<div class="container">
  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item">3</div>
  <div class="item">4</div>
  <div class="item">5</div>
  <div class="item">6</div>
</div>

body {
  font-family: Avenir, Helvetica, Arial, sans-serif;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  color: #2c3e50;
  position: relative;
  box-sizing: border-box;
}

.container {
  max-width: 960px;
  display: flex;
  overflow: scroll;
  margin: 0 auto;
  gap: 10px;
}

.item.focused {
  background-color: #9288FF;
  filter: grayscale(0);
}

.item {
  background-color: #dcdcdc;
  display: flex;
  flex-shrink: 0;
  width: 300px;
  height: 200px;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  padding: 0;
  margin: 0;
  text-align: center;
  filter: grayscale(1);
  transition: filter 300ms ease-in-out;
}

let root = document.querySelector(".container");
let elements = document.querySelectorAll(".container .item");

let rootWidth = root.offsetWidth;
let itemWidth = elements[0].offsetWidth;
let rootMargin = (rootWidth - itemWidth) / 2;

elements[0].style.marginLeft = rootMargin + "px";
elements[elements.length - 1].style.marginRight = rootMargin + "px";

// hide left filler item
root.scrollLeft = rootMargin;

let centerObserver = new IntersectionObserver(
  function (entries) {
    entries.forEach((entry) => {
      if (entry.isIntersecting) {
        entry.target.classList.add("focused");
      } else {
        entry.target.classList.remove("focused");
      }
    });
  },
  {
    root: root,
    threshold: [0.51],
    rootMargin: `0px -${rootMargin}px 0px -${rootMargin}px`
  }
);

elements.forEach((element) => {
  centerObserver.observe(element);
});

let documentObserver = new IntersectionObserver(
  ([root]) => {
    // when items go out of viewport, hide left filler item
    if (!root.isIntersecting) root.scrollLeft = rootMargin;
  },
  { threshold: 0 }
);
documentObserver.observe(root);