This happens fairly regularly to me on another discourse forum I use (where the minimise composer arrow occasionally goes off screen). I’m afraid I don’t know how too repro it though.
I’m using an Honor 9, with Android 9 and Chrome browser.
Edit: Sorry, this may be a different issue. I’ve just had a fiddle, and it’s only the minimise arrow that gets pushed offscreen for me, not the right side of the composer as well.
Mine appears to be when I reply to a topic with a long title. And it only seems to affect the other forum I use, and not meta.