[ticket/12966] Sort by post_time and post_id where applicable

This change was not applied to the search as this would require a larger
rewrite of the search methods.

PHPBB3-12966

phpBB/phpbb/feed/topics.php

@@ -71,7 +71,7 @@ class topics extends \phpbb\feed\topic_base
 			),
 			'WHERE'		=> 'p.topic_id = t.topic_id
 							AND ' . $this->db->sql_in_set('p.post_id', $post_ids),
-			'ORDER_BY'	=> 'p.post_time DESC',
+			'ORDER_BY'	=> 'p.post_time DESC, p.post_id DESC',
 		);
 
 		return true;