The Unloaded event fires when a visual object is removed from the VisualTree. This does not always happen with virtualization enabled since containers are reused and may not be removed from the VisualTree.
There is a 'cleanup' event that you can handle.
and the handler:
sender, Telerik.Windows.Controls.TreeView.CleanUpVirtualizedItemEventArgs e)
Do you mind telling us why you need the unloaded event?
All the best,
the Telerik team
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