I was unable to reproduce this issue. You can test my application where I have a RadTreeView bound to collection of ViewModels. In the ItemTemplate we have StackPanel holding TextBox, StackPanel and MaskedNumericInput. The TabIndex of the MaskedInput is successfully bound to an integer property in the ViewModel. Please note that in some cases the TabIndex may not work for you and you may need to use the KeyDown event handler of the RadTreeViewItem and implement your own navigation. Could you please elaborate more on your scenario if it is more complex from the one in the project ?
the Telerik team
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