Thank you for your suggestion. It is indeed Classic view that I am after, and where (until yesterday) I had had no problems for years. Now I am limping along with whatever the alternative to Classic is, but I'd like to go back. However, without message Preview that is tedious, and that's what I'd like to fix. I have experimented with just about every Preference setting that looks like it could bear on the matter without success. At the end of the day, if I click on the dot on the bottom of the list window, I get the cursor that looks like it should allow me to drag the dividing bar up to open the Preview window (that is, a horizontal bar with an arrow up from it), but does nothing.
I did try powering off the Mac, rebooting, and holding down Shift while opening Mail. It did nothing that I could observe, and in any event the problem persists. Do you have any suggestions for a Plan B?
Much appreciate the help! This is quite frustrating.