More newbie questions - IDE side by side code tabs

Messing around with the latest MC.NET IDE to see how I can customize it. I noticed that sometimes when I click and hold on the names on the top tabs it will allow me to rearrange the tabs into a panel view between two files.
On the other hand, sometimes it will not respond to the click and hold at all. What's happening with this? Is there a setting that I have to modify or is it a bug?

EDIT: I am chalking this up to confusion on my initial installation and setup. Perhaps the IDE needed to be restarted. Perhaps I am all thumbs. At any rate, this has not come up again, and now I cannot repeat the initial issue.