I agree, its not too bad in Discourse, but I find it annoying in other places, Google Images is one example.
I think it is because I sometimes drag the scroll block when I want to scan through something quickly. So when more results appear, and I’m 75% down the bar, I remain at 75%, but because the page got longer, that means I’ve skipped a whole chunk and have to go back if I don’t want to miss that chunk.