This won't be easy to achieve since the RadBusyIndicator will disable the RadTreeView. You need first to ensure that the BusyIndicator and the RadTreeView are in different containers (or different Grid`s RowDefinitions or ColumnDefinitions). Then you have to determine the moment when al the RadTreeViewitems have been Expanded . You can use counter in the ViewModel or something similar.
Please let us know if you need further assistance on this.
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