bug#17585: 24.4.50; vertical-motion erroneously adds points

[Keith David Bershatsky]

The following test function demonstrates how vertical-motion erroneously adds 
points when dealing with long lines without spaces that go beyond the 
window-width:

    (defun test ()
    (interactive)
      (switch-to-buffer (get-buffer-create "foo.txt"))
      (text-mode)
      (linum-mode t)
      (visual-line-mode t)
      (dotimes (i (* (window-width) 4))
        (insert ";"))
     (dotimes (i 10)
        (insert "\n"))
      (goto-char (point-min))
      (vertical-motion 1)
      (message "Second Row + 2 points:  %s" (current-column))
      (vertical-motion 1)
      (message "Third Row + 4 points:  %s" (current-column)))

Additional incorrect behavior can be seen by moving the cursor to the beginning 
of the buffer after running the above test function and evaluating 
`(vertical-motion 1`).  Instead of moving to line number two, the cursor will 
move to column number 2.  I have written a workaround to compensate for the 
problem -- if point is on the first wrapped line, then ...

    (vertical-motion 1)
    (when (= (current-column) 2 )
      (move-to-column (+ (current-column) (- window-width 2))))

This bug affects the popular functions that rely upon the behavior of 
vertical-motion -- e.g., scrolling with the mouse; and using the arrow keys to 
navigate the buffer.

If the wrapped line is at the top of the buffer, and there are many such 
wrapped lines that don't break nicely at the far right edge (e.g., no space and 
no tab), then the mouse scroll becomes inoperable unless one uses the scroll 
bars.

Thanks,

Keith

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In GNU Emacs 24.4.50.1 (x86_64-apple-darwin10.8.0, NS apple-appkit-1038.36)
 of 2014-04-28 on MP.local
Repository revision: 117030 eggert@cs.ucla.edu-20140428165941-zk00wxiq6dy6xcg4
Windowing system distributor `Apple', version 10.3.1038
Configured using:
 `configure --with-ns'

Configured features:
ACL LIBXML2 ZLIB

Important settings:
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Major mode: Fundamental

Minor modes in effect:
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