I am not sure of what the exact name of the controller in key editor is but it is perhaps the most basic controller. It allows you to move between parts of the midi score - between different bars if you like.
I have found this is a very small thing to get hold of on the mouse and then can be very sensitive to moving too far. Is there some way of adjusting the size and sensitivity of this controller?
I hope I have explained this well enough for someone to understand and will appreciate any ideas someone has.