it happens to me too, i think it automatically logs you out after a certain time but i've kinda sussed it now... if you think you've took long enough for it to log out then hit preview reply not send reply, then when you log back in it's usually still there.
Or you could copy it, then if you log out all you have to do is paste it back and click send
When i high light it to copy it won't let me though
I have to press ctrl+c instead.
jayne x