Well, I thought I had most of this shared pwindow stuff figured out - was even able to get Syphon & Spout output working perfectly - but when I tested my patch on Windows, I got some very disappointing results. It looks like there is a weird erase bug or something - the drawing looks correct, but it appears the "erase" part is not erasing entirely. Of course, this is just what the issue looks like visually, and actually not have anything to do with the actual issue...
Here are some screen shots of both platforms - OS X works beautifully:
Here is a link to album on imgur:
This is the only rendering issue I have ever faced on Windows.
Max 7.3.1 (OS X)
Max 7.3.1 (x64 Win 10)
Mac Pro - Dual AMD D700 - 16GB RAM