Index: wp-includes/css/editor.css
===================================================================
--- wp-includes/css/editor.css	(revision 21877)
+++ wp-includes/css/editor.css	(working copy)
@@ -1046,7 +1046,8 @@
 }
 
 .wp-editor-container textarea.wp-editor-area {
-	width: 99.9%;
+	width: 100%;
+	margin: 0;
 	-webkit-box-shadow: none;
 	box-shadow: none;
 }