In nested RadDocking scenarios if the inner RadPane's content contains a ScrollViewer pressing "Tab" or "Tab + Shift" (keyboard navigation) the focus is moved out of the RadPane's content into the parts of the RadDocking.
Fixed in Q1 2016 release.
Workaround: Set the KeyboardNavigation.TabNavigation of the Content of the ScrollViewer to Local or Cycle. For example if the ScrollViewer has a Grid inside it set the TabNavigation property to that Grid.