diff --git a/wp-admin/admin-header.php b/wp-admin/admin-header.php
index 30c9e89d08..f00fab9bce 100644
--- a/wp-admin/admin-header.php
+++ b/wp-admin/admin-header.php
@@ -186,6 +186,15 @@ if ( is_network_admin() ) {
 
 $admin_body_class .= ' no-customize-support no-svg';
 
+if ( $current_screen->is_block_editor() ) {
+	// Default to is-fullscreen-mode to avoid jumps in the UI.
+	$admin_body_class .= ' block-editor-page is-fullscreen-mode wp-embed-responsive';
+
+	if ( current_theme_supports( 'editor-styles' ) && current_theme_supports( 'dark-editor-style' ) ) {
+		$admin_body_class .= ' is-dark-theme';
+	}
+}
+
 ?>
 </head>
 <?php
diff --git a/wp-includes/version.php b/wp-includes/version.php
index d1812099a9..e5c4e1f220 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.1-alpha-44132';
+$wp_version = '5.1-alpha-44133';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.