As per title.

As far as I know phpBB is supposed to support things like adding posts to a topic with a lower timestamp than the last post (i.e. adding posts between two other posts in the topic). For example a modification might do that.

While viewtopic etc. order fine by post_time instead of post_id, it looks like the sync() function doesn't take into account that the lowest post_id does not necessarily need to be the topic's first_post_id.

Some snippets ...

$sql = 'SELECT t.topic_id, t.post_approved, COUNT(t.post_id) AS total_posts, MIN(t.post_id) AS first_post_id, MAX(t.post_id) AS last_post_id

$topic_data[$topic_id]['first_post_id'] = (!$topic_data[$topic_id]['first_post_id']) ? $row['first_post_id'] : min($topic_data[$topic_id]['first_post_id'], $row['first_post_id']);