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HTML

              
                <!-- Learn about this code on MDN: https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API -->

<section class="global_wrapper">
  <main><h2>Main Contents</h2></main>
  <aside class="sidebar">
    <h2>Sidebar Contents</h2>
    <p> scroll down↓</p>
  <div class="sampleBox">
    <div class="label topLeft"></div>
    <div class="label topRight"></div>
    <div class="label bottomLeft"></div>
    <div class="label bottomRight"></div>
    <div class="mark top">trigger→</div>
        <div class="mark second">trigger→</div>
        <div class="mark middle">trigger→</div>
            <div class="mark fourth">trigger→</div>
        <div class="mark bottom">trigger→</div>
  </div>

  </aside>
</div>
              
            
!

CSS

              
                .contents {
  position: absolute;
  width: 700px;
  height: 1725px;
}

.global_wrapper{
  display: flex;
  max-width: 100%;
}
main{
  background: #c1c1c1;
  flex-basis: 60%;
  border: gray 5px solid;
}
aside{
  flex-basis: 40%;
  border: gray 5px solid;
  height: 300px;
  overflow: scroll;
}

.mark{
  color: red;
  font-size: 1.3em;
  position: absolute;
  top: 0%;
  left: -80px;
  transform: translateY(-50%);
}
.second{
  top: 25%
}
.middle{
  top: 50%
}
.fourth{
  top: 75%
}
.bottom{
  top: 100%
}
.sampleBox {
  position: relative;
  width: 300px;
  background-color: rgb(245, 170, 140);
  border: 2px solid rgb(201, 126, 17);
  padding: 4px;
  margin: 250px 100px;
  height: 200px;
}

.label {
  font: 14px "Open Sans", "Arial", sans-serif;
  position: absolute;
  margin: 0;
  background-color: rgba(255, 255, 255, 0.7);
  border: 1px solid rgba(0, 0, 0, 0.7);
  width: 3em;
  height: 18px;
  padding: 2px;
  text-align: center;
}

.topLeft {
  left: 2px;
  top: 2px;
}

.topRight {
  right: 2px;
  top: 2px;
}

.bottomLeft {
  bottom: 2px;
  left: 2px;
}

.bottomRight {
  bottom: 2px;
  right: 2px;
}

              
            
!

JS

              
                

startup();

function startup() {
  let sidebar = document.querySelector("aside");

  // Options for the observers

  const option = {
    root: sidebar,
    rootMargin: "0px",
    threshold: [0, 0.25, 0.5, 0.75, 1]
  };
    let observer = new IntersectionObserver(intersectionCallback, option);
    observer.observe(document.querySelector(".sampleBox"));
}

function intersectionCallback(entries) {
  entries.forEach(function(entry) {
    let box = entry.target;
    let visiblePct = (Math.floor(entry.intersectionRatio * 100)) + "%";

    box.querySelector(".topLeft").innerHTML = visiblePct;
    box.querySelector(".topRight").innerHTML = visiblePct;
    box.querySelector(".bottomLeft").innerHTML = visiblePct;
    box.querySelector(".bottomRight").innerHTML = visiblePct;
  });
}

              
            
!
999px

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