Issue #3320055 by nod_, alexpott: Use document in bigpipe first attachBehaviors call

  const observer = new MutationObserver(processMutations);

  // Attach behaviors early, if possible.

  Drupal.attachBehaviors(document.body);

  Drupal.attachBehaviors(document);

  // If loaded asynchronously there might already be replacement elements

  // in the DOM before the mutation observer is started.