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Thank you, Frank. Before I try to experiment with your suggestions, let me mention a couple of other things that I noticed that might be a clue to what's going on. I have Stellarium downloaded onto my sons laptop. I noticed today that when Stellarium is displayed on his laptop screen , it works as it should (i.e. as it did before I downloaded the latest version). When I drag the Stellarium window to a larger screen, then I have the problems mentioned in my initial post. Another quirk is that when I have the S  Cardinal Point located in the middle of the screen side-to-side, the azimuth line (180°) at S doesn't run straight up the screen as it does on the laptop screen, but rather up to the right and down to the right. Does this info give you any further insight into my problem? My son doesn't have anything of these issues when he uses his lapatop and larger screen for his work.