When focus is lost for a fullscreened window, it minimizes. Again, the window manager handles stuff like that with NetWM fullscreen and may not be required or desired... When you have multiple monitors and you click into a different window, it ends up minimizing the fullscreened window. I assume this was to allow for alt+tabbing with an actual fullscreened window. However, once restored, the cursor is unable to leave the window. I assume this is a bug and the cursor is just always supposed to be unable to leave the window...which would mean the minimizing window when focus is lost would actually make sense.
edit: However, some applications would want to have the cursor able to leave the window....So I am unsure which is the bug..perhaps this should only happen for an actual exclusive fullscreen window?