requires you to use the ListBox ScrollViewers instead of the backstage menu scroll viewer, you can create a custom ControlTemplate for the RibbonBackstage and remove the ScrollViewer element. I attached a sample project demonstrating this approach. Please give it a try and let me know if it helps.
All the best,
Tina Stancheva
the Telerik team
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