gistfile1.cpp

  // Now delve down in to the message to get the HTML representation of the selection
  nsCOMPtr<nsIDocShell> rootDocShell;
  rv = aMsgWindow->GetRootDocShell(getter_AddRefs(rootDocShell));
  NS_ENSURE_SUCCESS(rv, rv);

  nsCOMPtr<nsIDocShellTreeNode> rootDocShellAsNode(do_QueryInterface(rootDocShell, &rv));
  NS_ENSURE_SUCCESS(rv, rv);

  nsCOMPtr<nsIDocShellTreeItem> childAsItem;
  rv = rootDocShellAsNode->FindChildWithName(NS_LITERAL_STRING("messagepane").get(),
                                             true, false, nullptr, nullptr, getter_AddRefs(childAsItem));
  NS_ENSURE_SUCCESS(rv, rv);