Source

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

public function tally_sidebars_via_dynamic_sidebar_calls( $has_widgets, $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 'is_active_sidebar' if we want to ensure there is an area to
		 * drop widgets into, if the sidebar is empty.
		 */
		return $has_widgets;
	}