How to reopen the Preview Pane in Mail
I use Apple Mail (Version 11.5 on macOS 10.13.6) in Classic View. The Preview Pane was always open at the bottom of the Mail window. But now the Preview pane has disappeared! Only the thin "Drag Bar" is visible at the bottom. See the following screenshot:
I ahve tried to drag up the Drag Bar (marked up in the screenshot with the red ellipse), but it just does do anything.
How can I make the Preview Pane reappear? It's pretty vital for my workflow!
iMac, macOS High Sierra (10.13.6)