I’m writing this as an experience report for now:
I have just edited the translation of this post: Introducing our new composer, making writing on Discourse easier than ever. (Somehow some parts were missing in the German version 
).
First, the border box that moves with the scrolling was a bit irritating. I would have expected to scroll within it.
Second, I would have liked to check the preview of my translation several times in between. Especially with the quotations, but also to integrate the images correctly into the list. In the end, I simply copied everything and looked at it in a normal composer and then pasted it back into the translation.
While we’re on the subject of using the composer. In between, I wanted to make notes for this post. It was a bad idea. I made the mistake of minimizing the composer in which I was editing the translation and then I clicked on ‘new topic’. If the composer is not minimized, the typical question appears whether you want to discard or continue. When the composer is minimized, this does not appear. Everything I had already done was gone 
So I went back to inserting all the missing images by quoting and removing the quote formatting and adding the block quotes. Adding the images would have been easier if I could also look at the raw version of the original post. Then I could have copied them from there. Also finding the correct emoji would have been easier then.
So I guess I would like an original post/preview toggle and a button to view the raw of the OP, maybe similar to the one in the edit history. And I would like to get a warning when I minimize the composer and clicking new topic discards my work.