Happy New Year!
I debugged it and found two issues.
- You need to make sure that the selection actor is not selectable by doing SetPickable(0) on it. Otherwise, if you highlight an already selected area, it will select cells from both actors.
- You need to use vtkPolyDataMapper for the tube and not vtkDataSetMapper. For some reason that I did not investigate, vtkDataSetMapper transforms the original tube so what is being selected is different. If you switch to vtkPolyDataMapper, the tube is directly rendered and the selection matches.
Cheers.