I want to place “already open comment box” below last reply so user can reply quickly and have more attractive on this. How to do it?
AI-generated summary
A user tran_tien requested a feature to place an always-open comment box below the last reply for quicker responses, noting it would improve UX. justin confirmed it’s technically possible but complex, recommending custom development via the Marketplace category — with a warning that such changes may require ongoing maintenance due to potential core Discourse updates. ddf expressed surprise that Discourse lacks this feature, common on social networks. awesomerobot clarified that Discourse is designed for threaded, long-form discussions — not social media — and that the current composer allows users to browse while composing, which is intentional. merefield joked that such a change would make the ‘Fakebook’ theme even more authentic, but agreed it’s best suited as a third-party plugin. The consensus is that while the feature is desirable for some, it contradicts Discourse’s core design philosophy and would require custom development with maintenance overhead.
It’s possible, but fairly complex to do. The best route is probably to have it custom developed. You can talk to a developer in the Marketplace category about possibly doing that. One word of advanced warning – doing something like this may require a lot of long-term maintenance, as a major interactive change can break easily if any methods you’re using from Discourse core changes.
Alot of social network site have this function by default, very surprise that Discourse do not.
While there are some similarities Discourse isn’t exactly a social network and is meant to be focused on longer discussions.
The composer gives you the benefit of being able to scroll through the current topic or even visit other topics while you’re writing. We have no plans to change this, so if it’s a necessity you’ll need some custom work.
It would make ‘Fakebook’ even more authentic, though, no? 
But agree, this is screaming third party plugin 
Marketplace