thand target
	 * lists keyed by different fields. Casting to string keeps numeric
	 * identities matching whether they arrive as an int or a string. Anything
	 * else (e.g. a nested list) has no identity and never matches, so it is
	 * always appended.
	 *
	 * @since 7.1.0
	 *
	 * @param mixed $item The list member.
	 * @return string|null The identity, or null when the member has none.
	 */
	private function list_item_identity( $item ) {
		if ( is_scalar( $item ) ) {
			return (string) $item;
		}

		if ( is_array( $item ) && ! array_is_list( $item ) ) {
			foreach ( array( 'id', 'slug', 'field' ) as $key ) {
				if ( isset( $item[ $key ] ) && is_scalar( $item[ $key ] ) ) {
					return (string) $item[ $key ];
				}
			}
		}

		return null;
	}
}