Aha! Folks on the Plant ID forum agreed it was some type of Campanula, and one said Campanula Elizabeth. As soon as I read "Campanula," I knew it had come from my daughter Stacy (sstateham on DG), I remembered her talking about them, and she has given me quite a few plants, mostly perennials. I called her and it's Campanula Elizabeth. It's in almost total shade with about 2 hours of morning sun, and seems to be reseeding itself there among the hardy begonias.
Now I know! I think I'll try to start some of it in another shady area on the north side and see how it does. Thanks!