Thanks Kris, that fixed it. I have noticed a couple of other issues though. .topic-navigation needs to be pushed down 40px, and .topic-post.sticky-avatar .topic-avatar seems to be calculated incorrectly when you scroll down, and it ends up behind the d-header. Also, it’s possible to expand the composer and chat window too high and lose the controls behind the d-header as well. I think that should cover everything!
Edit: one more thing, when you visit an anchor link the heading is under the header as well.