The Mail app:

In the "view" tab I deselected "Use Column Layout" to see what it looked like, I then reselected "Use Column Layout" - but now I cannot preview messages. Specifically, with the "Show Side Preview" unchecked the extra bar at the bottom of the page appears, with the central dot, and if I grab it the up-pointing arrow appears, but does not function, i.e. it will not pull up in order to change the size of the preview window. the same happens when I try side view. i have restarted my brand new iMac (Catalina) but it did not affect anything.