Index: wp-admin/js/common.dev.js
===================================================================
--- wp-admin/js/common.dev.js	(revision 18984)
+++ wp-admin/js/common.dev.js	(working copy)
@@ -195,7 +195,7 @@
 
 		if ( body.hasClass('folded') ) {
 			body.removeClass('folded');
-			deleteUserSetting('mfold');
+			setUserSetting('mfold', 'o');
 		} else {
 			body.addClass('folded');
 			setUserSetting('mfold', 'f');