Aplle Mail 9.3 > messageviewer window suddenly gone

In Mail 9.3 you have a large windowbox where all received messages are in.

When you click on one message it opens automatically in another window beneath it.

But this window, where you can read one message only, is gone.

Which is very annoying of course.

Now I can only double-click on a message > then a new window opens.

And so on... I have to open a new window for every message.

And close them again by hand. Very timeconsuming.

When the automatic messageviewer was working, this was not necessary.

You just clicked one message and you immediatly see it.

Clicking on another message, and you see that message.

Without the need to click the former message away.

Who knows the cause of this problem AND hopefully a solution?

Untill now I tried all possible menu's and preferences in Mail, but found nothing.

I don't even know if this (now magic) 'messagewindow' has a special name.