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

[Dani Moncayo <dmoncayo@gmail.com>]

> 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.

From an user point of view, the <backspace> key (from Info buffers) is
indeed a movement command.  In this case the movement was from the to
of one info node to the bottom of another one (the previous one).

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

That makes little sense to me, and is not what I interpret from the
documentation:

     The variable `scroll-margin' restricts how close point can come to
  the top or bottom of a window (even if aggressive scrolling specifies a
  fraction F that is larger than the window portion between the top and
  the bottom margins).  Its value is a number of screen lines; if point
  comes within that many lines of the top or bottom of the window, Emacs
  performs automatic scrolling.  By default, `scroll-margin' is 0.

As I see it, this variable guarantees the users to _always_ see some
context lines around point, which is an important feature to me.
Without this feature, I would be sometimes unsure about whether the
current line is the one I am looking for (because I have no context
lines below/above the current one).  That's the very reason I set this
variable in my init file, and it makes no sense to me to honor this
variable in some situations and not in others.

And BTW, one symptom of the abnormal location of the current line in
my recipe is this: just after the last step, if you minimize the Emacs
frame and restore it again, the current line is then centered in the
window.  What sense does that make?  The current line should not
change because of that, definitely.

-- 
Dani Moncayo