Re: Popup when buffer file is changed on disk

[Richard Stallman]

    !   "When non-nil Auto-Revert Mode stops checking files on user input.

I don't understand that sentence.  Does it mean,

  "When nil, Auto-Revert Mode checks files on user input--otherwise, it doesn't?

If so, what does it mean to "check files on user input"?

    ! This prevents Auto Revert from interfering with your normal Emacs usage.

That is so general that it doesn't mean much.  What kind of
"interference" does it refer to?