IIRC the captcha field was autofocused and the position is ok, but if I press enter nothing happens, I have to go to the "Reply" button to submit. Pressing enter should submit, for consistency with e.g. the reason field in action=edit and special pages.

(The captcha field appeared after the first submission attempt because I included an URL, but that's not a Flow-specific issue, see T90866: "Incorrect or missing CAPTCHA" should be closely associated to the captcha field.)