The only way that more-than-2-dimensional UIs will work will be if there's an extra dimension _only_ for each of the elements ("in-and-out" for the window stack

The only way that more-than-2-dimensional UIs will work will be if there's an extra dimension _only_ for each of the elements ("in-and-out" for the window stack, etc)... otherwise people will get frustrated with mousing... Don't even get me started on 3D mice...