Mine requires refreshes and calculations at every movement and since I'm running back and forth through the serial connection, it's a little slower than I'd like it to be.
Here's a suggestion, I am not completely sure it will work and you'll have to play about with it to get it working.
Create several separate text boxes, one above the other, one for each line of text you want to scroll, so if you want to display 8 lines create 8 boxes.
Create 2 buttons, these should be hidden out of sight by making them small (1 pixel x 1 pixel) and with the colour to match the background.
Under 1 button put something like
t7.txt=t6.txt //Moves text 6 to text 7
t6.txt=t5.txt //Moves text 5 to text 6
etc...
To activate this send from the Arduino
click b0,1
click b0,0
Which clicks the button (you have to send click press and click release, try without and you'll see why)
Do the same for the other button but in the opposite order for scrolling the other way.
You then only have to send either the top or bottom line of text.
I think this should work, but you will have to play with it.