I find myself toggling Output checkbox often for testing/debugging purposes. It is tedious to click on a step, scroll the mouse to the Output checkbox to toggle, and scroll back to the step.
Please consider making the spacebar key to toggle the Output checkbox on the highlighted step.
Generally I think far too much mouse-clicking is needed when working on server actions and extra keyboard shortcuts would make a big improvement in efficency etc. However, switching output on/off is not something I find too much of a problem; you don't need to scroll as you describe - you can use the button here:
(thought a shortcut key would be welcome).
I agree the spacebar could be put to good use - eg to respond to the current step (whatever it is) in the most useful/expected way: jumping to the text input for a comment step or to open the relevant options panel for a database action etc.