Not disagreeing with your overall point regarding the usefulness UX of this sort of interface, but unlike the Famo.us demo, Space.js seems to maintain the scroll bar on the right window edge, which allows you to keep a sense of where you are within the document and to jump where you want within it. Also, the down arrow alleviates the strain on your middle finger nicely, as will clicking and dragging the scroll bar.
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Not disagreeing with your overall point regarding the usefulness UX of this sort of interface, but unlike the Famo.us demo, Space.js seems to maintain the scroll bar on the right window edge, which allows you to keep a sense of where you are within the document and to jump where you want within it. Also, the down arrow alleviates the strain on your middle finger nicely, as will clicking and dragging the scroll bar.