Show whether a thread is read, partially read, or unread

[Colonel_mortis]

At the moment, a thread is bold and has a dot if there are any unread posts, and not if you have read the whole thing. I think it would be really useful to be able to also see if you have viewed it at all, even if you didn't get to the end or there have been responses since. Sometimes a title can be a bit ambiguous but still interesting, and after clicking it you can realise that you're not actually interested in it, but next time you come back to it, it shows as unread, and you have forgotten that you had already read it, and thus read it again. Also, there can be times where a thread is more about the first post than the responses, so being able to track whether you've looked at the thread would help there.

I don't know how it could be visually differentiated (maybe some shape other than a circle?), but personally I think it would be really useful.