I can't reproduce any issue on Windows 10 64-bit with an nVidia GTX 1070 - it looks like this, which AFAICT is correct: https://dl.dropboxusercontent.com/u/15217362/9patchrendering.png
It never changes as I scroll/zoom around.
A key thing missing from that screenshot is having another, large project loaded into the editor at the same time. I thought I made that pretty clear. Load multiple large projects, just to be sure.
http://i.imgur.com/AWnaaEL.png
Even the grid is missing (which is unusual for this bug).
2) it ought to reproduce in both the 32-bit and 64-bit versions identically if it's a logic problem with C2's code, since we compile the same source code for both builds.
So this seems like it could be some kind of 64-bit driver issue, but I have to admit, I'm not sure. These kinds of problems are notoriously borderline-impossible to solve. We've had a bunch of them in the past. At least this isn't a serious one - we've had "C2 crashes on startup on all Radeon cards of this model" in the past...
While I've never seen it myself, there are potential problems that can arise when compiling for 64-bit. The only thing I can think of though, is making an assumption that pointers are 4 bytes long. Perhaps a pointer is being stored in a variable that is only 4 bytes long in the 64-bit build? If I were you, I would take a close look at any pointers that may be stored on the same struct or class that contains any OpenGL transformation matrices.