I tried disabling hardware graphics acceleration first without effect. But then I tried changing from Pulse to Thatch for the design and the problem magically disappeared!

Thank you, John! As you say, the Pulse is very old, and I have used it for a long time. I really like it, but it looks like I will have to move on. I can always change the colors to get that rich blue that I so like, but Thatch may grow on me (not literally). Thanks, again for your help.