I downloaded the full version of Minecraft today, and it seems to be really fun. The only problem I have, is that the mouse doesn't seem to stay focused in the Minecraft window. Usually, you would have to hit Esc to pause the game, and go to a different window. On my computer, I move my mouse around, and it doesn't stay with the crosshairs in the middle of the screen. I'm not really sure what happens, but my guess is that it thinks I clicked outside of the window, so I lose control of the mouse. I can still use the keyboard, just no mouse. To fix it temporarily, I have to hit the "e" key for inventory, and hit it again. Then I have control over my mouse until it happens again. Can anyone help me with this, and sorry if this is unclear. I've tried my best to explain it, but I really don't know whats going on.

Note: I played the exact same version on Windows, and it ran fine. No mouse/window problems at all. Oh, and my OS is Lubuntu.