In general the unexpanded already-read system works, but there are anomalies, and I cannot define the exact situation when they happen.
As of this moment, when I open the thread http://chowhound.chow.com/topics/661072
I see all the posts expanded, including ones I wrote. I scroll through to the end so I have 'seen' them all. I am not sure that makes any difference, but I did it anyway. If I navigate away and then navigate to the same thread it is still expanded. If I close all instances of the browser (Chrome) and re-open, and navigate to this same thread then then all the posts are still expanded.
This is not true of all threads.