Fix fill-paragraph combining text with preceding comment

* lisp/textmodes/fill.el (fill-paragraph): Handle the case
when a non-comment line follows a comment line with
non-nil 'fill-paragraph-handle-comment' (bug#80449).
Skip such a comment line before filling a non-comment line.

* test/lisp/textmodes/fill-tests.el
(fill-test-fill-paragraph-handle-comment): Add new test.

* test/lisp/textmodes/fill-resources/fill-paragraph-handle-comment.erts:
New file.

lisp/textmodes/fill.el

@@ -911,6 +911,7 @@ region, instead of just filling the current paragraph."
             (fill-comment-paragraph justify)))
      ;; 4. If it all fails, default to the good ol' text paragraph filling.
      (let ((before (point))
+           (paragraph-start-orig paragraph-start)
            (paragraph-start paragraph-start)
            ;; Fill prefix used for filling the paragraph.
            fill-pfx)
@@ -933,6 +934,18 @@ region, instead of just filling the current paragraph."
              (setq fill-pfx "")
            (let ((end (point))
                  (beg (progn (fill-forward-paragraph -1) (point))))
+             ;; If the paragraph starts with a comment line preceding point
+             ;; on a non-comment line, skip such comment lines, so they
+             ;; are not filled together (bug#80449).
+             (when (and fill-paragraph-handle-comment comment-start-skip
+                        (< beg before))
+               (save-excursion
+                 (goto-char beg)
+                 (when (looking-at paragraph-start-orig)
+                   (goto-char (1+ (match-end 0))))
+                 (when (looking-at comment-start-skip)
+                   (forward-line 1)
+                   (setq beg (point)))))
              (goto-char before)
              (setq fill-pfx
                    (if use-hard-newlines

test/lisp/textmodes/fill-resources/fill-paragraph-handle-comment.erts

@@ -0,0 +1,37 @@
+Point-Char: |
+
+Name: fill-paragraph-handle-comment - non-comment line before comment line
+Code:
+  (lambda ()
+    (setq-local comment-start "# ")
+    (setq-local fill-paragraph-handle-comment t)
+    (setq-local fill-column 42)
+    (fill-paragraph))
+
+=-=
+
+this is not part of the comment this is not part of the comment|
+# this is a comment this is a comment this is a comment
+
+=-=
+
+this is not part of the comment this is
+not part of the comment
+# this is a comment this is a comment this is a comment
+
+=-=-=
+
+Name: fill-paragraph-handle-comment - non-comment line after comment line
+
+=-=
+
+# this is a comment this is a comment this is a comment
+this is not part of the comment this is not part of the comment|
+
+=-=
+
+# this is a comment this is a comment this is a comment
+this is not part of the comment this is
+not part of the comment
+
+=-=-=

test/lisp/textmodes/fill-tests.el

@@ -163,6 +163,10 @@ eius. Foo")))
   (skip-unless (functionp 'markdown-mode))
   (ert-test-erts-file (ert-resource-file "fill-paragraph-semlf-markdown-mode.erts")))
 
+(ert-deftest fill-test-fill-paragraph-handle-comment ()
+  "Test the `fill-paragraph-handle-comment' variable."
+  (ert-test-erts-file (ert-resource-file "fill-paragraph-handle-comment.erts")))
+
 (provide 'fill-tests)
 
 ;;; fill-tests.el ends here