This is because not all the topics in between are loaded: it only loads a couple of topics around the one you want to look & leaves gaps in between. As you scroll up, those gaps would also be loaded.

However, it seems we have a bug in there. The first time I just tested, the topic I clicked was added slightly too low, prompting the above gap to load the missing topics & cause scrolling (after that, I tried reproducing it 20+ times and it worked fine every single time, then)

Also: when it does load a gap above, it probably shouldn't stay on the position of the page where new topics were just inserted, but rather jump back to the end of the newly inserted topics (where the topic you scrolled up from still is) - we never really implemented that.

It's true we never fully implemented smooth upwards scrolling. However, this scenario "click on non-loaded topic and end up there" was working before.