WP_Customize_Setting::_preview_filter( mixed $original )

	public function _preview_filter( $original ) {
		if ( ! $this->is_current_blog_previewed() ) {
			return $original;
		}

		$undefined = new stdClass(); // Symbol hack.
		$post_value = $this->post_value( $undefined );
		if ( $undefined !== $post_value ) {
			$value = $post_value;
		} else {
			/*
			 * Note that we don't use $original here because preview() will
			 * not add the filter in the first place if it has an initial value
			 * and there is no post value.
			 */
			$value = $this->default;
		}
		return $value;
	}