Is there a way to prevent this from happening when the browser’s scrollbar is used? You see the normal undocked header until replies load, and then the extra-info header comes back again.
Without customizations this is just a minor title flicker, but if you have a larger customized header that goes away on scroll (a secondary nav for example), it’s much more disruptive. I’d expect to always see the extra-info header unless I’ve reached the top of the topic (right now I assume we’re relying on reaching the top of the viewport).