Edit in JSFiddle

// // Intersection Observer V1
// const logIfIsInViewPort = (entries) => {
//   for (const entry of entries) {
//     if (entry.isIntersecting) {
//       console.log(entry);
//     }
//   }
// };
// 
// const observer = new IntersectionObserver(logIfIsInViewPort);
// observer.observe(document.querySelector('.observed'));

 // Intersection Observer V2
 const handler = (changes) => {
  for(const change in changes) {
    // feature detection
    if(typeof change.isVisible !== 'undefined') {
      if(change.isIntersecting && change.isVisible) {
        console.log(`Visible since ${change.time}`);
      } else {
        // fallback to v1 if not supported
        change.isVisible = true;
        console.log(`v2 not supported, fallback to v1`);
      }
    }
  }
}

const observer = new IntersectionObserver(handler, {
  threshold: [1.0],  
  trackVisibility: true, // this will give you the isVisible property
  delay: 100
});

observer.observe(document.querySelector('.observed'));

<div class="viewport">
  <div class="element">before scroll</div>
  <div class="observed">after scroll</div>
</div>

.viewport {
  height: 200px;
  background-color: blue;
  overflow-y: scroll;
  padding: 16px;
}

.element {
  height: 300px;
  background-color: red;
}

.observed {
  height: 100px;
  background-color: green;
}