Index: wp-includes/css/editor.css
===================================================================
--- wp-includes/css/editor.css	(revision 24487)
+++ wp-includes/css/editor.css	(working copy)
@@ -270,6 +270,8 @@
 }
 
 .wp_themeSkin .mceListBoxMenu {
+	width: auto !important;
+	max-width: 350px;
 	overflow: auto;
 	overflow-x: hidden;
 }