A few weeks back I asked about a problem that was being caused by my mouse on the screen:

https://talk.newagtalk.com/forums/thread-view.asp?tid=780768&posts=5&highlight=mouse&highlightmode=1#M6751053

It seems the most popular suggestion was that it was probably my old mouse (Logitech wireless) was giving up. I ordered a new Logitech. Luckily the new one is 99% similar to the old one, but maybe just a "bit" smaller. Not much but noticable. Plug & play is so much easier than it was 20 yrs ago. New mouse solved all the screen & photo editing problems I was having.

Thanks everyone.