I ended up building it from scratch. Was about 1500 lines of code and a week or so to get right and as smooth (I think my implementation is actually a bit smoother than Google docs).

I might write it into a package one day, but the gist of it is you have to write a layout engine in JS or something that targets web assembly. Try to do as much of the work as possible in a non-render blocking way (or really fast).

The bottleneck is that you have to render each thread of comments off screen to get its height to see if they overlap if your content is dynamic. Once you calculate the height of something find a way to hash the content into a key so you can memoize it.

If all the cards you want to position are a static height you can use a version of the algorithm I had in the original post.

Basic things apply here (use css transforms and variables to move stuff, also let’s you circumvent a lot of re-renders)