Index: wp-includes/js/tinymce/plugins/wpfullscreen/editor_plugin_src.js
===================================================================
--- wp-includes/js/tinymce/plugins/wpfullscreen/editor_plugin_src.js	(revision 20795)
+++ wp-includes/js/tinymce/plugins/wpfullscreen/editor_plugin_src.js	(working copy)
@@ -121,7 +121,7 @@
 				if ( tinymce.isIE || tinymce.isWebKit )
 					myHeight = d.body.scrollHeight;
 				else
-					myHeight = d.body.offsetHeight;
+					myHeight = d.documentElement.offsetHeight;
 
 				// Don't make it smaller than 300px
 				resizeHeight = (myHeight > 300) ? myHeight : 300;