Many thanks; I did try the Hardware Accelerated Rendering, and also the Animations checked but the only difference was that with Animations on, the borders of the Rectangle Select are animated which IMHO is the most annoying thing imaginable, LOL. No effect on the checkerboard flash.
I love this laptop (circa 2010 Dell Vostro) for many reasons, and unless it spontaneously combusts into a pile of rubble someday, don't have the slightest wish or reason to replace it. The 2GB of RAM has always been more than enough for everything I do until I downloaded this new version of paint.net :-(
Guess it is time I start learning how to use Gimp, which I fiddled with about a year ago but never sat down to learn the ins and outs of. A real shame that I can't get back 3.5.11 which worked perfectly (which is why I never bothered to go further with Gimp) unless I go to one of those shady, probably-malware-infested Other Sites. Don't really want to do that.
My own fault, I guess, for not saving 3.5.11 to a CD back in the day, to guard against future "improvements", LOL But I didn't realize that unlike Mozilla, older versions wouldn't continue to be available here.