bug#65015: 29.1; align-to on wrapped line regression

[Eli Zaretskii]

> From: Axel Forsman <axelsfor@gmail.com>
> Date: Wed, 2 Aug 2023 15:22:35 +0200
> 
> I noticed that the interpretation of the hpos given to the :align-to
> space specification property changed in Emacs 29.1 compared to 28.2,
> without it being documented anywhere. In version 28 it counts relative
> to the visual start of the line, whereas in version 29 it starts at the
> logical start of the line.
> 
> That is, the following MWE exhibits different visual behavior in Emacs
> 28 contra 29:
> 
>     (insert
>      (concat
>       "\n"
>       (make-string (round (* 1.25 (window-text-width))) ?x)
>       (propertize " " 'display `(space :align-to ,(round
> (window-text-width) 2)))
>       "foo\n\n"))
> 
> (In 28 the text "foo" is centered correctly by the space. In 29 the
> space has zero-width and no effect.)
> 
> The previous behavior makes more sense in the context of section 41.16.3
> Pixel Specification for Spaces in the Emacs manual, and it would be
> quite the breaking change so I am hoping it was unintentional.

It was intentional, since :align-to counts columns, and columns in
Emacs continue being counted in continuation lines, they don't start
from zero again at the point where the line wraps.  Cf current-column
and move-to-column.  What you seem to expect would make it impossible
to wrap lines with :align-to space display specs without losing the
alignment when the line wraps.

So this change fixed a bug, and it is therefore here to stay.  That is
also the reason why it is not in NEWS: we don't include bug fixes
there.