When the RadTabControl is placed in a RadBusyIndicator, one of the navigation repeat buttons is initially disabled.
As a workaround, you can invoke the ScrollToHorizontalOffset method of the ScrollViewer inside the RadTabControl in a Dispatcher in the Loaded event:
private void RadTabControl_Loaded(object sender, RoutedEventArgs e)
var tabControl = sender as RadTabControl;
var scrollViewer = tabControl.ChildrenOfType<ScrollViewer>().FirstOrDefault();
Dispatcher.BeginInvoke(new Action(() =>