Index: wp-admin/css/wp-admin.dev.css
===================================================================
--- wp-admin/css/wp-admin.dev.css	(revision 20998)
+++ wp-admin/css/wp-admin.dev.css	(working copy)
@@ -1893,10 +1893,6 @@
   8.0 - Layout Blocks
 ------------------------------------------------------------------------------*/
 
-body.admin-bar {
-	padding-top: 28px;
-}
-
 .narrow {
 	width: 70%;
 	margin-bottom: 40px;
Index: wp-admin/includes/template.php
===================================================================
--- wp-admin/includes/template.php	(revision 20998)
+++ wp-admin/includes/template.php	(working copy)
@@ -1899,3 +1899,16 @@
 
 	return WP_Screen::get( $hook_name );
 }
+
+/**
+ * Add necessary padding for the admin bar.
+ *
+ * @since 3.4.0
+ *
+ */
+function _admin_bar_admin_bump_cb() { ?>
+<style type="text/css" media="screen">
+	html { padding-top: 28px; -webkit-box-sizing: border-box; -moz-box-sizing: border-box; box-sizing: border-box; }
+</style>
+<?php
+}
Index: wp-includes/class-wp-admin-bar.php
===================================================================
--- wp-includes/class-wp-admin-bar.php	(revision 20998)
+++ wp-includes/class-wp-admin-bar.php	(working copy)
@@ -45,8 +45,10 @@
 		if ( empty($header_callback) )
 			$header_callback = '_admin_bar_bump_cb';
 
-		add_action('wp_head', $header_callback);
+		add_action( 'wp_head', $header_callback );
 
+		add_action( 'admin_head', '_admin_bar_admin_bump_cb' );
+
 		wp_enqueue_script( 'admin-bar' );
 		wp_enqueue_style( 'admin-bar' );