code: function updateStepAboveIO() {
      io.stepAbove = stepEl.map((el, i) => {
        const marginTop = -offsetMargin + stepOffsetHeight[i];
        const marginBottom = offsetMargin - viewH;
        const rootMargin = `${marginTop}px 0px ${marginBottom}px 0px`;
        const options = { rootMargin };
        // console.log(options);
        const obs = new IntersectionObserver(intersectStepAbove, options);
        obs.observe(el);
        return obs;
      });
    }
docs: look above for intersection