Index: wp-admin/js/common.js
===================================================================
--- wp-admin/js/common.js	(revision 21780)
+++ wp-admin/js/common.js	(working copy)
@@ -202,6 +202,9 @@
 			if ( parseInt( m.css('top'), 10 ) > -5 )
 				return;
 
+			if ( $(this).hasClass('wp-menu-open') )
+				return;
+
 			menutop = $(this).offset().top;
 			wintop = $(window).scrollTop();
 			maxtop = menutop - wintop - 30; // max = make the top of the sub almost touch admin bar