Can you explain how these work? Does the server send small subrectangles of the large grid when the user scrolls to new regions of the grid? Does the browser actually have a two-dimensional array in memory with a billion items, or is there some other data structure?
Yeah the server only sends what the user is currently looking + plus a buffer around their view. There's no actual checkbox state on the client. When the user clicks a checkbox a depress animation is started and a request is made (which the server responds to with no data and a 204). The user then gets the html for the next view down a long lived SSE connection that started when they first loaded the page. Because, there's a long lived connection, it has really good compression. Same thing happens when the user scrolls. If they scroll far enough a new view is rendered.
The billion items themselves are just in a server on the backend, stored in a sqlite database.
