account the fact that there might be a need for the other scrollbar too.
it used to be that if it knew that it *did* need a vert/horz scrollbar it
would take the other one into account properly. it was just in making the
initial decision that there was an error.
git-svn-id: svn://10.0.0.236/trunk@15290 18797224-902f-48f8-a5cc-f745e15eee43