Index: wp-admin/css/customize-widgets.css
===================================================================
--- wp-admin/css/customize-widgets.css	(revision 32126)
+++ wp-admin/css/customize-widgets.css	(working copy)
@@ -342,10 +342,11 @@
 #available-widgets {
 	position: absolute;
 	overflow: auto;
-	top: 46px;
+	top: 0;
 	bottom: 0;
 	left: -301px;
 	width: 300px;
+	padding-top: 46px;
 	margin: 0;
 	z-index: 1;
 	background: #fff !important;