Redirect to the same page after voting

Looks good to me.

A fallback is needed if the referer isn't set since we can't rely on the referer always being there?

Just a thought: How about something like

  drupal_goto($_GET['q'], drupal_query_string_encode($_GET, array('q')));

Shouldn't that work in all cases?

Actually, my suggestion does not work because the button goes to /plus1/vote. However, would that not ensure that a referer is set?

I don't think referers can be relied on - why not use Drupal's destination system instead?

How about

      drupal_set_message(t('Thank you for your vote.'));
      
      $path = "node/$nid";
      if (isset($_REQUEST['destination'])) {
        $path = $_REQUEST['destination'];
      }
 
      drupal_goto($path);

and

      $output_content .= '<form class="' . $vote_class . '" action="'
        . url("plus1/vote/$node->nid",
          array('query' => 'token=' . drupal_get_token($node->nid) . '&' . drupal_get_destination()))
        . '" method="post"><div>';
      $output_content .= '<button type="submit"><span>' . t($vote_text) . '</span></button>';
      $output_content .= '</div></form>';

I'm not sure, but I think the query string should use drupal_urlencode().

The drupal_goto('node/'. $nid) doesn't need to be changed - we only need to add the destination to the url. We could do that as an extension of the patch in #322735: Vote link is executed by search engines?

Too fast; we cross-posted.

Committed to 6.x-2.x-dev.