On full-width themes the composer will take full-width on smaller screens and push the sidebar up (default behaviour as before), and position next to the sidebar on larger screens:
I very much hope that this design, if it will be pushed to every discourse platform - will be switchable. It’s the norm for me to write very long articles with pictures on forums (while having to not open full screen because I need to quote other posts). Reducing the width makes it hard for me to scroll up and down
I wonder: have you tried our full-width theme by any chance? I personally like the feel of that much better, and it gives you, as implied, also more space for everything, including the composer.