First: You can remove horizontal scrollbar when you set the property
ScrollViewer.HorizontalScrollBarVisibility="Hidden" of RadTreeViewControl.
Second: If you don't need virtualization, you can change the ItemsPanel property of RadTreeViewItem.
Please find attached a project that demonstrates how to change the panel.
If you have further questions please do not hesitate to ask us.
I hope this will help you.
All the best,
the Telerik team
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