Hi JL82, that's a type of display artifact. It's not really feasible for the viewport display to show thing with perfect accuracy because it has a big focus on displaying things quickly rather than perfectly. Those goals are usually at odds with each other.

Please see here for some more information on this particular type of artifact: http://moi3d.com/forum/index.php?webtag=MOI&msg=4987.2

I do have some ideas on how to possibly eliminate that particular artifact on solids for some future work, the thing that remains to be seen is how much of a slowdown the additional processing will incur.

- Michael