Here's a puzzler. Adobe support suggested I roll back the graphics driver to an earlier release with Photoshop's OpenGL Drawing disabled and then to reinstall the current driver and try again. That didn't work.
As promised above I installed the 186.08 beta driver and guess what. That didn't fix the issue either.
As I prefer not to run beta software these days I decided to roll back to 185.85 and guess what? OpenGL Drawing in Photoshop CS4 is fixed.
This problem has to be laid entirely at NVIDIA's door in my estimation. What I didn't mention above, though Adobe's support team were aware, is that an OpenGL application I wrote myself a few years ago (Gravity 6
) had also needed Desktop Composition disabled in order to work correctly and the steps performed above have fixed that application as well. That would seem to point the finger very squarely at the NVIDIA installation routines for their driver(s).
It gets worse. When I next started the computer the OpenGL Drawing wasn't working again. This ain't funny! I'm going to try my luck with NVIDIA support now.