Subject: bug#41897: 28.0.50; JavaScript comment filling with mhtml-mode

From: Alan Mackenzie <[email-masked]>
Date: 20 Jun 2020 17:18:27 -0000

Hello, Simen.

In article <[email-masked]> you wrote:
> In js-mode, the following JavaScript comment is filled as expected when
> fill-paragraph (M-q) is used with point within it:

> /*
>  * This is a long comment that should break over multiple lines when fill-paragraph is used.
>  */

> Is filled as:

> /*
>  * This is a long comment that should break over multiple lines when
>  * fill-paragraph is used.
>  */

> In mhtml-mode however, the same JavaScript comment:

> <html>
>   <script>
>     /*
>      * This is a long comment that should break over multiple lines when fill-paragraph is used.
>      */
>   </script>
> </html>

> Is filled as:

> <html>
>   <script>
>     /* * This is a long comment that should break over multiple lines
>      when fill-paragraph is used.  */
>   </script>
> </html>

Yes.  This is happening because the mechanism mhtml-mode is using to
"switch to" js-mode is imperfect.  Having more than one major mode in a
buffer is a difficult problem for Emacs, and there are currently no
really satisfactory solutions.

"Switching to" another mode essentially means setting buffer local
variable copies.  In the existing code, a variable essential to filling,
adaptive-fill-regexp, isn't getting set to a value suitable for js-mode.

Also, there was an error in the js-mode setting of the variable
comment-start-skip.

The patch below fixes both these errors, and seems to allow your test
case to work.  However, it is an incomplete solution - in particular,
filling caused by typing at the end of a long line still isn't working
properly.  Still, it may be better than nothing, at least for now.  I
intend to carry on working on this bug.

You haven't said what version of Emacs you're running.  The patch
applies cleanly both to the upcoming emacs-27 branch and the master
branch at GNU savannah.  To apply it, cd into ..../emacs/lisp, and
execute:

    $ patch -p2 < this-email

.  Then byte-compile the two changed files and load them (possibly by
restarting Emacs).

Here's the patch:

diff --git a/lisp/progmodes/js.el b/lisp/progmodes/js.el
index 04b449ecd2..f5b6a4c260 100644
--- a/lisp/progmodes/js.el
+++ b/lisp/progmodes/js.el
@@ -4570,7 +4570,7 @@ js-mode
 
   ;; Comments
   (setq-local comment-start "// ")
-  (setq-local comment-start-skip "\\(//+\\|/\\*+\\)\\s *")
+  (setq-local comment-start-skip "\\(?://+\\|/\\*+\\)\\s *")
   (setq-local comment-end "")
   (setq-local fill-paragraph-function #'js-fill-paragraph)
   (setq-local normal-auto-fill-function #'js-do-auto-fill)
diff --git a/lisp/textmodes/mhtml-mode.el b/lisp/textmodes/mhtml-mode.el
index 1ae07c0a30..069329f4e4 100644
--- a/lisp/textmodes/mhtml-mode.el
+++ b/lisp/textmodes/mhtml-mode.el
@@ -73,7 +73,8 @@ mhtml-tag-relative-indent
 
 (defconst mhtml--crucial-variable-prefix
   (regexp-opt '("comment-" "uncomment-" "electric-indent-"
-                "smie-" "forward-sexp-function" "completion-" "major-mode"))
+                "smie-" "forward-sexp-function" "completion-" "major-mode"
+                "adaptive-fill-" "fill-"))
   "Regexp matching the prefix of \"crucial\" buffer-locals we want to capture.")
 
 (defconst mhtml--variable-prefix

-- 
Alan Mackenzie (Nuremberg, Germany).