Comment 6 for bug 250358

I’m sorry that you had problems with that command. The problem is presumably
that nvidia-xconfig wrote the previous version of the file. Perhaps there is
a bug in nvidia-xconfig that means it doesn’t write out the Emulate3Buttons
option. If so, then this might be considered to be a bug in that package.

Middle-clicking will often bypass the clipboard. Ubuntu (the X Window System,
to be more precise) has a “primary selection” as well as a clipboard. The
primary selection is filled whenever you select some text; the clipboard is
filled only when you choose copy. This means that you can use the primary
selection without affecting the clipboard, which can be useful. (If you
select some text and then copy it, you can either middle-click or paste!)