Right; reply being a distinct action you have to trigger, rather than a prefilled input box is a highly intentional design element from the earliest days of Discourse. We want to encourage reading, not immediate replying because “someone is wrong on the internet”. I think a lot of current problems on the internet can be traced to too many people responding too quickly, with not enough thought.
The introduction of the chat feature should also take pressure off the “I must immediately respond with the first emotion that comes to mind” impulse as well, and direct it more appropriately.
If you want sidebar, that’s a Teams feature.