As discussed in the separate thread, it seems that there is no possibility to view all posts in a thread when replying.
If you click Reply (on top of the thread, just beside Topic Options) you see just the first post of a thread. If you click Reply in any post, you see just the post you're replying to. If you click Quick Reply in any post, a small frame in the post you're replying to is opened but it is not the full featured reply (no emoticons, links, pictures etc.). Another option is the link to "View Discussion in a Popup" on the reply screen but it still opens another window/tab.
Therefore I would like to see all posts in a thread when replying regardless on the manner I reply (to a post or to a thread). In order to save space, posts could be listed in a plain text (without emoticons, pictures etc.).