In chrome and firefox, when I hit preview or post, (If log-in time is expired) it'll shoot me over to the log-in screen, log-in, it takes you to an empty post box, hit the back button a couple of times, you land on your previous written non-previewed/non-posted post, hit preview or post and you're all set.
Also, at log-in, you can specify the number of minutes you stay logged in. I think. Maybe that's the other site.