diff --git a/src/wp-includes/js/tinymce/wp-mce-help.php b/src/wp-includes/js/tinymce/wp-mce-help.php
index cd5e6be8ca..1f4a11f861 100644
--- a/src/wp-includes/js/tinymce/wp-mce-help.php
+++ b/src/wp-includes/js/tinymce/wp-mce-help.php
@@ -95,10 +95,8 @@ header('Content-Type: text/html; charset=' . get_bloginfo('charset'));
 </head>
 <body class="windows wp-core-ui">
 <script type="text/javascript">
-var win = window.dialogArguments || opener || parent || top;
-
-if ( win && win.tinymce && win.tinymce.isMac ) {
-	document.body.className = document.body.className.replace(/windows/, 'macos');
+if ( navigator.userAgent.indexOf( 'Mac OS' ) > -1 ) {
+	document.body.className = document.body.className.replace( /windows/, 'macos' );
 }
 </script>
 
