Re: Using :align-to with non-spaces

[Eli Zaretskii]

> From: Alex <agrambot@gmail.com>
> Cc: help-gnu-emacs@gnu.org
> Date: Tue, 10 Oct 2017 12:09:53 -0600
> 
> > 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.
> 
> It does to me as well, but it would be nice to have similar
> functionality to :align-to in arbitrary strings.
> 
> Something like the following, that would align the string similar to
> :align-to does a space:
> 
> (propertize "test"
>             'display
>             '(align-to EXPR))
> 
> where EXPR can be the same as in :align-to.

:align-to doesn't align the space, it produces a stretch of white
space that _ends_ at the place which :align-to defines.  That's why it
makes sense to put this property on a SPC character: it makes that SPC
character's width on display be as you need it.

> I don't understand. The Info mode header isn't aligned at all (for
> example, toggling fringe and linum-mode (for the margin) doesn't change
> the position of the info header). Only headers with an `:align-to num'
> property would be affected.

Ah, you want :align-to take the line numbers into consideration!

It doesn't because neither does that happen with line-prefix or
wrap-prefix.  IOW, :align-to measures from the edges of the window's
text area.

> Here is what the manual says about :align-to:
> 
>    For example, ‘:align-to 0’ in a header-line aligns with the first
>    text column in the text area.
> 
> I would consider "the first text column" to be column 0, so the current
> behaviour is incorrect.

The "text area" is everything inside the fringes/display margins
(whichever comes last), so it includes the space used for the
line-number display.

> If line-number display is treated specially

It isn't: it is treated the same as line-prefix and wrap-prefix.

> then there should also be a `line-number' element for the :align-to
> and :width specs.

The solution I can suggest is to use the value returned by
line-number-display-width.