Source

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

public function get_widget_control( $args ) {
		$args[0]['before_form']           = '<div class="form">';
		$args[0]['after_form']            = '</div><!-- .form -->';
		$args[0]['before_widget_content'] = '<div class="widget-content">';
		$args[0]['after_widget_content']  = '</div><!-- .widget-content -->';
		ob_start();
		wp_widget_control( ...$args );
		$control_tpl = ob_get_clean();
		return $control_tpl;
	}