Somehow I had hit a keystroke combination which barely engaged a zoom feature -- just about 1% or so, barely off of zero. I didn't even know I had a zoom feature! I went to the System Preferences » System » Universal Access, and turned Zoom off (how did it get turned on?). This immediately brought my monitor back to the razor-sharp images I had been looking at for years. Whew! A close call, based on a very simple thing that many of us forget is even there.
[robg adds: The keyboard combo is Command-Option-Equals, and I think Zoom is enabled by default.]

