I have used that for a quite some time on RS. There is a problem however if you only have time to read a couple of posts and not all and then when you come back new posts will show up and you will miss the previous ones you didn't read. Overall it is an awesome feature and I personally would be lost without it!
Yes, if you keep hitting the New Posts button, that is how it will work.
A better way to use it is to keep hitting the Back button to go back to your original new posts list, so you can go through the entire list.
The best way is to use Firefox (or IE 7) and use tabbed browsing: keep your search list in the original tab, and then open several threads in their own tabs.
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