Bug:  Scrolling can leave behind traces of old text.
The traces appear to be cyan outlines of the right hand
side of characters which previously occupied the same location.

For example, a "Y" will leave behind a small cyan dot from its upper
right, an "A" will leave behind a small cyan dot from its lower right.

Steps to reproduce:

1)  /Applications/Emacs.app/Contents/MacOS/Emacs -Q
2)  C-x C-f helix.pqr [Ret]
3)  Rapidly scroll up and down with two-finger drag gesture

I have attached the file helix.pqr, for which the effect is
very pronounced, and a screenshot faithfully reproducing
the effect.  Note the small cyan marks on the left of
many numbers and letters.

This version of emacs came from http://emacsformacosx.com/

In GNU Emacs 23.4.1 (x86_64-apple-darwin, NS apple-appkit-1038.36)
 of 2012-01-29 on bob.porkrind.org
Windowing system distributor `Apple', version 10.3.1138
configured using `configure  '--host=x86_64-apple-darwin'
'--build=i686-apple-darwin' '--with-ns'
'build_alias=i686-apple-darwin' 'host_alias=x86_64-apple-darwin'
'CC=gcc -mmacosx-version-min=10.5''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Fundamental

Minor modes in effect:
  tooltip-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
<wheel-up> <double-wheel-up> <triple-wheel-up> <triple-wheel-up>
<triple-wheel-up> <triple-wheel-up> <triple-wheel-up>
<triple-wheel-up> <triple-wheel-up> <triple-wheel-up>
<triple-wheel-up> <wheel-down> <double-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<wheel-up> <double-wheel-up> <triple-wheel-up> <triple-wheel-up>
<triple-wheel-up> <triple-wheel-up> <triple-wheel-up>
<triple-wheel-up> <triple-wheel-up> <triple-wheel-up>
<triple-wheel-up> <triple-wheel-up> <triple-wheel-up>
<triple-wheel-up> <triple-wheel-up> <wheel-down> <double-wheel-down>
<triple-wheel-down> <wheel-up> <double-wheel-up> <triple-wheel-up>
<wheel-up> <double-wheel-up> <wheel-down> <double-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<triple-wheel-down> <triple-wheel-down> <wheel-down>
<double-wheel-down> <wheel-down> <double-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<wheel-up> <double-wheel-up> <triple-wheel-up> <triple-wheel-up>
<triple-wheel-up> <triple-wheel-up> <triple-wheel-up>
<triple-wheel-up> <triple-wheel-up> <triple-wheel-up>
<triple-wheel-up> <wheel-up> <double-wheel-up> <wheel-down>
<wheel-down> <double-wheel-down> <wheel-up> <wheel-down>
<wheel-down> <wheel-up> <double-wheel-up> <wheel-up>
<wheel-up> <double-wheel-up> <triple-wheel-up> <up>
<up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up>
<up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up>
<up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up>
<up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up>
<up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up>
<up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up>
<up> <down> <down> <down> <down> <down> <down> <down>
<down> <down> <down> <down> <down> <down> <down> <down>
<down> <down> <down> <down> <down> <down> <down> <down>
<down> <down> <down> <down> <down> <down> C-x b <return>
<wheel-up> <double-wheel-up> <triple-wheel-up> <triple-wheel-up>
<wheel-down> <double-wheel-down> <triple-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<triple-wheel-down> <wheel-up> <double-wheel-up> <triple-wheel-up>
<triple-wheel-up> <wheel-down> <double-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<triple-wheel-down> <wheel-up> <double-wheel-up> <wheel-down>
<double-wheel-down> <triple-wheel-down> <wheel-down>
<double-wheel-down> <triple-wheel-down> <wheel-up>
<double-wheel-up> <triple-wheel-up> <triple-wheel-up>
<wheel-down> <double-wheel-down> <triple-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<wheel-up> <double-wheel-up> <triple-wheel-up> <triple-wheel-up>
<triple-wheel-up> <wheel-down> <double-wheel-down>
<triple-wheel-down> <triple-wheel-down> <triple-wheel-down>
<triple-wheel-down> <triple-wheel-down> <wheel-up>
<double-wheel-up> <triple-wheel-up> <triple-wheel-up>
<wheel-down> <double-wheel-down> <triple-wheel-down>
<wheel-up> <double-wheel-up> <triple-wheel-up> <wheel-down>
<double-wheel-down> <wheel-down> <double-wheel-down>
M-x b u g <tab> <backspace> <backspace> <backspace>
<backspace> <backspace> <backspace> <backspace> <backspace>
<backspace> <backspace> <backspace> <backspace> <backspace>
<backspace> r e p <tab> o r <tab> <return>

Recent messages:
Saving file /Users/fvm/.emacs...
Wrote /Users/fvm/.emacs
byte-code: Beginning of buffer [8 times]
byte-code: End of buffer [13 times]
byte-code: Beginning of buffer [13 times]
byte-code: End of buffer [26 times]
byte-code: Beginning of buffer [14 times]
byte-code: End of buffer [2 times]
Making completion list...
byte-code: End of buffer [4 times]

Load-path shadows:
None found.

Features:
(shadow sort mail-extr message ecomplete rfc822 mml mml-sec
password-cache mm-decode mm-bodies mm-encode mailcap mail-parse rfc2231
rfc2047 rfc2045 qp ietf-drums mailabbrev nnheader gnus-util netrc
time-date mm-util mail-prsvr gmm-utils wid-edit mailheader canlock sha1
hex-util hashcash mail-utils emacsbug help-mode view vc-hg cc-mode
cc-fonts cc-menus cc-cmds cc-styles cc-align cc-engine cc-vars cc-defs
regexp-opt tooltip ediff-hook vc-hooks lisp-float-type mwheel ns-win
easymenu tool-bar dnd fontset image fringe lisp-mode register page
menu-bar rfn-eshadow timer select scroll-bar mldrag mouse jit-lock
font-lock syntax facemenu font-core frame cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese hebrew
greek romanian slovak czech european ethiopic indian cyrillic chinese
case-table epa-hook jka-cmpr-hook help simple abbrev loaddefs button
minibuffer faces cus-face files text-properties overlay md5 base64
format env code-pages mule custom widget hashtable-print-readable
backquote make-network-process ns multi-tty emacs)