_detectNearestEmptySortable = function _detectNearestEmptySortable(x, y) {
      var ret;
      sortables.some(function (sortable) {
        var threshold = sortable[expando].options.emptyInsertThreshold;
        if (!threshold || lastChild(sortable)) return;
        var rect = getRect(sortable),
          insideHorizontally = x >= rect.left - threshold && x <= rect.right + threshold,
          insideVertically = y >= rect.top - threshold && y <= rect.bottom + threshold;
        if (insideHorizontally && insideVertically) {
          return ret = sortable;
        }
      });
      return ret;
    }
Detects first nearest empty sortable to X and Y position using emptyInsertThreshold.
@param  {Number} x      X position
@param  {Number} y      Y position
@return {HTMLElement}   Element of the first found nearest Sortable