React for Two Threads

For over a decade, we've lived in a universe where programming UIs with React on the client-side, from event handling to rendering, has been ruled by a single JavaScript thread. But what if React had been born differently? What if, from day one, it had been designed to fully embrace two threads? What benefits would that unlock? What challenges would we face? And what new features, abstractions, and patterns might emerge?

What if this isn't just imagination... but something that exists today?