This is a feature of the RadTabControl. Both items look selected (in fact one is selected and the other one is in state MouseOver) because the UI thread is busy with the calculations and therefore no redrawing is performed until they are over. The solution in that case is to handle the event (e.Handeled = true) and start the time consuming operations in different thread. After calculations are finished you need to change the selected tab to the initially selected one. You can add an indication that some work is going on by using the BusyIndicator.
I'm attaching a sample project demonstrating how to implement the suggested solution. You can extend and improve it to fit your needs. Notice that I'm storing only one tab header, so if you quickly click several tabs while the calculation is taking place the item headers will be updated with wrong values.
All the best,
the Telerik team
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