GNU bug report logs - #18285
24.3.92; A combination of `display' on text and `invisible' and `before/after-string' leads to the before/after string being displayed twice

Previous Next

Full log

Message #29 received at 18285 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Dmitry Gutov <dgutov <at> yandex.ru>
Cc: 18285 <at> debbugs.gnu.org
Subject: Re: bug#18285: 24.3.92;
 A combination of `display' on text and `invisible'
 and	`before/after-string' leads to the before/after string being	displayed
 twice
Date: Fri, 22 Aug 2014 09:41:25 +0300

> Date: Fri, 22 Aug 2014 04:58:55 +0400
> From: Dmitry Gutov <dgutov <at> yandex.ru>
> CC: 18285 <at> debbugs.gnu.org
> 
> On 08/21/2014 08:06 PM, Eli Zaretskii wrote:
> 
> >> If the `invisible' starts even one character earlier, it *is* the other
> >> way around.
> >
> > Yes, because then there's no doubt about the order of evaluating the
> > various properties and acting upon them.  By contrast, when they all
> > begin at the same buffer position, the order is
> > implementation-defined.  The code is written to examine display
> > properties before the invisible properties.
> 
> Okay, but I'll take this to mean that hitting the changed behavior in 
> existing code would be pretty rare.

I have no idea how rare it will be.  FWIW, for the past year, all the
display-related bugs are for pretty rare cases.  What does that tell
you about user expectations?

> Anyway, how about the other way around? I'll like this less, but why not 
> make `invisible' inactive when `display' is set?

That's what Emacs does already.  The only place where invisible still
matters in this situation is when deciding how and where to display
overlay strings.  I thought I explained that earlier in this thread.

> >> Maybe. But at least it's consistent with the overlay priority rules.
> >
> > The priority is _per_buffer_position_.  We tend to forget that text
> > properties and overlays in Emacs are _character_ properties, but the
> > display engine is designed and implemented to support that, and in
> > some obscure cases, like this one, it is impossible to understand its
> > logic, unless one remembers this simple fact.
> 
> I'm just quoting Stefan from that discussion:
> 
> """
> Same problem: for two overlays of equal `priority', the shorter one has
> higher priority, so your original example is already one of those
> cases, AFAIC.
> """

This again omits the basic fact that properties and overlays are per
character.

> (let ((pt (point)))
>    (insert "aaa")
>    (let ((o (make-overlay pt (point))) (v (make-overlay (1+ pt) (1- 
> (point)))))
>      (overlay-put o 'face 'bold)
>      (overlay-put v 'face 'default)))
> 
> the middle character has normal weight, even though it's also covered by 
> an outer overlay that sets `face' to `bold'.
> 
> So, if I had to pick which single string to show (STRING1 or STRING2), 
> the latter seems to be the more consistent choice.

My opinion is that users and Lisp programmers should not enter these
dark corners.

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.