bug#71605: 30.0.50; [PATCH] Support variable-width text in 'visual-wrap-prefix-mode'

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Jim Porter <jporterbugs@gmail.com> writes:

> (Note: I plan to merge this only after we cut the Emacs 30 release
> branch, since it seems a bit too substantial a change to sneak in
> right near the end. However, I think the patch is mostly done aside
> from one remaining issue, so any feedback is very welcome.)
>
> 'visual-wrap-prefix-mode' has one small issue: since the wrap prefix
> is just a string, the wrapped text may not line up for variable-width
> fonts. This is mainly in cases like so:
>
>   * here is some text that
>     got visually wrapped
>
> If the "* " is variable-width, the second line will probably be
> indented wrong by a few pixels.
>
> The attached patch adds a display spec in this case so that the text
> lines up perfectly. There's currently one problem though: I'm not sure
> how to regenerate the wrap prefix automatically if the face
> changes. It's not hard to handle for 'text-scale-adjust', but I don't

Actually, there's more than just this one problem.  To start with, the
existing format of the generated line prefix properties enables the
extracted fill prefix to be displayed consistently on any frame,
whatever the metrics of its default font/face.  This I consider a far
more critical capability than perfect alignment of wrapped text in the
presence of a variable-pitch default font, as the latter is virtually
unknown among programmers, and also unimplementable within reasonable
standards of performance with pixelwise spacers.  If this is to be
installed, please condition it behind a user option and restore the
existing logic that you have effaced as the default.