bug#13055: 24.3.50; `scroll-margin' not always honored in Info buffers

[Eli Zaretskii]

> Date: Sun, 2 Dec 2012 17:39:09 +0100
> From: Dani Moncayo <dmoncayo@gmail.com>
> 
> >From Emacs -Q:
> 
> 1. Eval: (setq scroll-margin 1)
> 2. Eval: (info "(emacs) Intro")
> 3. Type: <backspace>
> 
> --> The current line is the last visible one, but that should never
> happen, since `scroll-margin' is set to 1.  The current line should be
> the penultimate visible one.

Not a bug.  scroll-margin is only in effect when window is _scrolled_
due to cursor motion commands.  In this case, <backspace> causes the
buffer to be emptied, and an entirely different text be inserted into
it.  There's no scrolling here, thus scroll-related variables are
never consulted in this case.

The Emacs User manual clearly starts the section that describes these
variables with this:

  Emacs performs "automatic scrolling" when point moves out of the
  visible portion of the text.

IOW, scroll-margin determines when automatic scrolling is triggered,
but not where point can be legitimately located in a window.