The server will know how far you scrolled in a session by what images/iframes are being downloaded.

With server side implications. The worst case would be a website stacking 1px images vertically to detect good far your scrolled

I see.. but this would only make a difference for people who disabled JS, and these people can probably also disable lazy loading.

Assuming lazy loading remains a flag. Chrome just removed the flag to disable hyperlink auditing and that one was even blatantly about privacy.

It's that true of iFrames? I haven't tried to determine within a child frame if scrolled into view.