Source

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

public function add_panel( $id, $args = array() ) {
		if ( $id instanceof WP_Customize_Panel ) {
			$panel = $id;
		} else {
			$panel = new WP_Customize_Panel( $this, $id, $args );
		}

		$this->panels[ $panel->id ] = $panel;
		return $panel;
	}