在此处将this替换为lastEle：

function handleDragEnter(e) {
  //lastEle ? this.classList.remove('over'): '';
  lastEle ? lastEle.classList.remove('over'): '';
  this.classList.add('over');
  lastEle = this;
}

document.addEventListener('DOMContentLoaded', (event) => {
  var dragSrcEl = null;
  let lastElem;

  function handleDragStart(e) {
    this.style.opacity = '0.4';

    dragSrcEl = this;

    e.dataTransfer.effectAllowed = 'move';
    e.dataTransfer.setData('text/html', this.innerHTML);
  }

  function handleDragOver(e) {
    if (e.preventDefault) {
      e.preventDefault();
    }

    e.dataTransfer.dropEffect = 'move';

    return false;
  }

  function handleDragEnter(e) {
    lastElem ? lastElem.classList.remove('over') : '';

    this.classList.add('over');

    lastElem = this;
  }

  function handleDragLeave(e) {
    // this.classList.remove('over');
  }

  function handleDrop(e) {
    if (e.stopPropagation) {
      e.stopPropagation(); // stops the browser from redirecting.
    }

    if (dragSrcEl != this) {
      dragSrcEl.innerHTML = this.innerHTML;
      this.innerHTML = e.dataTransfer.getData('text/html');
    }

    return false;
  }

  function handleDragEnd(e) {
    this.style.opacity = '1';

    items.forEach(function (item) {
      item.classList.remove('over');
    });
  }

  let items = document.querySelectorAll('.container .box');
  items.forEach(function (item) {
    item.addEventListener('dragstart', handleDragStart, false);
    item.addEventListener('dragenter', handleDragEnter, false);
    item.addEventListener('dragover', handleDragOver, false);
    item.addEventListener('dragleave', handleDragLeave, false);
    item.addEventListener('drop', handleDrop, false);
    item.addEventListener('dragend', handleDragEnd, false);
  });
});

试试这个变化。它会工作。