Re: Using :align-to with non-spaces

[Eli Zaretskii <eliz@gnu.org>]

> From: Alex <agrambot@gmail.com>
> Cc: eliz@gnu.org
> Date: Sun, 08 Oct 2017 23:22:59 -0600
> 
> (setq header-line-format
>       (concat (propertize " "
>                           'display
>                           '(space . (:align-to 0)))
>               "test"))
> 
> (setq header-line-format
>       (propertize "test"
>                   'display
>                   '(space . (:align-to 0))))
> 
> It's not a big deal, but it's makes things look cleaner, especially when
> the string in question is also being propertized.

The 'space' display property is of the "replacing" kind, i.e. it
completely replaces the text which has that property.  So implementing
what you want would need a new property at least.  Or maybe you could
use the '(space-width FACTOR)' display spec instead, and prepending a
single SPC character that precedes the string characters?  (This
assumes the string itself doesn't have embedded SPC characters; if it
does, put this display property only on the initial SPC character.)

FWIW, having a 'space' display property on some whitespace character
sounds very intuitive to me, since this feature was supposed to be
used to display white space subject to pixel-level resolution
considerations of a GUI frame.

> P.S. I noticed that the above does not take into account the space taken
> by `display-line-numbers'.

That's because it isn't clear whether it should be done for every
header-line.  For example, the Info mode will certainly not want
that.  So this has to be done by the Lisp program, because only it
knows whether the text in the header line should be aligned with the
rest of buffer text.