Nor is it this:
Mavericks, though. So far...everything works! I'd heard of some minor audio issues so I started by testing Audacity, QLab, and Vox. All worked normally. Tried Scrivener. Tried Steam, Wine, and Steam Wine, and played a little Dead Space (a PC game). Then played Tomb Raider 2013 on the Mac side -- and it looked nicer than it had on the PC emulator (better color balance, basically).
And Fusion 360. Which was the main reason. Autodesk had pushed through a basically mandatory upgrade to Fusion, and the upgrade wouldn't run on my previous OS. And since the upgrade either stalled or required me to quit, the only recourse I had to finish the Raygun project was to take the computer off line every time I needed Fusion. But with it off line, it could not import, nor could it open the "cloud stored" (really, I have no idea where they are stored) other projects I wanted to work on. Like this:
I stil have yet to test Eagle, Cura, Arduino, Reaper...