org-comment-line-break-function: Avoid built-in Emacs comment machinery

* lisp/org.el (org-comment-line-break-function): Rely on Org
parser (`org-adaptive-fill-function') to determine comment filling.
Handle nil values of `fill-prefix' correctly.

* testing/lisp/test-org.el (test-org/default-indent-new-line): New
test.  Written by Kaushal Modi <kaushal.modi@gmail.com>
https://list.orgmode.org/CAFyQvY36DkBSNy2mPxDNZWeoTjUK8mAqgJM-zHxNamfReqGkuQ@mail.gmail.com/

Reported-by: Richard Lawrence <richard.lawrence@uni-tuebingen.de>
Link: https://list.orgmode.org/87lf18fue9.fsf@aquinas.i-did-not-set--mail-host-address--so-tickle-me/

lisp/org.el

@@ -19296,12 +19296,18 @@ filling the current element."
   "Break line at point and indent, continuing comment if within one.
 The inserted newline is marked hard if variable
 `use-hard-newlines' is true, unless optional argument SOFT is
-non-nil."
-  (if soft (insert-and-inherit ?\n) (newline 1))
-  (save-excursion (forward-char -1) (delete-horizontal-space))
-  (delete-horizontal-space)
-  (indent-to-left-margin)
-  (insert-before-markers-and-inherit fill-prefix))
+non-nil.
+
+This function is a simplified version of `comment-indent-new-line'
+that bypasses the complex Emacs machinery dealing with comments.
+We instead rely on Org parser, utilizing `org-adaptive-fill-function'"
+  (let ((fill-prefix (org-adaptive-fill-function)))
+    (if soft (insert-and-inherit ?\n) (newline 1))
+    (save-excursion (forward-char -1) (delete-horizontal-space))
+    (delete-horizontal-space)
+    (indent-to-left-margin)
+    (when fill-prefix
+      (insert-before-markers-and-inherit fill-prefix))))
 
 
 ;;; Fixed Width Areas

testing/lisp/test-org.el

@@ -1405,6 +1405,26 @@ CLOCK: [2022-09-17 sam. 11:00]--[2022-09-17 sam. 11:46] =>  0:46"
 	    (org-indent-region (point-min) (point-max))
 	    (buffer-string)))))
 
+(ert-deftest test-org/default-indent-new-line ()
+  "Test behavior of default binding `M-j'."
+  ;; Calling `M-j' when point is not in an Org comment:
+  (should
+   (equal "* Some heading\n"
+  (org-test-with-temp-text "* Some heading<point>"
+                           (call-interactively #'default-indent-new-line)
+                           (buffer-string))))
+  ;; Calling `M-j' when point is in an Org comment:
+  (should
+   (equal "# Some Org comment\n# "
+  (org-test-with-temp-text "# Some Org comment<point>"
+                           (call-interactively #'default-indent-new-line)
+                           (buffer-string))))
+  (should
+   (equal "# Some Org\n# comment"
+  (org-test-with-temp-text "# Some Org <point>comment"
+                           (call-interactively #'default-indent-new-line)
+                           (buffer-string)))))
+
 
 
 ;;; Editing