Why has an extra title/header bar appeared in Mail?

I had an issue where Mail suddenly started acting weird and unresponsive, so I quit it and when I reopened it, it had essentially lost all the mails although the IMAP account was still there, so I had to wait for it to re-download everything etc. It was annoying, but it's largely fixed now.

However something that also happened during this time was that a duplicate title bar (the one that says "Inbox (x messages)" underneath the favourites bar and above the area where you read the emails (From/To/Subject headers etc. in classic layout).

Does anyone know how to get rid of this bar? The only thing I seem able to do with it is, drag it up/down and it will minimise the whole window just as Safari does when you are moving a tab to a separate window. Then I let go and it goes back to full size.

Thanks in advance,

Rob