static _findElements(top, right, bottom, left) {
      let hits = [];
      for (let i = 0; i < ScrollSpy._elements.length; i++) {
        let scrollspy = ScrollSpy._elements[i];
        let currTop = top + scrollspy.options.scrollOffset || 200;

        if (scrollspy.$el.height() > 0) {
          let elTop = scrollspy.$el.offset().top,
            elLeft = scrollspy.$el.offset().left,
            elRight = elLeft + scrollspy.$el.width(),
            elBottom = elTop + scrollspy.$el.height();

          let isIntersect = !(
            elLeft > right ||
            elRight < left ||
            elTop > bottom ||
            elBottom < currTop
          );

          if (isIntersect) {
            hits.push(scrollspy);
          }
        }
      }
      return hits;
    }
Find elements that are within the boundary
@param {number} top
@param {number} right
@param {number} bottom
@param {number} left
@return {Array.<ScrollSpy>}   A collection of elements