I was playing around with the editor and trying to enter a Z offset with a negative number. I noticed that I was only able to enter the hyphen (-) if there was a 0 in the field, first. If there is any other number, I could not highlight it and press hyphen (-) -- I had to first delete the number, then enter 0, and then select 0 and then press hyphen (-).

Don't highlight the current number then. If you highly the current number the output is a -, not a number so 3dnes doesn't accept it.