The RadTreeView control doesn't support this functionality out-of-the-box. But you can edit the default ControlTemplate of the RadTreeView control and set a custom ControlTemplate for the ScrollViewer element.
In the ControlTemplate of the ScrollViewer you need to handle the MouseWheel event of the ScrollContentPresenter. I attached a sample project that demonstrates this approach. Let me know if it helps.
the Telerik team
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