If I've navigated away it doesn't
usually remove the text so I've gone through the same situation you've discussed. Sometimes the text's removed. I don't know why going forward or back in my browser nav buttons isn't consistent each time but I'm happy it's there most of the time so I don't have to retype all of my post.
Most forums and many websites (like gmail) are able to save your post in a cookie as you type. If you navigate away by mistake, the post will be waiting for you if you go right back.
Does the billion dollar forum have that feature? I got so used to it I forgot this forum is so old... :/