diff -r 490d5cc509ed -r cf61fcea0001 wp/wp-includes/customize/class-wp-customize-nav-menu-section.php
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/wp/wp-includes/customize/class-wp-customize-nav-menu-section.php	Mon Oct 14 17:39:30 2019 +0200
@@ -0,0 +1,41 @@
+<?php
+/**
+ * Customize API: WP_Customize_Nav_Menu_Section class
+ *
+ * @package WordPress
+ * @subpackage Customize
+ * @since 4.4.0
+ */
+
+/**
+ * Customize Menu Section Class
+ *
+ * Custom section only needed in JS.
+ *
+ * @since 4.3.0
+ *
+ * @see WP_Customize_Section
+ */
+class WP_Customize_Nav_Menu_Section extends WP_Customize_Section {
+
+	/**
+	 * Control type.
+	 *
+	 * @since 4.3.0
+	 * @var string
+	 */
+	public $type = 'nav_menu';
+
+	/**
+	 * Get section parameters for JS.
+	 *
+	 * @since 4.3.0
+	 * @return array Exported parameters.
+	 */
+	public function json() {
+		$exported = parent::json();
+		$exported['menu_id'] = intval( preg_replace( '/^nav_menu\[(-?\d+)\]/', '$1', $this->id ) );
+
+		return $exported;
+	}
+}