function eventAtOffset(event, $item, offsets) {
                    $item.trigger($.Event(event, {
                        pageX: $item.offset().left + offsets[0],
                        pageY: $item.offset().top  + offsets[1],
                        which: 1
                    }));
                }
Simulate the given event with clientX/clientY specified by the given
offsets by the left/top of the item.
@param {string} event The name of the event to simulate.
@param {object} $item A jQuery object to trigger the event on.
@param {Array.<number>} offsets Numbers the x and y positions of the
     event relative to the item's top and left.