[vtkusers] vtkPropAssembly with vtkInteractorStyleTrackballActor behaviour

[Dean Inglis]

David D. and I are trying to move vtkAxesActor by
pulling the individual actors into a collection and then
adding to a vtkPropAssembly.  The idea was to have 
a vtkRenderWindowInteractor set up with vtkInteractorStyleTrackballActor
so that the whole assembly would respond to mouse interaction
(scale, translate etc.). However, with this style, the assembly does not 
get transformed.  Adding an isolated test actor to the renderer shows 
that the assembly does not respond during mouse interaction even
though the test actor does.  Other events like picking (key "p" with
mouse click) or switching from wireframe (key "w") to solid (key "s")
work for the test actor but not the assembly.  Can someone
explain why the behavior is different for a single actor vs an assembly?

Dean