Do not hijack keybindings in Calendar mode

* lisp/org-compat.el (org-calendar-to-agenda-key):
(org-calendar-insert-diary-entry-key): Moved from "org.el".
(org--setup-calendar-bindings): New function.

Fixes: Bug#10289

etc/ORG-NEWS

@@ -57,6 +57,11 @@ should become
   ,#+END_SRC
 #+END_SRC
 
+*** Change ~org-calendar-to-agenda-key~ value
+
+Default value and accepted value types changed.  See [[doc:org-calendar-to-agenda-key][docstring]] for
+details.
+
 *** Change ~org-structure-template-alist~ value
 
 With the new template expansion mechanism (see

lisp/org-compat.el

@@ -25,7 +25,7 @@
 ;;; Commentary:
 
 ;; This file contains code needed for compatibility with older
-;; versions of GNU Emacs.
+;; versions of GNU Emacs and integration with other packages.
 
 ;;; Code:
 
@@ -842,6 +842,53 @@ ELEMENT is the element at point."
        "Make the position visible."
        (org-bookmark-jump-unhide))))
 
+;;;; Calendar
+
+(defcustom org-calendar-to-agenda-key 'default
+  "Key to be installed in `calendar-mode-map' for switching to the agenda.
+
+The command `org-calendar-goto-agenda' will be bound to this key.
+
+When set to `default', bind the function to `c', but only if it is
+available in the Calendar keymap.  This is the default choice because
+`c' can then be used to switch back and forth between agenda and calendar.
+
+When nil, `org-calendar-goto-agenda' is not bound to any key."
+  :group 'org-agenda
+  :type '(choice
+	  (const :tag "Bind to `c' if available" default)
+	  (key-sequence :tag "Other binding")
+	  (const :tag "No binding" nil))
+  :safe (lambda (v) (or (symbolp v) (stringp v)))
+  :package-version '(Org . "9.2"))
+
+(defcustom org-calendar-insert-diary-entry-key [?i]
+  "The key to be installed in `calendar-mode-map' for adding diary entries.
+This option is irrelevant until `org-agenda-diary-file' has been configured
+to point to an Org file.  When that is the case, the command
+`org-agenda-diary-entry' will be bound to the key given here, by default
+`i'.  In the calendar, `i' normally adds entries to `diary-file'.  So
+if you want to continue doing this, you need to change this to a different
+key."
+  :group 'org-agenda
+  :type 'sexp)
+
+(defun org--setup-calendar-bindings ()
+  "Bind Org functions in Calendar keymap."
+  (pcase org-calendar-to-agenda-key
+    (`nil nil)
+    ((and key (pred stringp))
+     (local-set-key (kbd key) #'org-calendar-goto-agenda))
+    ((guard (not (lookup-key calendar-mode-map "c")))
+     (local-set-key "c" #'org-calendar-goto-agenda))
+    (_ nil))
+  (unless (eq org-agenda-diary-file 'diary-file)
+    (local-set-key org-calendar-insert-diary-entry-key
+		   #'org-agenda-diary-entry)))
+
+(eval-after-load "calendar"
+  '(add-hook 'calendar-mode-hook #'org--setup-calendar-bindings))
+
 ;;;; Saveplace
 
 ;; Make sure saveplace shows the location if it was hidden

lisp/org.el

@@ -3771,25 +3771,6 @@ A nil value means to remove them, after a query, from the list."
   :group 'org-agenda
   :type 'boolean)
 
-(defcustom org-calendar-to-agenda-key [?c]
-  "The key to be installed in `calendar-mode-map' for switching to the agenda.
-The command `org-calendar-goto-agenda' will be bound to this key.  The
-default is the character `c' because then `c' can be used to switch back and
-forth between agenda and calendar."
-  :group 'org-agenda
-  :type 'sexp)
-
-(defcustom org-calendar-insert-diary-entry-key [?i]
-  "The key to be installed in `calendar-mode-map' for adding diary entries.
-This option is irrelevant until `org-agenda-diary-file' has been configured
-to point to an Org file.  When that is the case, the command
-`org-agenda-diary-entry' will be bound to the key given here, by default
-`i'.  In the calendar, `i' normally adds entries to `diary-file'.  So
-if you want to continue doing this, you need to change this to a different
-key."
-  :group 'org-agenda
-  :type 'sexp)
-
 (defcustom org-agenda-diary-file 'diary-file
   "File to which to add new entries with the `i' key in agenda and calendar.
 When this is the symbol `diary-file', the functionality in the Emacs
@@ -3800,17 +3781,6 @@ points to a file, `org-agenda-diary-entry' will be used instead."
 	  (const :tag "The standard Emacs diary file" diary-file)
 	  (file :tag "Special Org file diary entries")))
 
-(eval-after-load "calendar"
-  '(progn
-     (org-defkey calendar-mode-map org-calendar-to-agenda-key
-		 'org-calendar-goto-agenda)
-     (add-hook 'calendar-mode-hook
-	       (lambda ()
-		 (unless (eq org-agenda-diary-file 'diary-file)
-		   (define-key calendar-mode-map
-		     org-calendar-insert-diary-entry-key
-		     'org-agenda-diary-entry))))))
-
 (defgroup org-latex nil
   "Options for embedding LaTeX code into Org mode."
   :tag "Org LaTeX"