Engineering · Part 18 · the one rung above htmx

Reactive islands: Preact signals without the build step

uiarchitecture

Reference documentation for the one rung above htmx: a scoped reactive island — real reactive state (Preact signals) bound to server-rendered DOM, for the single component where a round-trip per interaction is too slow — with no build step, no TSX, no bundler, and every other page left as plain server-rendered htmx.

Part 9 drew the line: htmx covers the overwhelming majority of a UI, but it strains when dense, interdependent client state must update in real time without a network hop — a view where dragging one control live-updates several others, an editor recomputing derived values on every keystroke. The wrong response is to rewrite the app as a single-page application. The right one is to drop a small reactive layer into that one component. This document is exactly that layer: `@preact/signals-core` plus about forty lines of glue, usable from an inline `