Index: src/wp-admin/css/edit.css
===================================================================
--- src/wp-admin/css/edit.css	(revision 51498)
+++ src/wp-admin/css/edit.css	(working copy)
@@ -634,7 +634,8 @@
 
 #poststuff h3.hndle, /* Back-compat for pre-4.4 */
 #poststuff .stuffbox > h3, /* Back-compat for pre-4.4 */
-#poststuff h2 {
+#poststuff h2,
+.poststuff h2 {
 	font-size: 14px;
 	padding: 8px 12px;
 	margin: 0;
@@ -649,7 +650,8 @@
 	border-bottom: 1px solid #f0f0f1;
 }
 
-#poststuff .inside {
+#poststuff .inside,
+.poststuff .inside {
 	margin: 6px 0 0 0;
 }
 
Index: src/wp-admin/includes/post.php
===================================================================
--- src/wp-admin/includes/post.php	(revision 51498)
+++ src/wp-admin/includes/post.php	(working copy)
@@ -2278,8 +2278,8 @@
 	</form>
 	<?php foreach ( $locations as $location ) : ?>
 		<form class="metabox-location-<?php echo esc_attr( $location ); ?>" onsubmit="return false;">
-			<div id="poststuff" class="sidebar-open">
-				<div id="postbox-container-2" class="postbox-container">
+			<div class="poststuff sidebar-open">
+				<div class="postbox-container">
 					<?php
 					do_meta_boxes(
 						$current_screen,