Rufus,
Based on what you said, what is clear for me is that your session was timed out (aka automatically logged out due to 'inactivity') while you were composing the post. My best guess is you have NOT chosen the "Log me on automatically each visit" checkbox in the log in form. So, the server automatically logs you out if it didn't receive a request from you for about 15 minutes.
My browser tells me that (at least on the "compose" page) the page reloads all the time (I don't know the interval but the reload requests seem to stack up quickly).
If there was something in the composer page that sends anything to our server, it would have prevented your session getting timed out. But I don't think we have something like that, at least AFAIK. Can you please show us a screenshot of what your browser tells you?