I use iTerm2 as a terminal replacement on the Mac. It has some nice features that save time, especially when working on multiple servers remotely. One of the problems I would often encounter with the program (and be too lazy to fix) is when I somehow would accidentally "split" a window into two panes on the same tab. My solution would be to just close the tab and reopen a single pane.
Finally, I had enough and searched for the keyboard/mouse combination that would set my pane free. The answer was simple enough. However, I ran into a small issue that caused me a few minutes of consternation. I had followed the instructions, but attempted to drag the pane using the pane's bottom bar. As soon as I figured out that the mouse pointer belonged in the pane's window and not the bottom bar of the pane, my problem was solved.
Hold <cmd><shift><opt> on the keyboard and drag the pane's window to the terminal's tab bar (or away from the terminal altogether to create a new window).