system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why, despite all the advances in browser technology, does the screen still jump when an off-screen image or other part of the site loads?

If I'm reading a piece of text, I expect to stay on it. Something loading off-screen (common now that sites are so big and load lots of stuff via AJAX) shouldn't bounce me to a different position on the page.
gpt
Because your browser still doesn't know how big that image is going to be until it downloads it, so it has no way of knowing how much space to leave. The people who design the website could plan for it, but many of them don't.