I've been waiting to be able to have a fully responsive layout (where interleave sidebar content in with the page content on small screens, but have it in a sidebar on larger screens) without using any JavaScript, and this finally makes it possible.
Demo: https://codepen.io/pbowyer/pen/raLBVaV
Previous comment: https://news.ycombinator.com/item?id=46228993
Chromium still seems to be working on support it seems based on https://cr-status.appspot.com/feature/5149560434589696 so maybe it'll be useful soon? That page indicates that they're still discussing certain parts of the spec.
One of the biggest arguments over the last couple of years was what to call it. A lot, lot of ideas put forth as alternatives to "masonry" which wasn't thought to be great for non-English speakers. I'm glad they finally nailed a name for it!
The other big argument was over how to activate it. Is it a grid? Is it a flexbox? Is it a brand new animal?
Now I just need to figure out the best way to implement this semantically with a polyfill for the next 30 months until it's baseline.
I am a fan of: https://www.youtube.com/@KevinPowell
Kevin is a no-nonsense no-hype educator. He will keep you up to date without a lot of engagement hacking.
I don't think the smooth reflow made it into the current spec, so I guess watch this space?
The other posters have good answers. One thing to consider for a smooth interaction would be to eagerly load the next x elements before they scroll into view.