d3.event.sourceEvent.target in safari mobile

[gshrman]

There seems to be a discrepancy between desktop browsers and safari mobile browser when calculating d3.event.sourceEvent.target during a drag event. See example below:

https://jsfiddle.net/5ngf178o/10/

[mbostock]

Per https://www.w3.org/TR/touch-events/#touchevent-interface:

target of type EventTarget, readonly
The EventTarget on which the touch point started when it was first placed on the surface, even if the touch point has since moved outside the interactive area of that element.