This is amazing, and appeared exactly when I was thinking of taking another look at this functionality 
One thing I would consider is making the scroll bar somehow obvious. For example here: Understanding Discourse Trust Levels I see:
This looks like there is three comments. When I hover there I see the tiny silver slither of a default scroll bar top right(Firefox, linux).
So if I don’t know what Discourse is, I assume there is currently three replies, and that clicking the reply next to Jay’s name is the generic reply to topic button. The unfortunate reality is that Jay will now get notifications of a reply every time someone makes this mistake.
I wonder if there is:
a) a way to bring the Reply to Topic button into view at all times?
b) a way to make the scroll more obvious, whilst still staying elegant and non invasive