IntersectionObserver: scrollMargin property

<button id="reset" type="button">Reset</button>

<div id="root-container">
  <p>content before (scroll down to carousel)</p>

  <div class="flex-container">
    <div class="carousel">
      <img
        src=""
        data-src="ballon-portrait.jpg"
        class="lazy-carousel-img"
        alt="Balloon portrait" />
      <img
        src=""
        data-src="balloon-small.jpg"
        class="lazy-carousel-img"
        alt="balloon-small" />
      <img
        src=""
        data-src="surfer.jpg"
        class="lazy-carousel-img"
        alt="surfer" />
      <img
        src=""
        data-src="border-diamonds.png"
        class="lazy-carousel-img"
        alt="border-diamonds" />
      <img src="" data-src="fire.png" class="lazy-carousel-img" alt="fire" />
      <img
        src=""
        data-src="puppy-header.jpg"
        class="lazy-carousel-img"
        alt="puppy" />
      <img src="" data-src="moon.jpg" class="lazy-carousel-img" alt="moon" />
      <img src="" data-src="rhino.jpg" class="lazy-carousel-img" alt="rhino" />
    </div>
    <div id="margin-indicator"></div>
  </div>
  <p>content after</p>
</div>

<div class="controls">
  <label>
    Set the right margin of the scroll root:
    <input id="margin" type="number" value="0" step="5" />px
  </label>
</div>

<pre id="log"></pre>

#root-container {
  height: 250px;
  overflow-y: auto;
  border: solid blue;
}

p {
  height: 50vh;
}

.flex-container {
  display: flex;
}

#margin-indicator {
  position: relative;
  height: 100px;
  width: 1px;
  background-color: red;
  opacity: 0.5;
  display: flex;
}

.carousel {
  width: 300px;
  overflow-x: auto;
  scroll-snap-type: x mandatory;
  display: flex;
  border: solid;
  /* outline: 200px solid rgba(0, 0, 0, 0.1); */
}
.carousel img {
  scroll-snap-stop: always;
  scroll-snap-align: start;
  display: block;
  width: 195px;
  height: 99px;
  min-width: 195px;
  min-height: 99px;
  margin-right: 10px;
  background-color: #eeeeee; /* Placeholder background */
}

.controls {
  margin-top: 10px;
}

#log {
  height: 100px;
  overflow: scroll;
  padding: 0.5rem;
  border: 1px solid black;
}

const reload = document.querySelector("#reset");

reload.addEventListener("click", () => {
  window.location.reload(true);
});

const logElement = document.querySelector("#log");
function log(text) {
  logElement.innerText = `${logElement.innerText}${text}\n`;
  logElement.scrollTop = logElement.scrollHeight;
}

const rootContainer = document.getElementById("root-container");
const marginIndicator = document.getElementById("margin-indicator");
const carousel = document.querySelector(".carousel");
const lazyImages = carousel.querySelectorAll(".lazy-carousel-img");
let imageObserver;

function createImageObserver() {
  if (imageObserver) {
    imageObserver.disconnect();
  }

  let observerOptions = {
    root: rootContainer,
    rootMargin: "0px", // No extra margin
    scrollMargin: `${margin.valueAsNumber}px`, // No extra margin / Can be set
    threshold: 0.01, // Trigger when 1% of the image is visible
  };

  imageObserver = new IntersectionObserver((entries, observer) => {
    entries.forEach((entry) => {
      if (entry.isIntersecting) {
        const img = entry.target;
        log(`intersect: ${img.dataset.src}`); // Only on first intersection
        img.src = `https://mdn.github.io/shared-assets/images/examples/${img.dataset.src}`; // Load image by setting src
        img.classList.remove("lazy-carousel-img"); // Remove the class
        observer.unobserve(img); // Stop observing once loaded
      }
    });
  }, observerOptions);

  if (margin.valueAsNumber < 0) {
    marginIndicator.style.width = `${-margin.valueAsNumber}px`;
    marginIndicator.style.left = `${margin.valueAsNumber}px`;
    marginIndicator.style.backgroundColor = "blue";
  } else {
    marginIndicator.style.width = `${margin.valueAsNumber}px`;
    marginIndicator.style.left = "0px";
    marginIndicator.style.backgroundColor = "green";
  }

  lazyImages.forEach((image) => {
    imageObserver.observe(image); // Start observing each image
  });
}

if ("IntersectionObserver" in window) {
  createImageObserver();
  margin.addEventListener("input", () => {
    createImageObserver();
  });
} else {
  // Fallback for browsers that don't support Intersection Observer
  // Loads all images immediately if Intersection Observer is not supported.
  lazyImages.forEach((img) => {
    img.src = img.dataset.src;
    img.classList.remove("lazy-carousel-img");
  });
  console.warn(
    "Intersection Observer not supported. All carousel images loaded.",
  );
}