This page captures a config object (a Map and a Set survive the boundary) and passes it to the island below. The island mutates all three fields in onMount. Because the prop is a deserialized copy with no reactive link back here, those writes are a no-op — the host's config is untouched.

The warning is dev-only, so it is not faked here. Under vite dev the runtime wraps the captured prop in a deep Proxy and logs a warning once per mutated path — you would see something like config.count, config.meta.set(), and config.roles.add() in the console. A production build ships the plain object with no proxy and no warning, so the mutation is silent and free. Open this page under pnpm --filter docs dev with the console open to see the real warnings; this preview build intentionally shows none.

The fix is always the same: keep mutable state inside the island ($state seeded from the prop), not in the dead shell. And two islands never share reactive state — if they must agree on something, both read it from the server.