This doesn't happen every time that I switch between the game world tab and another tab or window, but several times I've had problems with focus after switching. Pressing enter takes focus off the game world and I think is putting it on the chat. Also, pressing space wont work either. I'm using Google Chrome, in case that makes a difference. Sometimes if I fiddle around I can get everything to work properly again, but generally I've just left the game world and come back to fix the problem.