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HTML

              
                 <header>
    <h1>DOM move detection</h1>
    <p>
      This shows off using <code>IntersectionObserver</code> to track movement.
      <strong>Try dragging the gif, and scrolling the page!</strong>
      The bounding box is invalidated and recreated only through an unrelated callback, and has no knowledge of the dragging code (and it's slowed down for this demo).
      See <a target="_blank" href="https://whistlr.info/2021/observing-dom/">Observing rendered DOM nodes</a> for how!
    </p>
    <div id="info"></div>
  </header>

  <div id="vizHolder">
    <div id="gifViz" class="viz"></div>
  </div>
  <img id="gif" src="https://storage.googleapis.com/hwhistlr.appspot.com/assets/thinking-face.gif" width="128" height="128" />
              
            
!

CSS

              
                body {
  font-family: Segoe UI,system-ui,-apple-system,sans-serif;
  font-size: 12px;
  line-height: 20px;

  position: relative;
  margin: 0;
  min-height: calc(100vh + 64px);
}

* {
  margin: 0;
}

#info {
  font-family: monospace;
}

.viz {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  pointer-events: none;
  z-index: -1;
  color: blue;
}
.viz.invalid {
  color: red;
}
.viz::before,
.viz::after {
  position: absolute;
  border: 2px solid currentColor;
  box-sizing: border-box;
  content: '';
}
.viz::before {
  top: -100vh;
  bottom: -100vh;
  left: -2px;
  width: calc(100% + 4px);
}
.viz::after {
  left: -100vw;
  right: -100vw;
  top: -2px;
  height: calc(100% + 4px);
}

header {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  padding: 8px;
  background: #eeee;
  border-bottom: 4px solid #ccc9;
  z-index: 10;
}
p {
  margin: 8px 0;
}

#vizHolder {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  overflow: hidden;
}

#gif {
  position: absolute;
  user-select: none;
  left: 128px;
  bottom: 128px;
}
              
            
!

JS

              
                


// watcher.js clone (see https://gist.github.com/samthor/37a8555218e8a69ebf9f1eaf20f751db)

const debug = false;
const root = document.documentElement;


/** @type {Set<() => void>)} */
const activeObservers = new Set();


const documentResizeObserver = new ResizeObserver((entries) => {
  activeObservers.forEach((handler) => handler());
});
documentResizeObserver.observe(root);


/**
 * @param {Element} element to observe
 * @param {(rect: {x: number, y: number, width: number, height: number}) => void} callback on move or resize
 * @param {(ratio: number, force?: true) => void} callbackRatio for debugging
 * @return {() => void} cleanup function
 */
function vizObserver(element, callback, callbackRatio) {
  /** @type {IntersectionObserver?} */
  let io = null;

  const viz = document.createElement('div');
  viz.className = 'viz';
  debug && document.body.append(viz);

  let refresh = (threshold = 1.0) => {
    io?.disconnect();
    io = null;

    const rect = element.getBoundingClientRect();
    const se = /** @type {HTMLElement} */ (document.scrollingElement);
    const top = rect.top + se.scrollTop;
    const left = rect.left + se.scrollLeft;

    if (!rect.width || !rect.height) {
      callback({x: 0, y: 0, width: 0, height: 0})
      return;  // Wait for the element to be resized to a sensible size.
    }
    callback({
      x: left,
      y: top,
      width: rect.width,
      height: rect.height,
    });

    // Calculate the exact position this element holds on the page.
    const x = (v) => Math.floor(v);
    const {offsetWidth: dw, offsetHeight: dh} = root;
    const insetTop = x(top);
    const insetLeft = x(left);
    const insetRight = x(dw - (left + rect.width));
    const insetBottom = x(dh - (top + rect.height));
    const rootMargin = `${-insetTop}px ${-insetRight}px ${-insetBottom}px ${-insetLeft}px`;

    const options = {root, rootMargin, threshold};
    let isFirstUpdate = true;
    callbackRatio(threshold, true);

    io = new IntersectionObserver((entries) => {
      const only = entries[0];

      debug && console.warn('got update', only.intersectionRatio, 'first?', isFirstUpdate, 'refresh', threshold !== only.intersectionRatio);
      callbackRatio(only.intersectionRatio);
      if (threshold !== only.intersectionRatio) {
        if (!isFirstUpdate) {
          return refresh();
        }

        // It's possible for the watched element to not be at perfect 1.0 visibility when we create
        // the IntersectionObserver. This has a couple of causes:
        //   - elements being on partial pixels
        //   - elements being hidden offscreen (e.g., <html> has `overflow: hidden`)
        //   - delays: if your DOM change occurs due to e.g., page resize, you can see elements
        //     behind their actual position
        //
        // In all of these cases, refresh but with this lower ratio of threshold. When the element
        // moves beneath _that_ new value, the user will get notified.

        let update = only.intersectionRatio;
        if (update === 0.0) {
          update = 0.0000001;  // just needs to be non-zero
        }
        refresh(update);
      }

      isFirstUpdate = false;
    }, options);
    debug && console.debug('watching', {element, rootMargin, threshold});
    io.observe(element);

    viz.style.margin = `${insetTop}px ${insetRight}px ${insetBottom}px ${insetLeft}px`;
  };

  activeObservers.add(refresh);
  refresh();

  // Always add a ResizeObserver. This does nothing but force a refresh of the
  // IntersectionObserver, since the element has now changed size.
  const ro = new ResizeObserver(() => refresh());
  ro.observe(element);

  return () => {
    ro.disconnect();
    activeObservers.delete(refresh);
    io?.disconnect();
  };
}

// demo code

(function() {
  /**
   * @param {HTMLElement} element
   * @param {DOMRect} rect
   */
  function updatePosition(element, rect) {
    const top = Math.floor(rect.y);
    const left = Math.floor(rect.x);
    const width = Math.ceil(rect.x + rect.width) - left;
    const height = Math.ceil(rect.y + rect.height) - top;

    element.style.top = `${top}px`;
    element.style.left = `${left}px`;
    element.style.width = `${width}px`;
    element.style.height = `${height}px`;
  }

  let usedThreshold = 1.0;
  let lastRect = {x: 0, y: 0, width: 0, height: 0};

  const doUpdate = () => {
    updatePosition(gifViz, lastRect);
  };

  const updateWithRatio = (ratio = 1.0) => {
    info.textContent = `intersectionRatio=${ratio.toFixed(4)}`;
  };


  const initWithin = performance.now() + 100;
  let updateId = 0;
  const cleanup = vizObserver(gif, (rect) => {
    lastRect = rect;
    window.clearTimeout(updateId);

    if (performance.now() < initWithin) {
      return doUpdate();
    }

    gifViz.classList.add('invalid');
    updateId = window.setTimeout(() => {
      updateId = 0;
      gifViz.classList.remove('invalid');
      updateWithRatio(usedThreshold);
      doUpdate();
    }, 1000);
  }, (ratio, force) => {
    if (force) {
      usedThreshold = ratio;
    } else if (updateId === 0) {
      updateWithRatio(ratio);
    }
  });



  const eventHandler = (event) => {
    if (!(event.buttons & 1)) {
      return;
    }
    document.body.addEventListener('pointermove', eventHandler);

    gif.style.top = `${event.pageY - 64}px`;
    gif.style.left = `${event.pageX - 64}px`;
  };
  gif.addEventListener('pointermove', eventHandler);
  gif.addEventListener('pointerdown', eventHandler);
  document.body.addEventListener('pointerup', (event) => document.body.removeEventListener('pointermove', eventHandler));
  gif.addEventListener('dragstart', (event) => event.preventDefault());
}());
              
            
!
999px

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