Does Steam have special treatment of screen lock?

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http://unix.stackexchange.com – When I play a game through Steam, the computer is obviously active and I don't expect the screen to be locked. However, when I'm closing the game I see a locked screen. If the screen is locked, it should mean that in-game events are not registered by Linux as "activity" and so screen gets locked... Does the game somehow override this because if so, the screen should have been locked 15 minutes after the game? Or is it locked right after I quit it? I'm using Linux Mint with Cinnamon 2.0 if that matters. (HowTos)