Source

File: wp-includes/class-wp-customize-widgets.php

public function tally_sidebars_via_is_active_sidebar_calls( $is_active, $sidebar_id ) {
		if ( is_registered_sidebar( $sidebar_id ) ) {
			$this->rendered_sidebars[ $sidebar_id ] = true;
		}

		/*
		 * We may need to force this to true, and also force-true the value
		 * for 'dynamic_sidebar_has_widgets' if we want to ensure that there
		 * is an area to drop widgets into, if the sidebar is empty.
		 */
		return $is_active;
	}