Allow commenting out white space lines in latex-mode

* lisp/newcomment.el (comment-region-default-1): Allow commenting
out whitespace-only regions (bug#41793).

* lisp/textmodes/tex-mode.el (latex--comment-region): Use it.
(latex-mode): Set a comment style shim.

lisp/newcomment.el

@@ -1221,21 +1221,33 @@ changed with `comment-style'."
     ;; FIXME: maybe we should call uncomment depending on ARG.
     (funcall comment-region-function beg end arg)))
 
-(defun comment-region-default-1 (beg end &optional arg)
+(defun comment-region-default-1 (beg end &optional arg noadjust)
+  "Comment region between BEG and END.
+See `comment-region' for ARG.  If NOADJUST, do not skip past
+leading/trailing space when determining the region to comment
+out."
   (let* ((numarg (prefix-numeric-value arg))
 	 (style (cdr (assoc comment-style comment-styles)))
 	 (lines (nth 2 style))
 	 (block (nth 1 style))
 	 (multi (nth 0 style)))
 
-    ;; We use `chars' instead of `syntax' because `\n' might be
-    ;; of end-comment syntax rather than of whitespace syntax.
-    ;; sanitize BEG and END
-    (goto-char beg) (skip-chars-forward " \t\n\r") (beginning-of-line)
-    (setq beg (max beg (point)))
-    (goto-char end) (skip-chars-backward " \t\n\r") (end-of-line)
-    (setq end (min end (point)))
-    (if (>= beg end) (error "Nothing to comment"))
+    (if noadjust
+        (when (bolp)
+          (setq end (1- end)))
+      ;; We use `chars' instead of `syntax' because `\n' might be
+      ;; of end-comment syntax rather than of whitespace syntax.
+      ;; sanitize BEG and END
+      (goto-char beg)
+      (skip-chars-forward " \t\n\r")
+      (beginning-of-line)
+      (setq beg (max beg (point)))
+      (goto-char end)
+      (skip-chars-backward " \t\n\r")
+      (end-of-line)
+      (setq end (min end (point)))
+      (when (>= beg end)
+        (error "Nothing to comment")))
 
     ;; sanitize LINES
     (setq lines

lisp/textmodes/tex-mode.el

@@ -1169,7 +1169,12 @@ subshell is initiated, `tex-shell-hook' is run."
   (setq-local outline-regexp latex-outline-regexp)
   (setq-local outline-level #'latex-outline-level)
   (setq-local forward-sexp-function #'latex-forward-sexp)
-  (setq-local skeleton-end-hook nil))
+  (setq-local skeleton-end-hook nil)
+  (setq-local comment-region-function #'latex--comment-region)
+  (setq-local comment-style 'plain))
+
+(defun latex--comment-region (beg end &optional arg)
+  (comment-region-default-1 beg end arg t))
 
 ;;;###autoload
 (define-derived-mode slitex-mode latex-mode "SliTeX"