This option is missing in Rhino 5. Can it come back please? I can't use Rhino without this!3D Mouse does not work in a parallel viewport in Rhinoceros 3D
Issue: The 3D Mouse does not roll or spin the model in a parallel viewport of Rhino.
Solution: In Rhino3D, go to menu "Options", select "View - Spaceball" and enable Rotate Plan Parallel Views.
I asked McNeel about this and they said to ask you.
Thanks!