Isn't that the same thing that the [-] sign doesn in HN? Works the same way for me.
If you click on the second minus sign on the left, it collapses the conversation up to that parent without having to scroll up to that level.
I am afraid that it is an issue with the way some browsers render the CSS sticky position. If you have any CSS knowledge, I would greatly appreciate your input!
For my information, which browser do you use?