mbostock/d3 — pull request #368 by jasondavies (2011-11-05 17:46:10+00:00)
Title: Dispatch "dragstart" on mousemove.

Previously this was being dispatched on mousedown, which doesn't necessary indicate a drag gesture has started (it might be a stationary click).

Also, call event.preventDefault() on mousedown (to prevent default behaviours like text selection).

This is backwards-incompatible if you relied on d3.event.{dx,dy} being zero on dragstart, but I doubt anyone would have relied on that.