When a post is typed into the editor window, everything stays local to your machine until the 'Add Posting' button is clicked. If the server times-out, or the network misbehaves, or your PC or Browser misfire, then – as far as I can tell – the content is wiped.
I prepare long posts with a word processor first and only paste text into the forum editor screen when it's ready to go. Much less risk of a time-out because pasting is quick and there's still a master copy if the link breaks. Also helps manage long posts: a size limit makes it necessary to split large contributions across two or more posts. Easier and safer to work from a master.
Dave