Thanks. Sorry I have not had the time to make the call. I have the display scaled to one size bigger text than the default, in case that is part of the issue, and my macOS launch bar is at the bottom and in the mode where it is hidden until I put the cursor at the bottom of the screen.
I also, fairly regularly find, again in preview mode, that pictures are no longer displayed: the thumbnails are on the right and you can scroll through them, but the preview window and the info strip (the bit shown in the above image) are blank.
Is there a way to restore the preview window to its default settings? (I restart Photo Mechanic, which usually works at least for a while.)