Thanks for your detective work, GBW.
I've just learned to live with this stupid thing.
If I hit my lottery numbers, I'll buy a couple of Apple monitors to replace my Samsungs on the left and right to see if that has any effect.
All the best.
I know that this thread is considered not fixed, but it solved my problem. I just moved the menus (or related files for anchor links) to the left of the file from which I was linking. Someone posted that you can point left but not right, so I arranged elements so that pointing was only in that direction. I'm sure there are reasons why this won't work for everyone, but for me, it made my life much easier, so the problem isn't solved, but the workaround was successful.