No. In our experience these windows do not leak. Any opening window has the possibility to let in a little bit of water now and then & to collect condensation on cold days - so they are designed to minimise the possibility of leaks and to allow rapid draining of any water ingress: The opening section is completely edged with a tight fitting seal to keep water & draughts out. The sliding section moves in a black metal rail, which has a hidden drain tube at the back so that any water caught in the rail can escape to the outside quickly.
Our most recent window deliveries are now equipped with both an invisible drain towards the rear of the window and an additional drain slot towards the front.