[Users] Messages are not displayed.

[Malachi Constant]

Out of nowhere, messages (in any folder) are no longer displayed when I
click on them.

I can see them by clicking on View --> Open in new window, but it is
annoying to have to go through this step.

I must have, in my clumsy way, inadvertently clicked on something that
triggered this new unwanted behaviour, but I have idea what.

I've looked at Configuration --> Preferences (and Preferences
for current account...) but can find nothing relevant.

Can anyone help me out?

sincerely,

Unk