The Preview pane of my Mail's Message Viewer window is stuck closed

Hi--

My Mail.app's Preview pane of Mail's Message Viewer window is stuck closed. 

I was using Zoom at the time. Oddly, my zoom-correspondent also got his Preview pane stuck closed at the same time. We're both still stuck. A third poster in this discussion group also got his Preview pane stuck closed, several years ago, and he was also using Zoom at the time. Coincidence, I suppose, but...

I'm using Mail Version 12.4 running under MacOS Mojave Version 10.14.6 on a 2015 MacBook Pro.

My zoom-correspondent was using a 2009 iMac 9,1 running El Capitan.

The third poster was using Sierra.

I have tried, without success...

Rebooting my machine several times

Double-clicking on the little dot at the bottom, dozens of times.

Dragging up from the bottom, also dozens of times.

(The drag icon (↥) appears, but the bar with the dot does not drag.)

(I can change the window height, but I cannot make the Preview pane appear.)

Changing Mail -> Preferences -> Viewing -> Use classic layout to OFF,

then dragging from the right, also a few dozen times

(The drag icon (↤) appears, but the bar does not drag.)

(There is no dot that appears on the right.)

(I can change the window width, but I cannot make the Preview pane appear.)

Quitting Mail, 

then moving the folder com.apple.mail from ~/Library/Containers/ to the desktop, 

then relaunching Mail.

(On launch, Mail created a new folder named com.apple.mail in that location;

on cursory inspection, the two folders appear to be the same, 

but Synchronize Pro X finds many differences.)

I would appreciate suggestions on what else to try.

Is there perhaps a .plist item somewhere that governs this Preview pane?

--Gil