MacMail Viewing Panel Hidden with Divider Bar Lockup

I'm using Mac.mail 11.5 (3445.9.1) with High Sierra 10.13.6 on my Mac mini.

In Mail -> Preferences -> Viewing -> we are using classic layout with show To/Cc label and show contact photo's.

For these preference settings we normally have a mail box list on the left hand side, a list of emails on the top right corner and view area of the selected email(s) on the bottom right corner when we have selected to show the mail box list.

I have moved the divider down to the bottom of the view area so that it is now touching the bottom of the mail.app window. Here it seems to have become stuck to the bottom of the window and there is nothing I can do to re-instate the view area.

When you switch to Mail -> Preferences -> Viewing -> non classic layout the divider is stuck to the right hand side of the mail.app window. Once again, I can't do anything to re-instate the view area.

I've fiddled with the preferences and options extensively and can't do anything to get the viewing plane back.

Does anyone know what configuration file might have been corrupted in order to create this symptom? And, how to go about fixing it?

If Ihave to re-install the whole Mail.app, how would I go about doing this? (Yes, I am using time machine)

Is it possible that we have a mail item in one of my mail boxes that is stopping the viewing area from functioning?

If I remember correctly, I was search bar around about the time this situation began to occur.