I'd be willing to hazard a guess that very few borders on earth between nation states have no disputed areas, but in any case, even if this were true it is a state of affairs no more than a few hundred years old and many borders were arbitrarily drawn by world powers sometimes even for sinister reasons (for a good example, read about the border between Armenia and Azerbaijan). Throughout most of history firm borders were simply impossible to define. I object strongly to raising them to the level of a "shared reality."