@jeff, so would you say that behaviour with the display conduit and the groundplane is desired ? I think not. You may add this line to one of the above example scripts under PostDrawObjects to see the bounding box my conduit uses:
e.Display.DrawBox(self.BBox, Color.Red, 2)
Even if i inflate this bounding box by 10000 units, the mesh boxes are not drawn properly. The clipping applied by Rhino when the groundplane is ON is wrong. Below shows what happens if i turn it ON, red is my Bbox.
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c.