Subject: 26.1; sgml-mode indents as comment inside of -- text blocks

From: Akkana Peck <[email-masked]>
Date: Sat, 15 Jun 2019 12:02:18 -0600

In emacs -Q, edit a file in html-mode (derived from sgml-mode),
e.g. visit /tmp/foo.html.

M-x auto-fill-mode <RET>

Type some nonsense words with ' -- ' in the middle, like this:

asd shdf ladshjkl sjdk -- asdfh jklsdfh

and keep typing until it wraps.

sgml-mode will wrap adding unwanted extra dashes and indentations,
like this:

asd shdf ladshjkl sjdk -- asdfh jklsdfh jasdklf hjsdkl hjsdaklf sdf --
		       -- jkldsfhj kalshfjkasdlfh jaksd

This happens because sgml-mode is confused about what constitutes a
comment in sgml/html.

At some point, someone clearly agreed that this was a bug and tried to
fix it. If you look at the sgml.el source and search for --,
you'll see comments like:

> (defvar sgml-specials '(?\")
>   "List of characters that have a special meaning for SGML mode.
> This list is used when first loading the `sgml-mode' library.
> The supported characters and potential disadvantages are:
>
>   ?\\\"	Makes \" in text start a string.
>   ?\\='	Makes \\=' in text start a string.
>   ?-	Makes -- in text start a comment.
>
> When only one of ?\\\" or ?\\=' are included, \"\\='\" or \\='\"\\=', as can be found in
> DTDs, start a string.  To partially avoid this problem this also makes these
> self insert as named entities depending on `sgml-quick-keys'.
>
> Including ?- has the problem of affecting dashes that have nothing to do
> with comments, so we normally turn it off.")

The dash is not included in sgml-specials by default, but the problem
affecting dashes that have nothing to do with comments happens anyway.
Setting sgml-specials to nil in the mode hook doesn't help either.

The problematic behavior seems to come from comment-line-break-function.
One workaround I've found is to add this in my html-mode and sgml-mode
hooks to prevent the special processing of line breaks inside comments:

    (kill-local-variable 'comment-line-break-function)

Maybe comment-line-break-function should check sgml-specials and return
without doing anything if - isn't one of the specials?

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