Ah well still a browser then. Not sure what the forum's timeout is, 10, 15, or 20 minutes.
I've had good success with the "keep me logged in" checkboxes (not sure if the forum has one--been a while since I've had to relogin). Usually they are there right when you log in (I also use Chrome which seems to also keep me logged in); thus I never have to re-login getting back to the forum.
Another option would be to compose your response in a local text editor on your Mac/iPad and copy paste it into the post when you're ready.