The mouse re-centers itself in the game window when attempting to leave the window with libX11 active.
see conversation: http://www.opentk.com/node/3512
[X11] Improved CursorVisible = false behavior
We now use an XGrabPointer to confine the cursor to the window,
instead of the XWarpPointer hack. Fixes issue #28 and #36.
Fixed via #120