This allows starship fleets to circumvent nebulas that extend to the map's borders (actually these nebulas appear to extend beyond the visible portion of the map, but their external dimensions can only be inferred).
Starships can travel long distances outside the map in some cases, allowing them to be moved across the galaxy without other players being able to detect them.
In some cases jumpship fleets can also exit the map when pathing around rift zones.
I think the "visible region" of maps should be circular rather than square to decrease the effects of starting in a corner (also discussed in http://ministry.kronosaur.com/record.hexm?id=59785). If nebulas are going to extend to the edge of the visible map, it should not be possible for fleets to pathfind around them through unexplorable space (possibly by creating an invisible nebula just outside the visible bounds of the map and going all around the edge of the map.)
The lambda of the random function producing the planet point pattern could drop off near the edges of the map, producing an attenuated peripheral zone containing relatively few planets. Players could start in and vie primarily for the rich galactic core, with the edges being a sparse backwater that fleets can navigate through as a strategic maneuver but that does not support extensive colonization under most strategies.
Another alternative is to reduce the probability of starting near an edge.
And I agree that you should not be able to pathfind beyond the edge of the map.
As suggested in the forums, a way of fixing this might be generating a solid square rift zone around the edges of the map just outside the visible area.
Fixed on server.