Fix filling in js-mode and mhtml-mode (js-mode parts), fixing bug #41897

* lisp/progmodes/js.el (js-mode): Use "\\(?:" in the value of
comment-start-skip rather than "\\(", fixing the second half of bug #41952.
Call c-foreign-init-lit-pos-cache and install c-foreign-truncate-lit-pos-cache
on before-change-functions, to connect up correctly with CC Mode's filling
mechanism.

* lisp/textmodes/mhtml-mode.el (mhtml--crucial-variable-prefix): Add prefixes
"adaptive-fill-", "fill-", "normal-auto-fill-function" and "paragraph-" to
pull in variables crucial to filling.
(mhtml-syntax-propertize): Read the current submode from the piece of text
being propertized rather than one character before it, and do so before
erasing the submode text-property.
(mhtml-mode): Set the js-mode value of auto-fill-function to js-do-auto-fill.
Correctly initialize and use CC Mode's filling facilities, as above.

lisp/progmodes/js.el

@@ -4570,7 +4570,7 @@ This function is intended for use in `after-change-functions'."
 
   ;; 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)
@@ -4591,6 +4591,8 @@ This function is intended for use in `after-change-functions'."
   (setq imenu-create-index-function #'js--imenu-create-index)
 
   ;; for filling, pretend we're cc-mode
+  (c-foreign-init-lit-pos-cache)
+  (add-hook 'before-change-functions #'c-foreign-truncate-lit-pos-cache nil t)
   (setq-local comment-line-break-function #'c-indent-new-comment-line)
   (setq-local comment-multi-line t)
   (setq-local electric-indent-chars

lisp/textmodes/mhtml-mode.el

@@ -73,7 +73,9 @@ code();
 
 (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-" "normal-auto-fill-function"
+                "paragraph-"))
   "Regexp matching the prefix of \"crucial\" buffer-locals we want to capture.")
 
 (defconst mhtml--variable-prefix
@@ -255,17 +257,14 @@ This is used by `mhtml--pre-command'.")
    sgml-syntax-propertize-rules))
 
 (defun mhtml-syntax-propertize (start end)
-  ;; First remove our special settings from the affected text.  They
+  (let ((submode (get-text-property start 'mhtml-submode)))
-  ;; will be re-applied as needed.
+    ;; First remove our special settings from the affected text.  They
-  (remove-list-of-text-properties start end
+    ;; will be re-applied as needed.
-                                  '(syntax-table local-map mhtml-submode))
+    (remove-list-of-text-properties start end
-  (goto-char start)
+                                    '(syntax-table local-map mhtml-submode))
-  ;; Be sure to look back one character, because START won't yet have
+    (goto-char start)
-  ;; been propertized.
+    (if submode
-  (unless (bobp)
+        (mhtml--syntax-propertize-submode submode end)))
-    (let ((submode (get-text-property (1- (point)) 'mhtml-submode)))
-      (if submode
-          (mhtml--syntax-propertize-submode submode end))))
   (sgml-syntax-propertize (point) end mhtml--syntax-propertize))
 
 (defun mhtml-indent-line ()
@@ -333,6 +332,18 @@ the rules from `css-mode'."
   ;: Hack
   (js--update-quick-match-re)
 
+  ;; Setup the appropriate js-mode value of auto-fill-function.
+  (setf (mhtml--submode-crucial-captured-locals mhtml--js-submode)
+        (push (cons 'auto-fill-function
+                    (if (and (boundp 'auto-fill-function) auto-fill-function)
+                        #'js-do-auto-fill
+                      nil))
+              (mhtml--submode-crucial-captured-locals mhtml--js-submode)))
+
+  ;; This mode might be using CC Mode's filling functionality.
+  (c-foreign-init-lit-pos-cache)
+  (add-hook 'before-change-functions #'c-foreign-truncate-lit-pos-cache nil t)
+
   ;; This is sort of a prog-mode as well as a text mode.
   (run-hooks 'prog-mode-hook))