The big difference between those pages is that the TRS page is /latest while ubuntu uses /categories. Latest scrolls infinitely, categories is limited to 20 posts. There will always be a bit of jerkiness when your browser polls the server for more posts.
If you need something from the source you can look directly at the pages, but I’m pretty confident that rather than having some super-secret smooth scrolling code in their pages the reason is the above.
Please don’t tag people directly, particularly when a small amount of research on your part can answer these questions.
This occurs because the header is docked at the top of #main-outlet while positioned absolutely and then it changes to position: fixed and so it “jumps” to the top of the page once you scroll past the top of #main-outlet.
This is not an issue if you don’t have custom markup added before the main-outlet, but if you do, you get the results you described.
I pushed a small fix to address the issue and the transition will now be like what you see on the Ubuntu community if you add custom markup