Ok, I believe you, it's been over 10 years since I tried it :-) Maybe I was thinking of the forever iframe, where a regular application/javascript document had content added to it incrementally over time.
It is actually pretty bizarre. If you point Chrome at a .jpeg that does multipart/replace it will stall, giving you roughly 0.2 fps. Point it at a. .html that contains an <img> tag and it works fine. Found this out while hacking on Hawkeye: https://igorpartola.com/projects/hawkeye/
