What do I need to do to remove the split pane sidebar on the right from the terminal window? Never used to have it and now I don't know how to get rid of it.
I found out what was triggering it. The split-pane sidebar shows up when you have a non-magic mouse plugged in. So my answer for you could be: use an Apple's Magic Mouse.
However I'll tell you how to get rid of it, this will change the scroll bar behavior to show up only when scrolling: