variable width space display property value has no effect in empty overlay

variable width space display property value has no effect in empty overlay

[Kevin Rodgers]

This bug report will be sent to the Free Software Foundation,
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Please write in English, because the Emacs maintainers do not have
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In GNU Emacs 21.3.1 (i386-pc-solaris2.7, X toolkit)
 of 2003-04-24 on briard
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: nil
  default-enable-multibyte-characters: t

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

emacs-21.3.1 -q --no-site-file
M-<
C-e
M-: (let ((lbp (line-beginning-position))) (overlay-put (make-overlay lbp lbp) 'display '(space :align-to 4)))

That creates an overlay spanning 0 characters at the beginning of the
*scratch* buffer, with the display property set to (space :align-to 4).
But the buffer display is not affected.

Recent input:
g i n n i n g - p o s i t i o n ) ) ) SPC ( o v e r 
l a y - p u t SPC ( m a k e - o v e r l a y SPC l b 
p SPC l b p ) SPC ' d i s p l a y SPC ' ( s p a c e 
SPC : a l i g n - t o SPC 4 ) ) ) <return> <help-echo> 
<help-echo> <help-echo> <help-echo> <help-echo> <help-echo> 
<help-echo> <help-echo> <help-echo> <menu-bar> <help-menu> 
<report-emacs-bug>

Recent messages:
(/usr/local/emacs-21.3/src/emacs-21.3.1 -q --no-site-file)
Loading tool-bar...done
Loading image...done
Loading tooltip...done
For information about the GNU Project and its goals, type C-h C-p.
Mark set
(space :align-to 4)
Loading emacsbug...done

Re: variable width space display property value has no effect inempty overlay

[Richard Stallman]

    That creates an overlay spanning 0 characters at the beginning of the
    *scratch* buffer, with the display property set to (space :align-to 4).
    But the buffer display is not affected.

That is because no character has the display property.

Re: variable width space display property value has no effect in empty overlay

[David Kastrup]

Richard Stallman <rms@gnu.org> writes:

>     That creates an overlay spanning 0 characters at the beginning
>     of the *scratch* buffer, with the display property set to (space
>     :align-to 4).  But the buffer display is not affected.
> 
> That is because no character has the display property.

An overlay spanning 0 characters with a display property set to an
image displays the image (when the move-on-insertion properties of the
overlay ends are equal, it serves more or less as a visible marker).
It does not seem consistent if other display properties intended to
mask the underlying character range (even if empty) do not behave
likewise.

-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum

Re: variable width space display property value has no effect in empty overlay

[Richard Stallman]

    An overlay spanning 0 characters with a display property set to an
    image displays the image (when the move-on-insertion properties of the
    overlay ends are equal, it serves more or less as a visible marker).

I wasn't aware of that.

    It does not seem consistent if other display properties intended to
    mask the underlying character range (even if empty) do not behave
    likewise.

I agree, it seems inconsistent, and perhaps it ought to be changed.
Finding someone who can change it and wants to change it is a harder
issue.  Do we have anyone who understands that code?