I think you can leave your reply field open as its own window or tab, and do things elsewhere with your browser. I do this, when I'm consulting other pages. The risk is, something may go wrong, and you may loose your message in progress. I know I've accidently close a message in progress, and had to start over.
The safest, surest way, is to compose your message in a word processor, and then copy/paste to the forum.
AM