+ let touchEvent = aEvent.type.indexOf('touch') != -1;
+
+ // Bail out if this is neither a touch nor left-click.
+ if (!touchEvent && aEvent.button != 0)
+ return;
+
+ // Bail out if the started touch can't be found.
+ if (touchEvent && zoomstart &&
+ !aEvent.changedTouches.identifiedTouch(zoomTouchID))
+ return;
+
+ let coordObj = touchEvent ?
+ aEvent.changedTouches.identifiedTouch(zoomTouchID) :
+ aEvent;
+