Ah, that's just where they snapped into place when I dragged them over.
So, basically, you can't click through the grid? i.e. if I go below the grid I can select the bits that are below and when I'm above the grid I can select the bits poking above?
Is that a design choice for a particular purpose? I can't immediately think of what that would be used for.
Also - wow, thanks for the quick response! :-)