Now, before, I had to actually HIT the Fullscreen box to uncheck it. Try using the arrow keys to navigate to the Options tab on the Stalker menu. If I move my mouse really fast on the menu screen, I can briefly see it. My steam updated the other day, and Stalker works for me as well as it did before: meaning the mouse is temporarily stuck at the bottom. I have Stalker: Shadow of Chernobyl on Steam.
I have an iMac 10.6.4, Nvidia GeForce 7300 GT, 128 vram, 1gb ram
