I don't know how you figured this out, but you did! I have frequently toggled back and forth from full screen to a window when playing CW3. I don't know if I'd just done that when everything went wrong or not. During this period of problems, the blue and white screen was in full screen mode. If I went to a window, I was getting just the top Mac bar and some flickering. I guess maybe there was a microscopic piece of the game showing beneath the top bar. But if I clicked back to full screen I got the blue and white again. I just now tried clicking and dragging the screen to make the window larger, and like magic everything is now appears to be fixed. And I can go back and forth between full screen and window. Thank you!
Just one more thing though. I found the game I was playing when things messed up. It was map #7035, Easter Island by stdout. I just tried loading that map and got a message that it was created with a more recent version of the game and I should update. A couple of other games say the same thing. Will that happen automatically through Steam? I've always kept up with the updates but I'm not seeing any update messages now. I did click on a very short game and was able to play it just fine.
Thanks so much to you, Karsten, and Builder for all your follow through and patience!