Re: Horizontal centering in the display engine

["Paul W. Rankin" via "Emacs development discussions." <emacs-devel@gnu.org>]

> From: sbaugh@catern.com
> Date: Wed, 14 Apr 2021 11:25:41 -0400
>
> Yes, that makes sense. So I'm asking about how difficult it would be to
> add a new element to the :align-to expression language, one which
> specifies the width of the current line.

I apologise in advance if I've misunderstood your aims, but you can use the
line-prefix text property and compute the value of :align-to based on the line
width and the window text width:

(defun center-align-line ()
  (let (line-width align-col)
    (setq line-width (- (line-end-position)
                        (line-beginning-position)))
    (setq align-col (/ (- (window-body-width) line-width)
                       2))
    (put-text-property (line-beginning-position)
                       (line-end-position)
                       'line-prefix
                       `(space :align-to ,align-col))))

Achieving this dynamically is dependent upon your major mode; if you're using a
markup syntax, you could use font-lock and a regular expression, or if manually
applied you could use a text property flag.