React and Next.js

File layout for one figure

components/iso/                 the kit: iso-kit.ts, iso.css, react/draw.tsx, react/live.tsx
components/figures/rig/
  model.ts       pure model: true numbers, springs, readout text (no DOM)
  view.ts        world layout constants, the projection P (fitProjection), helpers like at(point)
  geometry.ts    every static path, computed once at module load
  frame.ts       paths for the parts that change shape: frameOf(state) -> { glass, coils, needle, ... }
  art.tsx        server component: static parts as JSX constants, passed to the live component
  live.tsx       "use client": refs, loop, pointer and keyboard, aria, writes the readout
  rig.css        scoped tokens and styles (e.g. .rig { … })
  figure.tsx     <Plate fig title hint readout keys caption><RigArt /></Plate>

examples/test-rig/ follows this layout. It uses rig.ts, view.ts, geometry.ts, pose.ts, art.tsx, live.tsx and figure.tsx.

The server/client split

• Geometry is pure TypeScript, so it runs on the server and the client receives finished path strings. No geometry code is shipped unless a moving part needs to rebuild per frame. Then frame.ts is imported by live.tsx and ships to the client.
• art.tsx builds the static parts as JSX (<Solid paths={BASE_ART.slab} tone="mid" /> and so on) and passes them as element props, such as base, rail, carriage and dial, to the "use client" live component. The live component places them in painter's order and wraps the moving ones in <g ref>.
• Only the live component and kit/react/live.tsx are client code. draw.tsx is server-safe.

The large-page key-warning trap

On a big page the server payload is large, and React streams some element props as deferred (lazy) chunks. The client renders {props.dial} among siblings. It can't mark a lazy as key-checked, so once the lazy resolves to an element without a key, React logs "Each child in a list should have a unique key", pointing at your live component or at IsoFigure. The same figure on a small test page logs nothing, so checking one figure at a time misses it.

There are two fixes, and you should do both from the start:

1. Give the root of every element prop a stable key: const DIAL = (<g key="dial">…</g>), and <React.Fragment key="sled"> for fragments. Keys on a single element cost nothing.
2. Keep "use client" components out of server-built art. Every client component element inside a server tree becomes a lazy reference, which makes all of its ancestors lazy too. If parts of the art need interactivity (hover-to-highlight, for example), make them plain <g data-part="…"> and handle events with delegation in the live component.

Any array you build with .map needs keys as usual.

Styles

• Import iso.css once, globally or in the figure. Put className="iso" on a wrapper (or <body>) for the tokens, plus data-theme="light" for the light theme.
• Scope per-figure tokens to the figure's class (.rig { --rig-glass-top: … }) and override kit tokens inside .iso-plate only when the card needs it.
• Don't put figure tokens on :root.

Readout

useReadout() finds the nearest .iso-plate [data-readout] from its anchor, and say(text) writes it without re-rendering. Render the resting text on the server so it is right before hydration.

Hydration

Render the rest pose on the server and on the client's first render; start motion only in effects. Don't read window while rendering. asksForStillness() is safe because it is only called inside effects and loops.