When in an interactive GUI, the cursor is lost and the mouse is switched to camera control if the capslock key is pressed. This made me restart the game several times trying to fix the problem until I happened to accidentally hit the capslock again.
This is on a Macbook Pro.
EDIT: I've lost the cursor when in GUIs again, but the capslock isn't fixing it now, so there is something else causing this half-gui-mode state.