title: Drag dataTransfer data unavailable in ondragover event
body: <p>I'm trying to get the hang of html5 drag and drop events.</p>
  
  <pre><code>function rowDragStart(event) {
      let targetRowId = event.target.attributes.row.value;
      event.dataTransfer.setData("text/plain", targetRowId);
  }
  
  function rowDragOver(event) {
      event.preventDefault();
      let draggingId = event.dataTransfer.getData('text');
  }
  </code></pre>
  
  <p>For now I'm just trying to put something in the dataTransfer object in dragStart, and retrieve it in dragOver.  The problem is it's not present in drag over.  I've set breakpoints and these events <em>are</em> getting called, but <code>getData('text')</code> returns empty string, along with every other possibility I can think of.</p>