Re: Redisplaying text in both display margins?

[pwr--- via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org>]

On 2021-01-20 01:16, Eli Zaretskii wrote:
>> Feedback-ID: 791:353:null:purelymail
>> 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.

Thanks for the help. I had another read through and after some trial and 
error got something working how I like. However I've run into some 
behaviour that confuses me. I'll describe this first but this will be 
clearer with the example below.

To achieve the desired result of a string in the left display margin 
aligned with the body text I've put display text properties onto two 
regions of text, one a space to fill most of the margin, and then the 
string to display. However, when inserting enough body text to cause 
these regions to wrap, the space in the margin begins to shrink.

To reproduce:

emacs -Q
(erase-buffer)
(set-window-margins nil 20)
(insert "foobarbaz")
(put-text-property 4 7 'display '((margin left-margin) (space :width (- 
left-margin 2))))
(put-text-property 7 10 'display '((margin left-margin) "1"))

This should result in a window with something like:

           1 foo

Then assuming a frame of 80 columns and `truncate-lines' is nil (or 
visual-line-mode is enabled):

(goto-char (point-min))
(insert (make-string 45 ?x))

The "1" will appear to get pushed towards the left as more text is added 
until the line wraps the window:

So my question becomes, how should I apply these display text properties 
such that they will not be affected by line wrapping?

Thanks!