I'm working on an infinite-scroll widget, and I have a very specific insight into this: if you know the number of non-visible rows and their heights upfront, it

I'm working on an infinite-scroll widget, and I have a very specific insight into this: if you know the number of non-visible rows and their heights upfront, it can be implemented in a way that doesn't jump around. If you can estimate the heights closely, you can make the jumps less disturbing (but still noticeable in the trained eyes). Unfortunately this is rarely the case, with a few exceptions, e.g. a spreadsheet, where you know everything upfront.