tl;dr: Bonsai is Jane Street's OCaml-based UI library, inspired by Elm, used to build most of their internal web applications. Unlike React-style frameworks, it decouples state, incrementality, and rendering into composable primitives managed outside the component hierarchy, and leverages OCaml's type system across frontend and backend. It also features a robust testing system for programmatically manipulating UI and inspecting DOM diffs, plus variants for terminal UIs (Bonsai_term) built on a generic incremental state machine core.