comment_form_submit redirects to example.com/node/123#comment-123 regardless of whether the comments span multiple pages or not. This means that if you reply from page 3, you end up back at the original node rather than your own comment, and have to use the pager (and some scrolling often) to get back to it.
This patch adds a function comment_last_page, and changes the $form_state['redirect'] behaviour to go to the correct page, so users will still see their new comment if it's on a subsequent page.
If comments are moderated, then users don't see their comment, this is consistent with current behaviour, except they'll be on the page they replied from.
tested in 5.1, this is against 6.x though so hopefully I got the changes right.