Advice on troubleshooting function to "unscroll"

[Will Monroe]

Hello,

I'm studying Emacs Lisp so that I can become more proficient at 
customizing GNU Emacs for purposes, mainly using org-mode.  I'm studying 
alone using Bob Glickstein's *Writing GNU Emacs Extensions*.  So far, 
the book has been a terrific guide to basic concepts.  But I've run into 
a problem that I can't seem to troubleshoot any further and I'd 
appreciate advice on: 1) the best way to troubleshoot this problem 
(e.g., other forums, listservs, or IRC channels that might be more 
appropriate) and 2) troubleshooting the problem itself.  I apologize in 
advance if this query has failed to take into account any basics or if 
it would be better posed elsewhere.  I welcome your on the best way to 
proceed.

So, following Glickstein's advice in Chapter 3, I'm trying to create a 
function that will allow me to create an "unscroll" function that will 
allow me to return to a prior buffer position after (inadvertently) 
pressing C-v a few times.  The code to do this defines three new 
variables, defines advice for an extant Emacs function, scroll-up, and 
then defines a new function, unscroll.  The code is below.

#+BEGIN_SRC emacs-lisp
(defvar unscroll-point nil
  "Cursor position for next call to 'unscroll'.")

(defvar unscroll-window-start nil
  "Window start for next call to 'unscroll'.")

(defvar unscroll-hscroll nil
  "Hscroll start for next call to 'unscroll'.")

(defadvice scroll-up (before remember-for-unscroll
                             activate compile)
  "Remember where we started from, for 'unscroll'."
  (if (not (eq last-command 'scroll-up))
      (setq unscroll-point (point)
              unscroll-window-start (window-start)
              unscroll-hscroll (window-hscroll))))

(defun unscroll ()
  "Revert to 'unscroll-point' and 'unscroll-window-start'."
  (interactive)
  (goto-char unscroll-point)
  (set-window-start nil unscroll-window-start)
  (set-window-hscroll nil unscroll-hscroll))
#+END_SRC

I've loaded this code in Emacs under two different conditions:  1) by 
firing up a vanilla Emacs instance without any configuration file, and 
evaluating the code in scratch and 2) by saving the above code as the 
sole contents in a configuration file (~/.emacs.d/init.el) and then 
starting up Emacs.  I've done this on two different machines both 
running versions of Emacs 24.

The test was just opening a lengthy file, usually an info page or an 
existing org-mode file, pressing C-v a few times, and then using M-x 
unscroll.  In all cases, I found that M-x unscroll would return to the 
position just before the last C-v but not to the original position.  In 
other words, if I pressed C-v two times and then pressed M-x unscroll, 
in would only go back one C-v.  My intent, and that of the example in 
the book, is to return the point the position before any C-v key 
sequences were pressed.

I could move one but I suspect that the mistake I've made will come up 
again.  I'd really welcome your (gentle) feedback!

All the best,

Will