First, thanks to all on this thread for the attention to detail and iterative refinement -- really makes a difference.

One additional observation:  When zooming using Ctl-mousewheel, MR paints the diagram at every intermediate zoom factor -- so if I zoom five clicks, it repaints five times, which is slow, and doesn't return responsiveness to the UI until it's done.  Intermediate zooms are not painted when rapidly operating the Diagram Nav slider.... so it seems like the issue is already licked if only it could be applied to the mousewheel.

Thanks,   Graham