 `radian-directory'."
   (let ((truename (file-truename filename)))
     (string-prefix-p radian-directory truename
-                     (when (file-name-case-insensitive-p truename)
+                     (when (if (fboundp 'file-name-case-insensitive-p)
+                               (file-name-case-insensitive-p truename)
+                             (radian-with-operating-system macOS
+                               t))
                        'ignore-case))))
 
 (defmacro radian--with-silent-load (&rest body)


;; Feature `text-mode' provides a major mode for editing plain text.
(use-feature text-mode
  :config

  (add-hook 'text-mode-hook #'auto-fill-mode)

  (radian-defhook radian--atomic-chrome-switch-back ()
    atomic-chrome-edit-done-hook
    "Switch back to the browser after finishing with `atomic-chrome'."
    (when-let ((conn (websocket-server-conn
                      (atomic-chrome-get-websocket (current-buffer))))
               (browser
                (cond
                 ((eq conn atomic-chrome-server-ghost-text)
                  "Firefox")
                 ((eq conn atomic-chrome-server-atomic-chrome)
                  "Google Chrome"))))
      (cond
       ((radian-operating-system-p macOS)
        (call-process "open" nil nil nil "-a" browser))
