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                <ul id='groceries'>
   <li draggable="true">A</li>
  <li draggable="true">B</li>
  <li draggable="true">C</li>
  <li draggable="true">D</li>
  <li draggable="true">E</li>
  <li draggable="true">F</li>
  <li draggable="true">G</li>
  <li draggable="true">H</li>
  <li draggable="true">I</li>
  <li draggable="true">J</li>
  <li draggable="true">K</li>
  <li draggable="true">L</li>
  <li draggable="true">M</li>
  <li draggable="true">N</li>
  <li draggable="true">O</li>
  <li draggable="true">P</li>
  <li draggable="true">R</li>
  <li draggable="true">S</li>


                  body {
    font-family: "Helvetica Neue", sans-serif;

  ul {
    list-style-type: none;
    padding-left : 0;
    user-select: none;
    -webkit-user-select: none;
    display: flex;
    align-items: center;
    width: 80%;
    flex-wrap: wrap;

  ul li {
    border: 1px solid rgb(150,150,255);
    min-width: 72px;
    width: 72px;
    height: 72px;
    box-sizing: border-box;
    text-align: center;
    margin: 12px;
    background: rgb(240,240,240);
    padding: 3px;
    display: block;
    color: black;

  ul li.insertion-marker {
    position: relative;
    height: 72px;
    min-width: 1px;
    width: 6px;
    box-sizing: border-box;
    margin: 0 -3px;
    border-radius: 2px;
    background: rgb(150,150,255);
    border: none;
    z-index: 999999;

  ul li.selected {
    background: rgb(150,150,255);
    color: white;

  tt {
    display: inline-block;
    font-size: 1.05rem;
    color: red;
    background: rgb(250,230,230);
    padding: 1px 2px;
    border: 1px solid;
    border-radius: 2px;

  .test-good, .test-bad {
    font-family: monospace;
    border: 1px solid;
    border-left: 5px solid;
    padding: 2px 5px;
    margin: 1px 0;
  .test-good {
    color: green;
    background: rgb(220,255,220);
  .test-bad {
    color: red;
    background: rgb(255,220,220);


                  const ul = document.querySelector('ul#groceries');
  const lis = ul.querySelectorAll('li');

  // Compute the centroid of an element in _page_ coordinates
  // (from the top-left of the page, accounting for the scroll).
  // We need to account for the scroll here because it is not only possible,
  // but actually _used_ by many that with long lists you can scroll while
  // you drag - pick an item, focus over the destination drop area and then scroll
  // using the wheel to "reposition" the area for your drop. Check this out, really -
  // it works like this in native macOS controls since ages.
  // Also, one of the very good indications of web-engine based apps posing as native:
  // scroll-during-drag not working correctly. We will not be like those apps.
  function computeCentroid(element) {
    const rect = element.getBoundingClientRect();
    const viewportX = (rect.left + rect.right) / 2;
    const viewportY = ( + rect.bottom) / 2;
    return {x: viewportX + window.scrollX, y:  viewportY + window.scrollY};

  function distanceBetweenCursorAndPoint(evt, centroid) {
    return Math.hypot(centroid.x - evt.clientX - window.scrollX, centroid.y - evt.clientY - window.scrollY);

  const INTENT_BEFORE = Symbol();
  const INTENT_AFTER = Symbol();
  const DIRECTION_HORIZONTAL = Symbol();
  const DIRECTION_VERTICAL = Symbol();

  function predictDirection(a, b) {
    if (!a || !b) return DIRECTION_HORIZONTAL;
    const dx = Math.abs(b.centroid.x - a.centroid.x);
    const dy = Math.abs(b.centroid.y - a.centroid.y);

  function intentFrom(direction, evt, centroid) {
    if (direction === DIRECTION_HORIZONTAL) {
     return ((evt.clientX + window.scrollX) < centroid.x) ? INTENT_BEFORE : INTENT_AFTER;
    } else {
     return ((evt.clientY + window.scrollY) < centroid.y) ? INTENT_BEFORE : INTENT_AFTER;

  function startReorderWithElement(el, {debug}) {
    const parent = el.parentNode;
    const orderables = Array.from(parent.children).map((element, i) => {
      return {i, element, centroid: computeCentroid(element)};

    // Determine the dominant direction in the list - is it horizontal or vertical?
    const direction = predictDirection(orderables[0], orderables[1]);

    let closest = el;
    let intent = INTENT_AFTER;
    let marker = document.createElement(el.nodeName);

    const unstyle = () => {
      orderables.forEach(({element}) => {

    const mouseMoveHandler = (evt) => {

      const byDistance = => {
        return {distance: distanceBetweenCursorAndPoint(evt, orderable.centroid), ...orderable};
      }).sort((a, b) => a.distance - b.distance);

      closest = byDistance[0].element;
      intent = intentFrom(direction, evt, byDistance[0].centroid);


      byDistance.forEach(({element}) => {
        if (element !== closest) return;
        if (intent === INTENT_BEFORE) {
          marker = element.insertAdjacentElement('beforebegin', marker)
        } else {
          marker = element.insertAdjacentElement('afterend', marker)
    parent.addEventListener('dragover', mouseMoveHandler);

    const stopFn = () => {
      parent.removeEventListener('dragover', mouseMoveHandler);
      return {closest, intent};

    return stopFn;

  lis.forEach((li) => {
    li.addEventListener("dragstart", (evt) => {
      console.warn("reorder started")
      const stop = startReorderWithElement(li, {debug: true});

      li.parentNode.addEventListener("drop", (evt) => evt.preventDefault(), {once: true});
      li.addEventListener("dragend", (evt) => {

        console.warn("reorder ending");

        const {closest, intent} = stop();
        if (intent === INTENT_BEFORE) {
          closest.insertAdjacentElement('beforebegin', li);
        } else {
          closest.insertAdjacentElement('afterend', li);
      }, {once: true});