bug#5117: 23.1.50; Weird display when wrap-prefix contains tabs

[Matthew Dempsky <matthew@dempsky.org>]

Here's the contents of `foo.el':

    (require 'cl)
    (with-output-to-temp-buffer "bonk"
      (with-current-buffer standard-output
        (loop for i from 1 to 100 do
              (insert (make-string (+ (random 5) 3) ?x) " "))
        (visual-line-mode)
        (setq wrap-prefix "\t")))

If I run `Emacs -Q foo.el' and then `M-x eval-buffer', it creates a new
buffer "demo", but instead of each continuation line being consistently
indented by 8 spaces, they're each randomly indented between 1 and 8
spaces.

The issue appears to be due to the expansion of tabs being related to
the actual current column rather than the visual current column.
Literal tabs within a wrapped paragraph exhibit similar behavior.

This additionally sometimes confuses `visual-line-mode's line motion
keys.


In GNU Emacs 23.1.50.1 (i386-apple-darwin9.8.0, NS apple-appkit-949.54)
 of 2009-12-02 on admins-macbook-pro-2.local
Windowing system distributor `Apple', version 10.3.949
configured using `configure  '--with-ns' '--without-x''

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: Emacs-Lisp

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
  global-auto-composition-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
M-x e v a l - b u f f e r <return> M-` h s

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