Block Editor: Add extra body classes when the block editor is loaded.

Merges [43780] from the 5.0 branch to trunk.

See #45037.


Built from https://develop.svn.wordpress.org/trunk@44133


git-svn-id: http://core.svn.wordpress.org/trunk@43963 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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