before-string property has no effect if display property is empty string

[Joe Wells]

The documentation leads me to believe that the before-string and
display properties of an overlay should be orthogonal.  However, the
before-string is not displayed if the display property is the empty
string.

Reproduce by evaluating this code:

  (let ((buf (get-buffer-create "foo")))
    (with-current-buffer buf
      (display-buffer buf)
      (erase-buffer)
      (dolist (o (overlays-in (point-min) (point-max)))
        (delete-overlay o))
      (insert "ABC")
      (let ((o (make-overlay 2 3))
            (s (make-string 3 ?X)))
        (overlay-put o 'display "")
        (overlay-put o 'before-string s))))

You will see the “foo” window pop up and in the window you will see
“AC”.  I expected that instead “AXXXC” would be shown.

This seems like a bug.

Thanks for your time in considering this bug report.

Joe

======================================================================
In GNU Emacs 22.1.1 (i686-pc-linux-gnu, GTK+ Version 2.8.20)
 of 2007-06-27 on artemis
Windowing system distributor `The X.Org Foundation', version 11.0.70000000
configured using `configure  '--prefix=/home/jbw/local2' '--enable-debug' 
'--disable-nls' '--with-x-toolkit=gtk' 'CFLAGS=-O0 -g3 -ggdb''

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

Major mode: Lisp Interaction

Minor modes in effect:
  TeX-source-specials-mode: t
  auto-fill-function: do-auto-fill
  shell-dirtrack-mode: t
  outline-minor-mode: t
  desktop-save-mode: t
  url-handler-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
  temp-buffer-resize-mode: t
  size-indication-mode: t
  line-number-mode: t
  transient-mark-mode: t