execCommand('copy') does not work in Ajax / XHR callback?

(Tested using Chrome 44)

Desired behaviour: Make XHR request, put result in text area, select text, and copy to clipboard.

Actual behaviour: On successful XHR request, puts the result in text area and selects it, but fails to copy result to clipboard. But if I initiate the copy outside of the XHR callback, it works.

Example html page:

var selectAndCopy = function() {
  // Select text
  var cutTextarea = document.querySelector('#textarea');
  cutTextarea.select();
  // Execute copy
  var successful = document.execCommand('copy');
  var msg = successful ? 'successful' : 'unsuccessful';
  console.log('Cutting text command was ' + msg);
};

var fetchCopyButton = document.querySelector('#fetch_copy');
fetchCopyButton.addEventListener('click', function(event) {
  var xhr = new XMLHttpRequest();
  xhr.open('get', 'http://httpbin.org/ip');
  xhr.onreadystatechange = function() {
    if (xhr.readyState === 4) {
      if (xhr.status === 200) {
        // Set text
        var textarea = document.querySelector('#textarea');
        textarea.value = xhr.responseText;

        selectAndCopy();
      }
    }
  };
  xhr.send();
});

var copyButton = document.querySelector('#copy');
copyButton.addEventListener('click', function(event) {
  selectAndCopy();
});

<html>

<head>
</head>

<body>
  <p>
    <textarea id="textarea">Hello, I'm some text!</textarea>
  </p>
  <p>
    <button id="fetch_copy">Fetch Data and Copy Textarea</button>
    <button id="copy">Copy Textarea</button>
  </p>
</body>

</html>

If you press the "Fetch Data and Copy Textarea" button the data is successfully fetched but not copied. If you press the "Copy Textarea" button the text is copied as expected. I've tried many combinations of request/copy to try and get it to work but to no avail (including programmatically pressing the copy button after fetching data). Does anyone know what's going on here? Is this a security feature or something?

I don't want the user to have to press two buttons to fetch and copy if possible.

Solution

You can only trigger a copy to the system clipboard in direct response to a trusted user action, such as a click event.

Spec: http://www.w3.org/TR/clipboard-apis/#integration-with-rich-text-editing-apis