1.Is there any reason you can think of why not expending replies into a scrollable box. With max-height of approx the window height.
2.It is also worth considering with @sam suggestion and current behavior a scenario where the user expanded a post with several lengthy replies.
The user start scrolling down and reading the replies. Mid reading the user lost interest in that particular sub-conversation, or possibly decided to check the original post or to continue reading the next post.
what should the user do?
a.scroll all the way up to collapse to check original post.
b.scroll all the way down to collapse and continue reading or check the original post.