Only just discovered Scrivener and experimenting with it.
In a new document, whenever I click on an index card in the Corkboard or an item in the outliner, the screen splits into two with the thing I’ve clicked a document at the top. Incredibly irritating.
This didn’t happen with the tutorial so must be about how I created the cards/outline topics. The icons of each topic looks like an index card, if this means anything.
I’ve obviously done something daft and obvious as I can see any other reference to this. But what?
Okay, at the bottom of the editor, there should be a set of four buttons on the left side. The right-most button looks like two arrows pointing in opposing directions. Is this button blue by any chance? If it is, click it once to turn it off (it should be gray now). If that was the problem, you should be all set.
It is a special mode for people that like their screen to split off and act like a three-pane viewer.
P.S. As long as you start breathing again as soon as you’ve demonstrated your chromatic versatility! It occurs to me that you may misinterpret my last message as some sort of malevolent murder by proxy - not at all the intention. But I ask you, is it fair to wind up a hard-working developer who hasn’t slept for weeks???
Had to go out for a while. Hadn’t realised I was about to spark genocide with my query.
However, thanks for the answer. That’s EXACTLY the problem. I think I’d been clicking it – like lots of other icons – wondering what exactly it did without cottoning onto its purpose.
Have a few other queriers but I’ll post them separately.
Joke? What’s that, then? My sense of humour button is definitely switched to OFF at the moment - update releases are always stressful, but it’s compounded by the end of a loooooong term.
It occurs to me that you may misinterpret my last message as some sort of malevolent murder by proxy - not at all the intention. But I ask you, is it fair to wind up a hard-working developer who hasn’t slept for weeks??? 
