Folder and application windows don't span both screens?

I just plugged a secondary monitor into my new iMac (21" Retina display, Mojave), and noticed a couple of changes in screen behavior, going from my old Mac Mini (El Capitan) to the new iMac.

First, about the menu bar: the old system has one only on the primary screen. The new one has a menu bar in whichever screen received the most recent mouse click, and a "ghost" menu bar in the other screen. Not a problem, but can anybody tell me more about that?

The other change is that I can apparently no longer have an application span both screens. Now, granted, the auxiliary screen is not "Retina" resolution (it's a ViewSonic VA912b, 1280x1024), but is that the only reason for the inability to have a window span both screens?