In case someone hasn't noticed that : the mouse cursor makes strange jumps during gameplay, so one scrolls or loos accuracy .... for sure this manifest when the screen is larger , because the cursor has more pixels to travel. Those errors should be linear due to diagonal of the screen but it ain't .... the error function rises much quicker -- so on 24' inch screen it;'s almost unmanagable.
Edit: Moved into Bug branch.
Well , I think I got it , why it happens -- your CEGUI coursor cast Ray , not always perpendicular to the floor , but one pararrel with Z camera axis, that is if one Tile's mesh is hidden by another than it would never get >>focus<< of the mouse and although during cursor movment it should be focused -- only its neighbors get it .