I am currently using a trackball and I am not satisfied with the scroll wheel. I wonder if there is a key combination to enable "free roaming, scrolling" mode, which locks the mouse and scrolls the screen according to the mouse motion, just like two-finger scrolling in trackpads. So that I will map the extra key on the trackball with that combination and scroll around with the ball while pressing that button.
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Can you please provide some more information? Which version of OS X are you using? What is the model of trackball you are using? – user26787 Nov 17 '15 at 20:42
Okay I actually found it. I installed Keyremap4Macbook and mapped CursorMove to ScrollWheel.