org-goto-sibling: Fix when called from inlinetask

* lisp/org.el (org-goto-sibling): Do not try to find inlinetask
siblings.  Clarify the docstring.
* testing/lisp/test-org.el (test-org/goto-sibling): New test.

lisp/org.el

@@ -20637,7 +20637,7 @@ point before the first headline or at point-min."
 	(< l level)))))
 
 (defun org-goto-sibling (&optional previous)
-  "Goto the next sibling, even if it is invisible.
+  "Goto the next sibling heading, even if it is invisible.
 When PREVIOUS is set, go to the previous sibling instead.  Returns t
 when a sibling was found.  When none is found, return nil and don't
 move point."
@@ -20646,6 +20646,8 @@ move point."
 	(re org-outline-regexp-bol)
 	level l)
     (when (ignore-errors (org-back-to-heading t))
+      (when (org-element-type-p (org-element-at-point) 'inlinetask)
+        (org-up-heading-safe))
       (setq level (funcall outline-level))
       (catch 'exit
 	(or previous (forward-char 1))

testing/lisp/test-org.el

@@ -2449,6 +2449,65 @@ Text.
     (should-not (org-up-heading-safe))
     (should (looking-at-p "^\\*\\* H2"))))
 
+(ert-deftest test-org/goto-sibling ()
+  "Test `org-goto-sibling' specifications."
+  (org-test-with-temp-text
+      "* Parent
+** Heading 1
+** Heading 2 <point>
+** Heading 3"
+    (should (org-goto-sibling))
+    (should (looking-at-p "^\\*\\* Heading 3"))
+    (should-not (org-goto-sibling))
+    (should (org-goto-sibling 'previous))
+    (should (looking-at-p "^\\*\\* Heading 2"))
+    (should (org-goto-sibling 'previous))
+    (should (looking-at-p "^\\*\\* Heading 1"))
+    (should-not (org-goto-sibling 'previous)))
+  ;; Inside heading.
+  (org-test-with-temp-text
+      "* Parent
+** Heading 1
+** Heading 2
+Some text.<point>
+** Heading 3"
+    (should (org-goto-sibling))
+    (should (looking-at-p "^\\*\\* Heading 3")))
+  (org-test-with-temp-text
+      "* Parent
+** Heading 1
+** Heading 2
+Some text.<point>
+** Heading 3"
+    (should (org-goto-sibling 'previous))
+    (should (looking-at-p "^\\*\\* Heading 1")))
+  (org-test-with-temp-text
+      "* Parent
+** Heading 2
+Some text.<point>
+"
+    (should-not (org-goto-sibling))
+    (should-not (org-goto-sibling 'previous)))
+  ;; Ignore inlinetasks.
+  (let ((org-inlinetask-min-level 3))
+    (org-test-with-temp-text
+        "* Parent
+** Heading 1
+** Heading 2
+*** Inlinetask 1
+test <point>
+*** END
+*** Inlinetask 2
+** Heading 3"
+      (should (org-goto-sibling))
+      (should (looking-at-p "^\\*\\* Heading 3"))
+      (should-not (org-goto-sibling))
+      (should (org-goto-sibling 'previous))
+      (should (looking-at-p "^\\*\\* Heading 2"))
+      (should (org-goto-sibling 'previous))
+      (should (looking-at-p "^\\*\\* Heading 1"))
+      (should-not (org-goto-sibling 'previous)))))
+
 (ert-deftest test-org/get-heading ()
   "Test `org-get-heading' specifications."
   ;; Return current heading, even if point is not on it.