code: function(element, x, y) {
      if (this.includeScrollOffsets)
        return this.withinIncludingScrolloffsets(element, x, y);
      this.xcomp = x;
      this.ycomp = y;
      this.offset = this.cumulativeOffset(element);
  
      return (y >= this.offset[1] &&
              y <  this.offset[1] + element.offsetHeight &&
              x >= this.offset[0] &&
              x <  this.offset[0] + element.offsetWidth);
    }
docs: caches x/y coordinate pair to use with overlap