diff -r 490d5cc509ed -r cf61fcea0001 wp/wp-includes/customize/class-wp-customize-sidebar-section.php
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/wp/wp-includes/customize/class-wp-customize-sidebar-section.php	Mon Oct 14 17:39:30 2019 +0200
@@ -0,0 +1,58 @@
+<?php
+/**
+ * Customize API: WP_Customize_Sidebar_Section class
+ *
+ * @package WordPress
+ * @subpackage Customize
+ * @since 4.4.0
+ */
+
+/**
+ * Customizer section representing widget area (sidebar).
+ *
+ * @since 4.1.0
+ *
+ * @see WP_Customize_Section
+ */
+class WP_Customize_Sidebar_Section extends WP_Customize_Section {
+
+	/**
+	 * Type of this section.
+	 *
+	 * @since 4.1.0
+	 * @var string
+	 */
+	public $type = 'sidebar';
+
+	/**
+	 * Unique identifier.
+	 *
+	 * @since 4.1.0
+	 * @var string
+	 */
+	public $sidebar_id;
+
+	/**
+	 * Gather the parameters passed to client JavaScript via JSON.
+	 *
+	 * @since 4.1.0
+	 *
+	 * @return array The array to be exported to the client as JSON.
+	 */
+	public function json() {
+		$json = parent::json();
+		$json['sidebarId'] = $this->sidebar_id;
+		return $json;
+	}
+
+	/**
+	 * Whether the current sidebar is rendered on the page.
+	 *
+	 * @since 4.1.0
+	 *
+	 * @return bool Whether sidebar is rendered.
+	 */
+	public function active_callback() {
+		return $this->manager->widgets->is_sidebar_rendered( $this->sidebar_id );
+	}
+}