Right now the input field is tiny and missing lots of functionality like canned replies when you send a message to a user about one of this posts via the Flag icon.
I understand that it doesn’t make sense to improve that small input field and keep it limited, but a small “Switch to normal ‘Start a Message’ screen” link in the bottom right of the input field would solve these problems.
Clicking it you go to the “send message” field with the username and quoted post prefilled (what the system does in the background when you use the tiny input).