...when the server is otherwise down.

I rely on clicking the check mark-like icon to the left of the subject to continue reading long threads.

So, if the server is down, can't it simply not record the last visited time?

Sorry, it is not something we can set I am sorry currently. The good news is we really do not go down a lot unless we really have something going on. (Like today with the database.)

In the future we will have all traffic sent to a closed page thus it will not be an issue.