diff --git a/wp-admin/site-editor.php b/wp-admin/site-editor.php
index e5d149f235..14c190be8c 100644
--- a/wp-admin/site-editor.php
+++ b/wp-admin/site-editor.php
@@ -31,6 +31,14 @@ $parent_file = 'themes.php';
 $current_screen = get_current_screen();
 $current_screen->is_block_editor( true );
 
+// Default to is-fullscreen-mode to avoid jumps in the UI.
+add_filter(
+	'admin_body_class',
+	static function( $classes ) {
+		return "$classes is-fullscreen-mode";
+	}
+);
+
 $block_editor_context = new WP_Block_Editor_Context();
 
 $active_global_styles_id = WP_Theme_JSON_Resolver::get_user_custom_post_type_id();
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 6b2cdbbac1..c4484f5fc5 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-52070';
+$wp_version = '5.9-alpha-52071';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.