When you are notified about a new comment, or new comments, we generate a link which has all the comment IDs in it to highlight them.
There are two problems with this:
* That URL hash can get very long. After ~20 comments IDs it would probably break in most browsers
* Any new comments added between you opening your notifications and you clicking on the link will not be highlighted
Both these problems can be solved by using the ID of the subscribed thread, and the timestamp of the oldest comment in the set as the hash. We can then highlight all comments in that thread with a greater timestamp.
In T301602 it is proposed to warn the users if the number of comments highlighted doesn't match the number in the notification. If we implement the above, I think we only need to warn users if there are no comments on the page to highlight.