I have tried with the example with 1 million items. The RadTreeView hangs when select 50 and deselect them after.
The TreeViewItem should display the type name as well as the item's Header, e.g: "Telerik.Windows.Controls.TreeViewItem Header:Header Content Items.Count:20"
The AddedItems in the SelectionChanged handler contains an empty object in some cases when the virtualization is enabled and the virtualization mode is set to Hierarchical.
The expanding of items does not work as expected in a data binding scenario when the CollapseAll()/ExpandAll() methods are used There is a scenario where the RadTreeView is data bound and its items IsExpanded property is bound to a property from the view model. Then the CollapseAll() and ExpandAll() methods are called. When you try to collapse the treeview items through the property from their view model (that is bound to IsExpanded) only the items in the view port are collapsed.
Setting the EnhanceTabNavigation to False workarounds the issue in some scenarios. telerik:TreeViewAdvancedSettings.EnhanceTabNavigation="False"
Silverlight only bug. TreeView is bound to observable collection of strings. Theme is changed. Then 10 times is done (remove + add) operation in the source collection. IndexOutOfRangeException is thrown in Set method of IndexTree.cs.
TreeView with virtualization in SL. After ExpandAll and BringPathIntoView (or GetItemBypath / SelectItemByPath), a LayoutCycle exception occurs.