Get the current active element safely.

safeActiveElement.js

/**
 * Get the current active element safely.
 * Ref: https://github.com/jquery/jquery-ui/blob/2b84531ae9331f60e4d739fabca6d78abde89ae1/ui/safe-active-element.js
 */
function safeActiveElement(doc) {
  doc = doc || document;
  var activeElement;

  // Support: IE 9 only
  // IE9 throws an "Unspecified error" accessing document.activeElement from an <iframe>
  // Support: IE 9 - 11 only
  // IE may return null instead of an element
  // Interestingly, this only seems to occur when NOT in an iframe
  // Support: IE 11 only
  // IE11 returns a seemingly empty object in some cases when accessing
  // document.activeElement from an <iframe>
  try {
    activeElement = doc.activeElement;
    if (!activeElement || !activeElement.nodeName) {
      activeElement = doc.body;
    }
  } catch ( error ) {
    activeElement = doc.body;
  }

  return activeElement;
}

Thanks for this, note that IE 11 apparently also throws an "Unspecified error" accessing document.activeElement from an <iframe>... That is what led me to this.