OK:
1) Record a bar's worth of MIDI notes (e.g. 4 kick notes)
2) Extend the resultant clip to 2 bars and open it in the PRV
3) In the PRV, lasso all of the notes. Don't lasso over the clip boundary.
Does doing so select the entire clip in the clips pane? It does for me. And when I try to paste those notes into the empty part of the clip, it creates a whole new clip on top of the first one.
Now try selecting all of the notes but one. When I do that, the only part of the clip that appears selected in the clip pane, is the part that encompasses the notes I selected. It doesn't select the whole clip. And when I paste those notes into the empty part of the clip, it just pastes the notes without creating a new clip.
Hope that makes sense - I typed it on my phone :)