Auto-concede when hitting Alt+F4, or we have to wait!
It's common to see opponents get mad when they realise they've lost the match, like when he's 100% milled. Upon seeing he's about to lose at the next draw step, instead of conceding, he just rage quits (probably with Alt+F4). In that case, the game is not finished; as far as the game is concerned, it's just the same as if the app crashed or his connection was lost, so he can hopefully be back in time to resume the game.
So when I'm about to win and notice the turn isn't passed to the opponent, it's certain: he hit Alt+F4. Now I have to wait for all of his time extensions to expire. Not a good experience.
I also believe some do this on purpose to see if we'll get impatient and quit ourselves. Not cool at all.
Please, have the player who deliberately closes the game window (or kills the process, if possible) automatically concede, so the winner doesn't have to wait.