Currently, when using Script Debugger, the "current step" moves down as you step through a script, until the currently-running-step is the bottom of the list.
It would make much more sense to keep the current step at the SAME point in the window, and instead to scroll the script-lines themselves past that point. That way, rather than having to repeatedly scroll every time you hit the bottom of the currently-visible list while debugging, you can keep doing what you are actually trying to do: step through the script, knowing some amount of where you just were and some amount of where you are going.
It seems like this is obviously what you'd want a debugger to do. The current behavior quickly makes it difficult to keep looking at upcoming script steps, which is an important part of debugging code.