I received the bug report below for our visual-line-mode in Aquamacs  
Emacs, but I managed to reproduce the core issue on Emacs 23 (Nextstep  
port) as follows:

(fringe-mode 0)
C-x 3 C-x 2 C-x o C-x o
(we're now in the large window on the right, with two small windows in  
the left half of the frame).

Insert enough newlines to be vertically just above the horizontal  
divider on the left.

M-: (posn-at-x-y 0 (cdr (posn-x-y (posn-at-point)))) RET

Insert newline, now we're next to the divider line.

M-: (posn-at-x-y 0 (cdr (posn-x-y (posn-at-point)))) RET

--> wrong window (the window in the upper left) is given in the  
structure.
Instead of 0, the x value can be up to 6 ((frame-char-width) reports 7).




Begin forwarded message:

> From: Jin Choi <jsc@alum.mit.edu>
> Date: 18 November 2008 14:35:17 EST
> To: aquamacs-bugs@aquamacs.org
> Subject: [Aquamacs-bugs] visual-line-mode error with vertically  
> split windows
>
> Please describe exactly what actions triggered the bug
> and the precise symptoms of the bug:
>
> THE SETUP: vertically split window with the left half split
> horizontally. This is commonly used in, say, ECB, perhaps other modes:
> ---------
> | 1 | 3 |
> |   |   |
> ----|   |
> | 2 |   |
> |   |   |
> ---------
>
>
> THE BUG: in the right hand window, attempting to move the cursor (in
> column 1) through the row adjacent to the mode line of the upper left
> hand window moves the cursor to the upper left window. Also, placing
> the cursor in column 0 of that row then typing C-n or C-p gives the
> error "Position not in text area of window."
>
>
>
> TO REPLICATE:
> C-x 3 C-x 2
>
> Hit return in the right side window (3) enough to get you past the
> divider
> on the left.
>
> Then C-p back past the divider. Cursor jumps to upper left window (1).
>
> Now click on the row in window 3 adjacent to the divider. Type C-n or
> C-p. Gives above error.
>
>
> WORKAROUND:
> (global-visual-line-mode 0)
>
>
>
> In GNU Emacs 22.3.2 (i386-apple-darwin9.5.0, Carbon Version 1.6.0)
>  of 2008-09-21 on plume.sr.unh.edu - Aquamacs Distribution 1.5
> Windowing system distributor `Apple Inc.', version 10.5.5
> configured using `configure  '--without-x' '--prefix=/usr/local''
>
> 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: nil
>   locale-coding-system: iso-8859-1
>   default-enable-multibyte-characters: t
>
> Major mode: Text
>
> Minor modes in effect:
>   shell-dirtrack-mode: t
>   savehist-mode: t
>   aquamacs-styles-mode: t
>   smart-frame-positioning-mode: t
>   global-visual-line-mode: t
>   visual-line-mode: t
>   recentf-mode: t
>   encoded-kbd-mode: t
>   osx-key-mode: t
>   show-paren-mode: t
>   delete-selection-mode: t
>   pc-selection-mode: t
>   cua-mode: t
>   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
>   unify-8859-on-encoding-mode: t
>   utf-translate-cjk-mode: t
>   auto-compression-mode: t
>   column-number-mode: t
>   transient-mark-mode: t
>
> Recent input:
> <menu-bar> <help-menu> <bug-diagnosis> <start-vanilla-aquamacs>
> <menu-bar> <help-menu> <bug-diagnosis> <send-emacs
> -bug-report>
>
> Recent messages:
> on One-Buffer-One-Frame-Mode (Display Buffers in Separate Frames)!
> Loading /Users/jsc/Library/Preferences/Aquamacs Emacs/
> customizations.el (source)...done
> Loading /Users/jsc/Library/Preferences/Aquamacs Emacs/Preferences.el
> (source)...done
> One-Buffer-One-Frame-Mode disabled.
> For best results, turn off Frame Appearance Styles now.
> Loading /Users/jsc/Library/Preferences/Aquamacs Emacs/frame-
> positions.el (source)...done
> Truncate long lines disabled
> Mark set
> Aquamacs is based on GNU Emacs 22, a part of the GNU/Linux system. It
> is Free Software: you can improve and redistribute it under the GNU
> General Public License, version 3 or later. Copyright (C) 2008 Free
> Software Foundation, Inc. (C) 2008 D. Reitter. No Warranty.
> Loading emacsbug...done
>
>
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