I'm not sure why, but suddenly the "window_handles" method disappeared in cases, when this library is used on PHP 5.x (e.g. https://travis-ci.org/minkphp/MinkSelenium2Driver/jobs/394658845).
Maybe different versions of this library are getting picked by Composer for PHP 5.x (non-working) and PHP 7.x (working).
Please help.