the server automatically stops the timer when it detects that the opponent is idle.
either your opponent has a really bad internet connection or he is trying to abuse that feature.
there is a 5-10 minutes time limit that disconnects the player anyway when he is idle.
I'm working on a feature that disconnects the player earlier. It will take some days for this to be ready.
until then, you can open another client window and leave the old one open until you get the win.