-- MOB-to-Border collisions are evaluated on a 1-pixel-wide border around
    the visible screen.
      -- Horizontal and vertical delay are taken into account.

      -- The top and bottom edges are 1 *double-res* row thick.  This only
         matters for MOBs that have double vertical resolution.

      -- MOBs w/ X==0 never interact with anything, including the borders.

 -- MOB-to-MOB collisions are evaluated within the visible screen and the
    1 pixel border surrounding the screen.  

     -- This 1 pixel border is 1 double-res pixel tall on top and bottom. 
        It's the same border as is used for border-collision detect.

 -- MOB-to-BACKTAB collisions are evaluated within the visible screen and
    one column to the right of the visible screen.  That's it.  

     -- No MOB-to-BACKTAB collisions are calculated in the bottom edge.

     -- Horizontal and vertical delay are taken into account as with
        everything else.