John (Hunter), Paul (Kienzle), Alex (Mont),
I have fixed some of the problems with pcolormesh reported by George Nurser on May 4, but the remaining problems appear to involve Agg rendering with alpha != 1. To see the problem, run examples/quadmesh_demo.py, and resize the window a few times, or zoom on the masked region of the RH subplot. You will see that the redraw does not erase what was under the region that is masked in the redraw. The masked region is transparent by default. A bit of uncommenting and commenting in quadmesh_demo.py allow you to make the masked region any color and alpha; you will find that if alpha is 1.0, this problem does not occur.
Second question: Why does QuadMesh.draw() *not* call update_scalarmappable()? The draw method seems like the logical place to do this, as other collection classes do.