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AFAIK all browsers implement it just as 'Event'. Gary says the WG planned to change the spec (http://crbug.com/503280#c1). This sounds like a good idea to me (makes my life easier: https://github.com/RByers/InputDevice/issues/5). I could imagine there might be reasons to want to add to the scroll event in the future (eg. I've wanted velocity and maybe other properties on it), but we can cross that bridge if/when we come to it.
Now tracking as: https://github.com/w3c/uievents/issues/47