Chris: Thanks for the idea, and it may help others searching for a similar behavior; but as noted in the first sentence of my topic: feather is turned off. This is a bug, not beginner error. I've been using Photoshop since the mid-90s, and this behavior was new in CS6. I go back to my old iMac that still has CS3 sometimes, and it's a major relief in this regard! (And strangely, I find I don't miss much at all...)
Noel: Will do! I can take before/after screenshots (later when I have time) with both the marquee and Info panel in view.
Also, since the CC 2014 problem is intermittent (compared to CS6) I will continue to look for a pattern that makes it more reproducible. (It may also be that the retina display is what makes it intermittent: I don't have room on my non-retina Air for CC 2014 and only have CS6 on there. But I will try to hook up a non-retina display to the Mac Pro sometime and see if the CC2014 problem worsens. Normally I use a high-DPI Dell UP2414Q.)
Lastly, I'll try Safe Mode (not sure if I did that before) to totally eliminate the possibility of anything else messing with the system.
GPU-wise: it has happened on the years-old Air's Intel integrated graphics, and on the new Mac Pro's D700s. I do think it may well be Mac-specific, but I'm amazed if nobody notices: it's obvious when working with pixel-precise CSS web layouts. (You might not notice as it first happens, but you'll notice later when your numbers are wrong or the page has a missing row somewhere.)