Re: Redisplaying text in both display margins?

[Eli Zaretskii]

> Feedback-ID: 791:353:null:purelymail
> Date: Tue, 19 Jan 2021 19:27:34 +1000
> From: pwr--- via Users list for the GNU Emacs text editor 
> <help-gnu-emacs@gnu.org>
> 
> My intent is to redisplay the string marked here:
> 
>         HEADING. FOOBAR   #5-A#
>                        ^^^^^^^^
> as:
> 
>    5-A  HEADING. FOOBAR                            5-A
> 
> So far clarity, in this particular major mode, a heading may end with an 
> arbitrary string between two '#' characters (e.g. "5-A"). This string 
> should be redisplayed in the display margins (without the '#' 
> characters).
> 
> At the moment I'm happily redisplaying the heading number in the right 
> margin with the following code (where group 7 is the preceding space, 
> group 8 is the first '#', group 9 is the string to redisplay, and gropu 
> 10 is the second '#'):
> 
>     (put-text-property (match-beginning 7) (match-end 10)
>         'display (list '(margin right-margin)
>                     (match-string-no-properties 9)))
> 
> There are a couple of things I can't figure out:
> 
> 1. How to put multiple display text properties on the same text, so that 
> it may redisplay group 9 in both margins.

You can't, not directly: each text property of a given character can
have only one value in Emacs.  But perhaps you could do that with 2
overlay strings, if you put the overlays on the same text?

> 2. When redisplaying a string in the left margin, how to right-align it, 
> ideally padded with a couple of spaces away from the body text.
> 
> I have read over (info "(elisp) Display Property") a few times, but 
> haven't been able to quite grok it for this specific problem.

I think the node "Pixel Specification" there should tell you how to
accomplish that, but if you already read that, please ask more
specific questions, and/or show the code you tried that didn't work.