> The reason we needed a new event is that mousewheel is not dispatched for > anything but vertical scrolling and that authors are expecting it to be > dispatched for vertical scrolling only.
I don't think it is wise to complicate the API to cater to developers that don't handle horizontal scrolling in their web app. I prefer an easier-to-manage and simpler approach for the long term. For existing apps, it doesn't seem like it would be that hard to short-circuit the mousewheel event handlers to check for a non-zero wheelDelta before continuing with the code.
