Comment 6 for bug 1313610

1. We allow non-contiguous screen layouts.
2. Even with contiguous screen layouts the bounding rectangle of the outputs can contain areas that are not on any output.

On the Unity7 desktop the cursor appears to be restricted to the bounding rectangle. (But it can be confusing if it moves off all monitors.)

I suspect that if we simply ensured that the screen layout is contiguous then we would have reasonable behaviour.