I do understand your point, but it seems that in this case you need to extend Flarum as well.
Just one hint:
If you reply by clicking into the post reply area at the buttom, additional space is created at the bottom to allow the reply area to stay below the preview of the new post. In order not to cover this preview the page is scrolled down to the new (shifted) bottom.
If you click the Reply button below an existing post, the above mentioned additional space is created as well but there is no scrolling to the bottom. Try to find out how these two ways of replying differ programmatically.
