diff --git a/src/cities/reconcile.ts b/src/cities/reconcile.ts
index 4a55599..9668ec1 100644
--- a/src/cities/reconcile.ts
+++ b/src/cities/reconcile.ts
@@ -80,19 +80,132 @@ export function setReconcile(rules: boolean | readonly Rule[] | null): void {
   else forced = new Set(rules);
 }
 
+/**
+ * Read the rule set out of a flag value.
+ *
+ * ## Exact names, and a warning for anything else
+ *
+ * This used to prefix-match: `RULES.filter((rule) => wanted.some((part) =>
+ * rule.startsWith(part)))`. Three things were wrong with that, and all three
+ * corrupt measurements rather than breaking anything loudly.
+ *
+ *   - **A misspelling reads as "off".** `?reconcile=palette` selected nothing,
+ *     because no rule name begins with "palette" — and an empty set is exactly
+ *     what the flag being absent produces. So a photograph taken to judge a rule
+ *     was a photograph of the unreconciled board, captioned as the rule. There
+ *     is no way to tell those two apart from the picture.
+ *   - **One letter selected a rule.** `?reconcile=p` was `projection`, `r` was
+ *     `roads`, `g` was `ground`. Convenient until a second rule shares a letter,
+ *     at which point the same URL silently means something else.
+ *   - **It is order-dependent in a way nobody would guess.** The result is
+ *     built by filtering `RULES`, so the set is in declaration order however the
+ *     caller wrote it, which is fine — but a caller who writes two prefixes that
+ *     both hit one rule gets one rule and no complaint.
+ *
+ * Whatever replaces it has to fail *loudly*, because every number anyone takes
+ * from this module downstream — every RMSE, every pixel-diff bbox, every "the
+ * metro boards are unmoved to the digit" — is only as good as the flag having
+ * meant what the person typing it thought it meant.
+ */
 function parse(value: string | null): ReadonlySet<Rule> {
   if (value === null || value === "" || value === "0" || value === "false") return new Set();
   if (value === "1" || value === "true" || value === "all") return new Set(RULES);
   const wanted = value.split(/[,+\s]+/).filter((part) => part !== "");
-  return new Set(RULES.filter((rule) => wanted.some((part) => rule.startsWith(part))));
+  const known = new Set<Rule>();
+  for (const part of wanted) {
+    const rule = RULES.find((candidate) => candidate === part);
+    if (rule === undefined) {
+      // Not an exception. A bad rule name must not be able to stop a page from
+      // booting — this flag is read at module load on every visit, including
+      // from a URL somebody pasted — but it must never pass silently either.
+      warn(
+        `reconcile: unknown rule "${part}". Known rules: ${RULES.join(", ")}. ` +
+          "This rule is being ignored; the flag is NOT off.",
+      );
+      continue;
+    }
+    known.add(rule);
+  }
+  return known;
 }
 
-/** Which rules are on right now. */
+/**
+ * Where the warning goes.
+ *
+ * Split out so a test can assert the warning happened without owning a console,
+ * and because this module is imported by the terrain worker, where `console` is
+ * present but nobody is reading it.
+ */
+let warned: (message: string) => void = (message) => {
+  (globalThis as { console?: { warn?(m: string): void } }).console?.warn?.(message);
+};
+
+/** Redirect the unknown-rule warning. Pass `null` to restore the console. */
+export function setReconcileWarn(sink: ((message: string) => void) | null): void {
+  warned =
+    sink ??
+    ((message) => {
+      (globalThis as { console?: { warn?(m: string): void } }).console?.warn?.(message);
+    });
+}
+
+function warn(message: string): void {
+  warned(message);
+}
+
+/**
+ * The rules that are on when nobody has said otherwise.
+ *
+ * **Empty, and `roads` was tried here and taken back out.** The reason is worth
+ * keeping, because the argument for turning it on is persuasive and wrong:
+ *
+ * `roads` exists to stop California drawing US-101 and I-5 as 1,919 m and
+ * 2,034 m black ribbons — wider than the cities they join. That complaint is
+ * real and you can see it in any state frame. But `city.roads` **is not what
+ * draws them**. `scene.ts` reads
+ *
+ *     scene.add(options.roadTraffic ? createFreewayWorld(world, pack) : createRoads(world))
+ *
+ * and California is the one board that has `roadTraffic`, so it takes the first
+ * branch and `createRoads` — the only consumer of `Road.width` — is never called
+ * for it at all. The corridor you can see is `createFreewayWorld`, built from a
+ * `TransportPack` at hard-coded scene units (carriageways 1.03 wide at ±0.64,
+ * shoulders 1.18), which at 1,919 m to the unit is a corridor about 4.7 km
+ * across. That is deliberate: `main.ts` says so where it sets the corridor's
+ * altitude — "an atlas glyph just like the black cars: literal metres would put
+ * the chase camera inside the coarse hills" — because DRIVE mode has to be able
+ * to drive down it on a board where a real freeway is a fifth of a pixel.
+ *
+ * So on California the rule rewrites a column nothing reads, and on the metros
+ * `reconciledRoadWidth` returns the same number the pack already authored, to
+ * within float noise. Turned on and photographed at three chapters: Downtown LA
+ * pixel-identical (empty diff bbox), FiDi RMSE 0.0006 with zero pixels past
+ * 8/255, California 0.001%. The one measurable consequence anywhere was **−112
+ * triangles on Southern California**, from a ribbon width recomputing to a float
+ * that differs in its last bits.
+ *
+ * A default that changes no pixel and 112 triangles is not a feature; it is a
+ * second baseline for everyone who measures the other three rules afterwards.
+ * The rule stays available — `?reconcile=roads` — and the real fix for the state
+ * board's freeway lives in `structures.ts`, not here.
+ */
+export const DEFAULT_RULES: readonly Rule[] = [];
+
+/**
+ * Which rules are on right now.
+ *
+ * Order of precedence: an explicit `setReconcile()` from a node script, then the
+ * query string, then `DEFAULT_RULES`. The middle one includes `?reconcile=0`,
+ * which is how a measurement asks for the unreconciled board now that "no flag"
+ * no longer means that.
+ */
 export function activeRules(): ReadonlySet<Rule> {
   if (forced !== null) return forced;
   const search = (globalThis as { location?: { search?: string } }).location?.search;
-  if (search === undefined) return new Set();
-  return parse(new URLSearchParams(search).get("reconcile"));
+  if (search === undefined) return new Set(DEFAULT_RULES);
+  const value = new URLSearchParams(search).get("reconcile");
+  if (value === null) return new Set(DEFAULT_RULES);
+  return parse(value);
 }
 
 export function reconcileEnabled(rule?: Rule): boolean {
diff --git a/src/engine/boards.ts b/src/engine/boards.ts
index 762c7af..fdb2ad0 100644
--- a/src/engine/boards.ts
+++ b/src/engine/boards.ts
@@ -166,3 +166,171 @@ export function createBoardCache<T>(options: BoardCacheOptions): BoardCache<T> {
     },
   };
 }
+
+// ---- Which board to load before anybody asks for it -------------------------
+
+/** A board the prefetcher may load, described by facts a pack already states. */
+export interface PrefetchTier {
+  id: string;
+  /** The stand-off at which this tier takes over, in true metres. */
+  handoverStandoffM: number;
+  bounds: { minLat: number; maxLat: number; minLng: number; maxLng: number };
+}
+
+export interface PrefetchQuery {
+  tiers: readonly PrefetchTier[];
+  /** Where the camera is looking, in degrees. */
+  lat: number;
+  lng: number;
+  /** How far it is standing off what it is looking at, in true metres. */
+  standoffM: number;
+  /** Boards already in memory, however they got there. */
+  resident: readonly string[];
+  /** The board on screen. Never a prefetch target, never an eviction victim. */
+  visible: string | null;
+  capacity: number;
+  pinned?: readonly string[];
+}
+
+/**
+ * How much closer than the handover the camera must be before a tier loads.
+ *
+ * Three handovers. The trigger has to fire early enough that a metro is resident
+ * before the visitor reaches the stand-off where it would otherwise be built in
+ * front of them, and late enough that it never fires while they are looking at
+ * something else entirely.
+ *
+ * Three puts San Francisco's trigger at 213.6 km and Southern California's at
+ * 326.4 km. The first lands inside the **133 km dead band** between California's
+ * finest authored rung (`la-sf-i-5`, 242,206 m) and the first Bay rung
+ * (71,200 m) — a stretch of the ladder with nothing authored in it, so the load
+ * happens while the camera is on a state-scale rung and has somewhere to be.
+ * It is nowhere near the overview (1,551 km) or Shasta (501 km), which is the
+ * property that matters: landing on the state board must not load two metros.
+ */
+export const PREFETCH_HANDOVER_MULTIPLE = 3;
+
+/**
+ * How far outside a tier's own rectangle the camera may be and still arm it,
+ * as a multiple of the tier's half-diagonal.
+ *
+ * 1.5 gives San Francisco a 92 km disc and Southern California a 146 km one.
+ * The two metros are 313.9 km apart edge to edge and 92 + 146 = 238, so **the
+ * discs provably cannot overlap and at most one tier is ever a candidate**.
+ * That is the same covering-set argument `residentCapacity` already rests on,
+ * re-derived here rather than assumed, and it is what makes a two-deep handheld
+ * cache safe to prefetch into at all.
+ */
+export const PREFETCH_REACH_MULTIPLE = 1.5;
+
+/** Metres per degree of latitude. The longitude term is cosine-corrected below. */
+const M_PER_DEG_LAT = 111_320;
+
+/** Great-circle-ish distance from a point to a rectangle, in true metres. */
+function metresToBounds(lat: number, lng: number, b: PrefetchTier["bounds"]): number {
+  const dLat = Math.max(b.minLat - lat, 0, lat - b.maxLat);
+  const dLng = Math.max(b.minLng - lng, 0, lng - b.maxLng);
+  // Cosine taken at the *camera*, not at the rectangle's centre: the caller may
+  // be a long way north of a southern tier, and the squash that matters for how
+  // far it has to travel is the one where it is standing.
+  const east = dLng * M_PER_DEG_LAT * Math.cos((lat * Math.PI) / 180);
+  return Math.hypot(dLat * M_PER_DEG_LAT, east);
+}
+
+/** Half the diagonal of a tier's rectangle, in true metres. */
+function halfDiagonal(b: PrefetchTier["bounds"]): number {
+  const midLat = (b.minLat + b.maxLat) / 2;
+  const north = (b.maxLat - b.minLat) * M_PER_DEG_LAT;
+  const east = (b.maxLng - b.minLng) * M_PER_DEG_LAT * Math.cos((midLat * Math.PI) / 180);
+  return Math.hypot(north, east) / 2;
+}
+
+/**
+ * Whether a board may be admitted without evicting the one on screen.
+ *
+ * The policy lives here and the effects live in `main.ts`, so this module stays
+ * free of DOM, WebGL and `three` — see this file's header. A prefetch that
+ * evicted the visible board to make room for an invisible one would be the worst
+ * possible trade, and on a two-deep handheld cache it is one `put` away.
+ */
+export function canAdmit(query: {
+  id: string;
+  resident: readonly string[];
+  visible: string | null;
+  capacity: number;
+  pinned?: readonly string[];
+}): boolean {
+  if (query.resident.includes(query.id)) return true;
+  if (query.resident.length < query.capacity) return true;
+  const pinned = new Set(query.pinned ?? []);
+  // `ids()` is least-recently-shown first, so the victim is the first entry that
+  // is neither pinned nor on screen. If there is no such entry, admitting means
+  // evicting something that must not be evicted.
+  return query.resident.some((id) => !pinned.has(id) && id !== query.visible);
+}
+
+/**
+ * The one board worth loading right now, or `null` for "do nothing".
+ *
+ * Scalars only, never a camera: this has to be callable from a test with no
+ * `three` in it, and a function that took a `PerspectiveCamera` would drag the
+ * whole engine into this module's dependency graph and out of CONTRACT §1's
+ * "no DOM, no WebGL, no three" promise for it.
+ *
+ * Returns at most one id. Loading two metros at once is never right — they are
+ * 313.9 km apart and the camera cannot be descending into both — and returning
+ * one keeps the caller's abort handling to a single controller.
+ */
+export function prefetchTarget(query: PrefetchQuery): string | null {
+  const resident = new Set(query.resident);
+  /**
+   * Whether there is room for another board without dropping one.
+   *
+   * This is the difference between two genuinely different situations, and
+   * collapsing them was the first version's mistake.
+   *
+   * **With free room, proximity is the wrong question.** CONTRACT §1.1 already
+   * promises three resident boards on a desktop with California pinned, and
+   * measured that all three together are about 60 MB — the residency exists, and
+   * until now the only thing that ever filled it was a visitor waiting through a
+   * build. A desktop sitting on the state board with two empty slots should
+   * simply *have* both metros, because the overwhelmingly common path is to land
+   * on California and then click a metro, and on that path a proximity trigger
+   * fires exactly never: the camera is at 1,551 km, hundreds of kilometres above
+   * any threshold, and the next thing that happens is the click. A rule that only
+   * helps free-camera explorers is a rule that helps almost nobody.
+   *
+   * **With the cache full, proximity is the only question.** Admitting now means
+   * evicting something, and evicting is only worth it for a board the camera is
+   * actually descending toward. That is the handheld case — capacity two, one of
+   * them pinned — and it is why the discs and the dead band still exist.
+   */
+  const hasRoom = query.resident.length < query.capacity;
+  let best: { id: string; distanceM: number } | null = null;
+  for (const tier of query.tiers) {
+    if (tier.id === query.visible) continue;
+    if (resident.has(tier.id)) continue;
+    const distanceM = metresToBounds(query.lat, query.lng, tier.bounds);
+    if (!hasRoom) {
+      if (query.standoffM > tier.handoverStandoffM * PREFETCH_HANDOVER_MULTIPLE) continue;
+      if (distanceM > halfDiagonal(tier.bounds) * PREFETCH_REACH_MULTIPLE) continue;
+    }
+    if (
+      !canAdmit({
+        id: tier.id,
+        resident: query.resident,
+        visible: query.visible,
+        capacity: query.capacity,
+        pinned: query.pinned,
+      })
+    ) {
+      continue;
+    }
+    // Nearest wins. With non-overlapping discs there is never a second
+    // candidate, so this is a tie-break that should not be reachable — and it is
+    // written anyway, because "should not be reachable" is how a board gets
+    // chosen by declaration order the day somebody adds a fourth pack.
+    if (best === null || distanceM < best.distanceM) best = { id: tier.id, distanceM };
+  }
+  return best?.id ?? null;
+}
diff --git a/src/engine/environmentRig.ts b/src/engine/environmentRig.ts
index cb41904..5f3c19b 100644
--- a/src/engine/environmentRig.ts
+++ b/src/engine/environmentRig.ts
@@ -54,8 +54,41 @@ import type { LightingState } from "./types.ts";
 /** Which of the two worlds is being reflected. */
 export type EnvironmentKind = "city" | "office";
 
+/** Per-call options for {@link EnvironmentRig.apply}. */
+export interface ApplyOptions {
+  /**
+   * This scene is being built but is not on screen, and must not disturb one
+   * that is.
+   *
+   * ## Why the rig needs to be told
+   *
+   * The rig holds exactly one cached probe per kind. On a key miss it renders a
+   * new one, **disposes the one the visible board is using**, and repoints every
+   * applied scene of that kind at the replacement. That is exactly right when
+   * the miss is time passing: one clock, one sky, every city scene agrees.
+   *
+   * It is exactly wrong when the miss comes from a board nobody is looking at.
+   * A board built ahead of the camera is observed at *its own* centre — across
+   * California about forty minutes of apparent solar time — so its key differs
+   * by construction, and the visible board's sky would change because something
+   * invisible finished loading behind it.
+   *
+   * With this set, a key miss is not a rebuild: the off-stage scene borrows
+   * whatever probe is already cached, and the correct one arrives on its first
+   * on-stage `apply`, which by then is the same call every board already makes.
+   * Nothing is disposed, nothing already applied is repointed, and no PMREM
+   * convolution runs on the shared renderer inside somebody else's frame.
+   */
+  offstage?: boolean;
+}
+
 export interface EnvironmentRig {
-  apply(scene: THREE.Scene, lighting: LightingState, kind: "city" | "office"): void;
+  apply(
+    scene: THREE.Scene,
+    lighting: LightingState,
+    kind: "city" | "office",
+    options?: ApplyOptions,
+  ): void;
   /**
    * Forget a scene that is being torn down.
    *
@@ -198,12 +231,17 @@ export function createEnvironmentRig(renderer: THREE.WebGLRenderer): Environment
   }
 
   return {
-    apply(scene, lighting, kind) {
+    apply(scene, lighting, kind, options) {
       if (disposed) return;
       const key = environmentKey(kind, lighting);
       const current = built.get(kind);
+      // An off-stage board never rebuilds. See ApplyOptions.offstage: the
+      // rebuild path disposes the visible board's probe and repoints it, and a
+      // board nobody is looking at has no business doing either. It takes
+      // whatever is cached and gets the right one when it is presented.
+      const mayRebuild = options?.offstage !== true;
 
-      if (!current || current.key !== key) {
+      if (mayRebuild && (!current || current.key !== key)) {
         let target: THREE.WebGLRenderTarget;
         try {
           target = build(kind, lighting);
diff --git a/src/engine/scene.ts b/src/engine/scene.ts
index d70ec0d..d51a093 100644
--- a/src/engine/scene.ts
+++ b/src/engine/scene.ts
@@ -806,9 +806,31 @@ export async function createScene(
   // two disagree for the one frame before the app's first `setLighting`.
   const opening = options.lighting ?? cityDaylight(pal, boardSpan);
   kit.applyLighting(opening);
-  // The environment before the first layer is added, so the very first frame
-  // has a sky to reflect rather than acquiring one a `setLighting` later.
-  options.environment?.apply(scene, opening, "city");
+  /*
+   * The environment before the first layer is added, so the very first frame has
+   * a sky to reflect rather than acquiring one a `setLighting` later.
+   *
+   * `offstage` is the `present` flag, one line early. It has to be, because this
+   * call is two hundred lines above the `if (options.present !== false)` that
+   * decides whether anybody will ever look at this scene — and the rig holds one
+   * probe per kind, disposes it on a key miss, and repoints every applied scene
+   * at the replacement. A board built ahead of the camera is observed at its own
+   * centre, so its key differs by construction, and without this a board loading
+   * invisibly in the background would change the sky of the board on screen.
+   * `onEnter` below applies it again, for real, when it is presented.
+   */
+  /**
+   * The lighting this scene was last told about, and whether anybody is looking.
+   *
+   * Both are held because the environment is applied from three places — here at
+   * construction, from `setLighting` on every clock tick, and from `onEnter` on
+   * arrival — and all three have to agree about the same two facts. A board that
+   * is off screen must never rebuild the shared probe; a board that is on screen
+   * must always be allowed to.
+   */
+  let lighting = opening;
+  let presented = options.present !== false;
+  options.environment?.apply(scene, opening, "city", { offstage: !presented });
 
   scene.add(createWater(world));
   scene.add(createShorePlates(world));
@@ -1172,6 +1194,23 @@ export async function createScene(
     // stale detail card for something the pointer is nowhere near reads as a
     // bug.
     onExit: () => kit.resetPick(),
+    /*
+     * Claim the environment on arrival.
+     *
+     * A board built with `present: false` was applied `offstage`, which means it
+     * borrowed whatever probe was cached rather than building its own — right
+     * while it was invisible, wrong the moment it is not. This is the same call
+     * every board already makes; on a cache hit it costs no convolution, which
+     * is the case a prefetched board arriving at the same hour always hits.
+     *
+     * It runs on every entry, not only the first, and that is deliberate: a
+     * board returned to after an office visit or another board has been off
+     * screen while the clock moved, and re-asserting is how it catches up.
+     */
+    onEnter: () => {
+      presented = true;
+      options.environment?.apply(scene, lighting, "city");
+    },
     tick(dt) {
       // Exactly one subsystem owns the camera. In particular, OrbitControls
       // must stay disabled while the road layer writes its follow pose.
@@ -1347,6 +1386,7 @@ export async function createScene(
       arrival = { from, to: rest, elapsed: 0 };
     },
     setLighting: (state) => {
+      lighting = state;
       kit.applyLighting(state);
       clouds.setLighting(state);
       fireLayer?.setLighting(state);
@@ -1364,7 +1404,7 @@ export async function createScene(
        * rig, the scene is applying it, and the environment is derived from the
        * decision rather than being a second opinion about the light.
        */
-      options.environment?.apply(scene, state, "city");
+      options.environment?.apply(scene, state, "city", { offstage: !presented });
     },
     setAerialFog: ({ near, far }) => {
       // The three fog owners on a city board and no more. `ports`, `vessels`,
diff --git a/src/main.ts b/src/main.ts
index 2319cf7..8abbce9 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -41,11 +41,18 @@ import {
   type Atmosphere,
   type WeatherObservation,
 } from "./engine/atmosphere.ts";
-import { createBoardCache, residentCapacity, type BoardCache } from "./engine/boards.ts";
+import {
+  createBoardCache,
+  prefetchTarget,
+  residentCapacity,
+  type BoardCache,
+  type PrefetchTier,
+} from "./engine/boards.ts";
 import {
   activeRung,
   buildLadder,
   handover,
+  HANDOVER_STANDOFF_M,
   placesRows,
   REGION_LABELS,
   type LadderRung,
@@ -66,6 +73,7 @@ import { createStage, deviceProfile } from "./engine/stage.ts";
 import { daylightPhase } from "./engine/solar.ts";
 import type { Aircraft, City, Marker, MarkerPalette, Port, View } from "./engine/types.ts";
 import CALIFORNIA from "./cities/california.ts";
+import { reconciledCity } from "./cities/reconcile.ts";
 import SAN_FRANCISCO from "./cities/sf.ts";
 import SOCAL from "./cities/socal.ts";
 import CALIFORNIA_TRANSPORT from "./transport/california.ts";
@@ -243,7 +251,24 @@ const CALIFORNIA_DESTINATIONS = new Map<string, { cityId: string; officeId: stri
  * guards in `scripts/brand-assets` aim at. The ladder is a **view** over that
  * data and nothing more.
  */
-const LADDER = buildLadder(CITIES.map((entry) => ({ id: entry.id, city: entry.city })));
+/**
+ * The ladder is built from the **reconciled** packs, because `World` is.
+ *
+ * `world.ts:116` is `this.city = reconciledCity(city)`, so every number the
+ * engine draws with — `lngScale`, `verticalExaggeration`, road widths — is the
+ * reconciled one. The ladder reads `focus` to derive each rung's stand-off, and
+ * a ladder built from the raw packs is a ladder measuring a world nobody is
+ * looking at. With every rule off that is the same object by identity and this
+ * line is a no-op; with a rule on it is the difference between the rail agreeing
+ * with the camera and quietly disagreeing with it.
+ *
+ * Deliberately calling `reconciledCity` here rather than reading it off a
+ * `World`: the ladder is built at module load, before any board is mounted, and
+ * it must be — `PLACES` is the navigation and it exists before the first frame.
+ */
+const LADDER = buildLadder(
+  CITIES.map((entry) => ({ id: entry.id, city: reconciledCity(entry.city) })),
+);
 const PLACES = placesRows(LADDER);
 
 const JOURNEY_SESSION_KEY = "tera:journey:v1";
@@ -699,6 +724,23 @@ let satellitesVisible = true;
  * allocated no WebGL context for the abandoned board.
  */
 let mounting: AbortController | null = null;
+/**
+ * The build nobody asked for.
+ *
+ * A strictly lower-priority second lane. It exists because "one map" is a claim
+ * about continuity, and continuity is paid for by having the detail already on
+ * the GPU when the camera gets there: CONTRACT §1.1 keeps three boards resident
+ * on a desktop and pins California, and until now nothing ever *filled* that
+ * residency except a visitor waiting through a build.
+ *
+ * Held separately from `mounting` rather than reusing it, because the two have
+ * opposite rights. A foreground build may cancel a background one; a background
+ * build may never cancel or queue in front of a foreground one, and it must be
+ * abandonable the instant somebody touches anything. Keeping one controller for
+ * both would make "abort the build" ambiguous at exactly the moment it matters.
+ */
+let prefetching: AbortController | null = null;
+
 let godmode: Godmode | null = null;
 let poseEditor: PoseEditor | null = null;
 /**
@@ -1517,6 +1559,16 @@ async function mountCity(id: string): Promise<void> {
    * build, and a built board belongs to the cache from the moment it lands in
    * it.
    */
+  /*
+   * The background lane goes first, and the order is the whole guard.
+   *
+   * `createScene` awaits a heightfield the terrain worker is building; if the
+   * prefetch is still holding that worker when a real request arrives, the
+   * foreground build queues behind work nobody asked for. Aborting it here —
+   * before `mounting?.abort()`, before anything else — is what makes a visitor's
+   * click always the most important thing in the process.
+   */
+  cancelPrefetch();
   mounting?.abort();
   const mount = new AbortController();
   mounting = mount;
@@ -1534,22 +1586,41 @@ async function mountCity(id: string): Promise<void> {
   }
 
   showSwitchProgress(entry.label, null);
-  const record = await buildBoard(entry, mount);
-  if (record === null) {
-    pendingPlace = null;
-    hideSwitchProgress();
-    return;
+  try {
+    const record = await buildBoard(entry, mount);
+    if (record === null) {
+      pendingPlace = null;
+      hideSwitchProgress();
+      return;
+    }
+    if (mount.signal.aborted || wantedCity !== id) {
+      // Somebody changed their mind while this was in the air. It is a complete,
+      // correct board — but nothing asked for it, and parking an unrequested
+      // board in a cache with a ceiling would evict one somebody did ask for.
+      disposeBoard(record);
+      pendingPlace = null;
+      hideSwitchProgress();
+      return;
+    }
+    await presentBoard(record);
+  } finally {
+    /*
+     * A finished mount is not a mount in flight.
+     *
+     * `mounting` used to be replaced and never cleared, which was harmless while
+     * the only thing that read it was the next `mountCity` wanting something to
+     * abort — an already-resolved controller aborts nothing and costs nothing.
+     * It stopped being harmless the moment a second lane asked the obvious
+     * question "is a foreground build happening right now?", because the honest
+     * answer after the first mount of the session was permanently "yes". The
+     * background lane armed exactly never, and the only symptom was a feature
+     * that silently did nothing.
+     *
+     * Guarded on identity: a build that was superseded must not clear the
+     * controller belonging to the build that superseded it.
+     */
+    if (mounting === mount) mounting = null;
   }
-  if (mount.signal.aborted || wantedCity !== id) {
-    // Somebody changed their mind while this was in the air. It is a complete,
-    // correct board — but nothing asked for it, and parking an unrequested
-    // board in a cache with a ceiling would evict one somebody did ask for.
-    disposeBoard(record);
-    pendingPlace = null;
-    hideSwitchProgress();
-    return;
-  }
-  await presentBoard(record);
 }
 
 /**
@@ -1565,6 +1636,19 @@ async function mountCity(id: string): Promise<void> {
 async function buildBoard(
   entry: { id: string; label: string; city: City },
   mount: AbortController,
+  options: {
+    /**
+     * Build without saying so.
+     *
+     * The background lane needs this and nothing else does. `onProgress` below
+     * ends in `bootProgress`, which raises the switch pill — so a board loaded
+     * ahead of the camera would put "Building The Bay Area… terrain 42%" on
+     * screen over a visitor who did nothing, which is exactly the "you are
+     * somewhere else now" chrome that removing the tab strip and the boot card
+     * was for. A prefetch that announces itself is worse than no prefetch.
+     */
+    quiet?: boolean;
+  } = {},
 ): Promise<MountedBoard | null> {
   const id = entry.id;
   /**
@@ -1804,6 +1888,7 @@ async function buildBoard(
     // An abandoned build keeps its worker running for a tick or two after the
     // abort; its percentages must not land on the card the new city is using.
     onProgress: (p) => {
+      if (options.quiet) return;
       if (!mount.signal.aborted) bootProgress(entry.label, p.fraction, p.onMainThread);
     },
     ...(access.subject !== null
@@ -1890,9 +1975,13 @@ async function buildBoard(
 
   const minimap = createMinimap({
     world: handle.world,
-    city: entry.city,
+    // The pack the *World* is drawing, not the one the module imported. These
+    // are the same object whenever no reconciliation rule is on, and when one
+    // is, the plan view drawn from the raw pack is a plan of a different
+    // California than the one under the camera.
+    city: handle.world.city,
     // The plan stops changing shape when the board does. See `planFrame`.
-    frame: planFrame(entry.city),
+    frame: planFrame(handle.world.city),
     camera: handle.stageScene.camera,
     controls: handle.stageScene.controls,
     markerPalette: palette,
@@ -2029,6 +2118,15 @@ async function presentBoard(record: MountedBoard): Promise<void> {
 
   activateBoard(record);
   hideSwitchProgress();
+  /*
+   * Arriving is itself a reason to look ahead.
+   *
+   * The camera listeners below arm the lane as the visitor moves, but landing on
+   * a board is a pose change nothing fired a `change` event for — and it is the
+   * single most likely moment for the next board to already be worth having. The
+   * old prefetch, if any, was cancelled by `mountCity` before this build began.
+   */
+  maybePrefetch(record);
 
   // Before the lift, so the incoming board's first drawn frame is already inside
   // the haze the outgoing one ended in rather than snapping to clear and then
@@ -2310,6 +2408,7 @@ function activateBoard(record: MountedBoard): void {
       lastAltitude = altitude;
       applyCameraFog();
       maybeHandover(record);
+      maybePrefetch(record);
     };
     const onDragStart = () => { dragging = true; };
     const onDragEnd = () => {
@@ -2318,6 +2417,7 @@ function activateBoard(record: MountedBoard): void {
       // refuses while a pointer is down, so without this a descent that ends
       // below a threshold would sit there until the next event.
       maybeHandover(record);
+      maybePrefetch(record);
     };
     controls.addEventListener("change", onCameraMoved);
     controls.addEventListener("start", onDragStart);
@@ -2362,6 +2462,17 @@ let dragging = false;
  */
 const FREE_HANDOVER = new URLSearchParams(location.search).get("handover") === "1";
 
+/**
+ * Whether a board may be built before anybody asks for it.
+ *
+ * On, and this is the single constant that turns it off — the rollback for the
+ * whole background lane, per the plan this was built from. It is a constant and
+ * not a query flag deliberately: a query flag is a thing that ships in one state
+ * and is measured in another, and every number quoted for this lane was measured
+ * with it in the state it ships in.
+ */
+const PREFETCH_ENABLED = true;
+
 function maybeHandover(record: MountedBoard): void {
   if (!FREE_HANDOVER) return;
   if (inside || fogDip !== null || record !== visibleBoard) return;
@@ -2383,6 +2494,166 @@ function maybeHandover(record: MountedBoard): void {
   if (next !== null && next !== wantedCity) switchCity(next);
 }
 
+/**
+ * The metro tiers the background lane may load, and the stand-off each takes
+ * over at. Derived from the ladder's own handover table, so there is exactly one
+ * place in the product that says where a board stops being the right one.
+ */
+const PREFETCH_TIERS: readonly PrefetchTier[] = CITIES.flatMap((entry) => {
+  const handoverStandoffM = HANDOVER_STANDOFF_M[entry.id];
+  return handoverStandoffM === undefined
+    ? []
+    : [{ id: entry.id, handoverStandoffM, bounds: entry.city.bounds }];
+});
+
+/**
+ * Drop whatever the background lane is doing.
+ *
+ * Called before every foreground abort and on every board present, because a
+ * prefetch that outlives the reason it started is a build competing with a
+ * visitor for one terrain worker. `createScene` honours the signal by dropping
+ * the heightfield and resolving `null` without allocating a renderer, so an
+ * abandoned prefetch costs nothing to abandon.
+ */
+/**
+ * How long a board must be on screen and undisturbed before anything loads
+ * behind it, in milliseconds.
+ *
+ * The board that just arrived is still linking shaders, uploading buffers and
+ * settling its first frames; starting another build inside that window is the
+ * one way this feature can make the product measurably worse. Deliberately not
+ * `requestIdleCallback`: Safari did not ship it until recently and it is exactly
+ * the browser most likely to be on the tightest budget, so the deferral is a
+ * timer everywhere rather than a good mechanism on some platforms and none on
+ * others.
+ */
+const PREFETCH_IDLE_MS = 2_000;
+
+let prefetchTimer: ReturnType<typeof setTimeout> | null = null;
+
+function cancelPrefetch(): void {
+  if (prefetchTimer !== null) {
+    clearTimeout(prefetchTimer);
+    prefetchTimer = null;
+  }
+  prefetching?.abort();
+  prefetching = null;
+}
+
+/**
+ * Load the board the camera is heading for, before it is asked for.
+ *
+ * ## Why this is the whole of "loads in the background"
+ *
+ * Everything else was already there. `present: false` already exists on
+ * `SceneOptions` and is already honoured; `boards` already holds three on a
+ * desktop with California pinned; `presentBoard` already runs a fog dip over a
+ * cache hit at zero shader links and zero blocked milliseconds. The only missing
+ * piece was something that decides to build a board nobody has clicked on, and
+ * the reason it was missing is that it is the piece that can hurt: it competes
+ * for the terrain worker and it can relight the board on screen.
+ *
+ * Both hazards are closed rather than hoped about. The competition is closed by
+ * `cancelPrefetch()` running before every foreground abort. The relighting is
+ * closed in `environmentRig.ts` — an off-stage apply borrows the cached probe
+ * instead of convolving a new one and disposing the visible board's — and by
+ * `scene.ts` passing `offstage: !presented` on every clock-driven apply.
+ *
+ * ## Why the decision is not made here
+ *
+ * `prefetchTarget` is pure, takes scalars, and lives in `boards.ts` with the
+ * eviction policy it has to agree with. This function is the effects half: read
+ * three numbers off the camera, ask, and act. Keeping the two apart is what lets
+ * `prefetchPolicy.test.ts` assert against the real pack bounds — including that
+ * the two metros' trigger discs provably cannot overlap — with no DOM, no WebGL
+ * and no `three` anywhere near it.
+ */
+function maybePrefetch(record: MountedBoard): void {
+  if (!PREFETCH_ENABLED) return;
+  // Never behind a visitor. A foreground build, an office, or a dip in progress
+  // all mean something is already happening that matters more than this.
+  if (inside || mounting !== null || fogDip !== null) return;
+  if (record !== visibleBoard) return;
+  if (prefetching !== null) return;
+
+  const target = record.handle.stageScene.controls.target;
+  const [lat, lng] = record.handle.world.unproject(target.x, target.z);
+  const id = prefetchTarget({
+    tiers: PREFETCH_TIERS,
+    lat,
+    lng,
+    standoffM: record.handle.cameraStandoffMetres(),
+    resident: boards.ids(),
+    visible: visibleBoard?.id ?? null,
+    capacity: residentCapacity(deviceProfile().handheld),
+    pinned: ["california"],
+  });
+  if (id === null) return;
+  const entry = CITIES.find((candidate) => candidate.id === id);
+  if (entry === undefined) return;
+
+  /*
+   * Armed now, started later.
+   *
+   * The timer is what separates "the camera is somewhere a load would be useful"
+   * from "and it has been there long enough that nothing else needs the machine".
+   * Every gesture cancels it through `cancelPrefetch`, so a visitor sweeping the
+   * camera across the state re-arms it repeatedly and starts nothing until they
+   * stop — which is the behaviour a background lane has to have to be invisible.
+   */
+  if (prefetchTimer !== null) clearTimeout(prefetchTimer);
+  prefetchTimer = setTimeout(() => {
+    prefetchTimer = null;
+    startPrefetch(entry);
+  }, PREFETCH_IDLE_MS);
+}
+
+/** The build itself, once the deferral above has decided it is safe to start. */
+function startPrefetch(entry: { id: string; label: string; city: City }): void {
+  const id = entry.id;
+  // Re-checked after the wait, because two seconds is long enough for every one
+  // of these to have changed.
+  if (inside || mounting !== null || fogDip !== null) return;
+  if (prefetching !== null || boards.has(id) || visibleBoard?.id === id) return;
+
+  const controller = new AbortController();
+  prefetching = controller;
+  void (async () => {
+    const built = await buildBoard(entry, controller, { quiet: true });
+    // Four ways this stops mattering between the ask and the answer, and all
+    // four have to be checked *after* the await rather than before it: the
+    // visitor may have clicked something, the lane may have been cancelled, the
+    // board may have arrived by the front door in the meantime, or the cache may
+    // no longer have room for it without evicting what is on screen.
+    if (built === null) {
+      if (prefetching === controller) cancelPrefetch();
+      return;
+    }
+    const stale =
+      controller.signal.aborted ||
+      prefetching !== controller ||
+      boards.has(id) ||
+      visibleBoard?.id === id;
+    if (stale) {
+      disposeBoard(built);
+      if (prefetching === controller) prefetching = null;
+      return;
+    }
+    /*
+     * Parked, not presented.
+     *
+     * `presentBoard` writes the cache itself at the moment a board becomes
+     * visible; this one never becomes visible, so it writes its own entry — and
+     * it goes in as the *least* recently shown, which is what `put` already does
+     * for a board that has never been shown. Anything evicted to make room is
+     * disposed here, exactly as `presentBoard` does, because the rig's ledger is
+     * a strong reference and a board left in it retains the whole scene graph.
+     */
+    for (const evicted of boards.put(id, built)) disposeBoard(evicted);
+    prefetching = null;
+  })();
+}
+
 /**
  * The rectangle the plan view draws, which is no longer the board's own.
  *
diff --git a/src/test/integration/sceneWiring.test.ts b/src/test/integration/sceneWiring.test.ts
index 2d19645..d913d8a 100644
--- a/src/test/integration/sceneWiring.test.ts
+++ b/src/test/integration/sceneWiring.test.ts
@@ -632,7 +632,27 @@ test("main.ts makes exactly one call into the chrome, and writes no visibility i
 test("the city scene is handed the same environment rig the office is", () => {
   const scene = readFileSync(path.join(ROOT, "src/engine/scene.ts"), "utf8");
   const office = readFileSync(path.join(ROOT, "src/interiors/officeScene.ts"), "utf8");
-  assert.ok(scene.includes('options.environment?.apply(scene, state, "city")'));
+  /*
+   * The call now carries a fourth argument, and this assertion got *stronger*
+   * rather than looser when it did.
+   *
+   * `apply` used to be unconditional everywhere it appeared, which is what let a
+   * board being built off-stage dispose the visible board's probe and repoint it
+   * — the rig holds one per kind and rebuilds on a key miss, and a board built
+   * ahead of the camera is observed at its own centre, so its key differs by
+   * construction. So the thing worth asserting is not merely that the city hands
+   * its lighting to the rig; it is that every clock-driven apply is *guarded* by
+   * whether anyone is looking. A future edit that drops the guard to "simplify"
+   * fails here.
+   */
+  assert.ok(
+    scene.includes('options.environment?.apply(scene, state, "city", { offstage: !presented })'),
+    "the city's clock-driven apply lost its off-stage guard",
+  );
+  assert.ok(
+    !/options\.environment\?\.apply\(scene, state, "city"\)/.test(scene),
+    "an unguarded clock-driven apply is back in scene.ts",
+  );
   assert.ok(scene.includes("options.environment?.release(scene)"));
   assert.ok(office.includes('options.environment?.apply(scene, state, "office")'));
   assert.ok(office.includes("options.environment?.release(scene)"));
diff --git a/src/test/prefetchPolicy.test.ts b/src/test/prefetchPolicy.test.ts
new file mode 100644
index 0000000..7ed2d52
--- /dev/null
+++ b/src/test/prefetchPolicy.test.ts
@@ -0,0 +1,235 @@
+/**
+ * When a board loads before anybody asks for it.
+ *
+ * ## What this is for
+ *
+ * "One map" is a claim about continuity, and continuity is paid for by having
+ * the detail already in memory when the camera arrives. CONTRACT §1.1 already
+ * keeps three boards resident on a desktop and pins California; what was missing
+ * was anything that *fills* that residency before a visitor demands it. This is
+ * the policy half of that — the half with no DOM, no WebGL and no `three` in it,
+ * so the decision can be argued about in a test instead of in a browser.
+ *
+ * ## The two numbers, and why they are safe
+ *
+ * `PREFETCH_HANDOVER_MULTIPLE = 3` and `PREFETCH_REACH_MULTIPLE = 1.5` are not
+ * taste. The first puts San Francisco's trigger at 213.6 km, inside the 133 km
+ * dead band between California's finest authored rung (242,206 m) and the first
+ * Bay rung (71,200 m) — so it fires on a state-scale rung with nothing authored
+ * below it, and never at the 1,551 km overview. The second gives discs of 92 km
+ * and 146 km around two metros that are 313.9 km apart, and 92 + 146 = 238 <
+ * 314, so **at most one tier is ever a candidate**. Both properties are asserted
+ * below against the real bounds, because both are load-bearing for a two-deep
+ * handheld cache and both would rot silently if a pack's bounds moved.
+ */
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+
+import CALIFORNIA from "../cities/california.ts";
+import SAN_FRANCISCO from "../cities/sf.ts";
+import SOCAL from "../cities/socal.ts";
+import { HANDOVER_STANDOFF_M } from "../engine/ladder.ts";
+import {
+  PREFETCH_HANDOVER_MULTIPLE,
+  PREFETCH_REACH_MULTIPLE,
+  canAdmit,
+  prefetchTarget,
+  residentCapacity,
+} from "../engine/boards.ts";
+import type { PrefetchTier } from "../engine/boards.ts";
+
+const TIERS: readonly PrefetchTier[] = [
+  { id: "sf", handoverStandoffM: HANDOVER_STANDOFF_M.sf!, bounds: SAN_FRANCISCO.bounds },
+  { id: "socal", handoverStandoffM: HANDOVER_STANDOFF_M.socal!, bounds: SOCAL.bounds },
+];
+
+const DESKTOP = residentCapacity(false);
+const HANDHELD = residentCapacity(true);
+
+/** The camera parked over each metro's own middle. */
+function centre(b: PrefetchTier["bounds"]) {
+  return { lat: (b.minLat + b.maxLat) / 2, lng: (b.minLng + b.maxLng) / 2 };
+}
+
+const SF_MID = centre(SAN_FRANCISCO.bounds);
+const SOCAL_MID = centre(SOCAL.bounds);
+
+function ask(over: { lat: number; lng: number }, standoffM: number, extra: Partial<Parameters<typeof prefetchTarget>[0]> = {}) {
+  return prefetchTarget({
+    tiers: TIERS,
+    lat: over.lat,
+    lng: over.lng,
+    standoffM,
+    resident: ["california"],
+    visible: "california",
+    capacity: DESKTOP,
+    pinned: ["california"],
+    ...extra,
+  });
+}
+
+describe("what the prefetcher arms on", () => {
+  it("loads the Bay while the camera is descending toward it", () => {
+    assert.equal(ask(SF_MID, HANDOVER_STANDOFF_M.sf! * 2), "sf");
+  });
+
+  it("loads the Southland while the camera is descending toward it", () => {
+    assert.equal(ask(SOCAL_MID, HANDOVER_STANDOFF_M.socal! * 2), "socal");
+  });
+
+  /**
+   * The common path, and the reason the free-room rule exists at all.
+   *
+   * A visitor lands on the state overview at 1,551 km and then clicks a metro.
+   * On that path a proximity trigger fires exactly never — the camera is
+   * hundreds of kilometres above every threshold and the next thing that happens
+   * is the click. A desktop has two empty slots and CONTRACT §1.1 already
+   * promises they may be filled, so they are.
+   */
+  it("fills the free residency from the state overview, where the visitor actually is", () => {
+    assert.equal(ask(SF_MID, 1_551_000), "sf");
+    // And having taken one, it goes back for the other rather than stopping.
+    assert.equal(ask(SF_MID, 1_551_000, { resident: ["california", "sf"] }), "socal");
+    // Until there is nothing left to want.
+    assert.equal(ask(SF_MID, 1_551_000, { resident: ["california", "sf", "socal"] }), null);
+  });
+
+  it("never proposes the board on screen, or one already resident", () => {
+    assert.notEqual(ask(SF_MID, HANDOVER_STANDOFF_M.sf! * 2, { visible: "sf" }), "sf");
+    assert.notEqual(
+      ask(SF_MID, HANDOVER_STANDOFF_M.sf! * 2, { resident: ["california", "sf"] }),
+      "sf",
+    );
+  });
+
+  /**
+   * With the cache full, proximity is the only thing that justifies an eviction —
+   * and this is the handheld case, capacity two with California pinned. A phone
+   * parked on the state board must load nothing at all.
+   */
+  it("arms nothing on a full handheld cache away from either metro", () => {
+    const full = {
+      resident: ["california", "sf"],
+      visible: "california",
+      capacity: HANDHELD,
+      pinned: ["california"],
+    };
+    // Redding, in the far north: nowhere near the Southland.
+    assert.equal(ask({ lat: 40.58, lng: -122.39 }, HANDOVER_STANDOFF_M.socal!, full), null);
+    // And the state overview, which is above every threshold.
+    assert.equal(ask(SOCAL_MID, 1_551_000, full), null);
+  });
+
+  it("arms on a full cache only while descending toward the tier it would evict for", () => {
+    const full = {
+      resident: ["california", "sf"],
+      visible: "california",
+      capacity: HANDHELD,
+      pinned: ["california"],
+    };
+    assert.equal(ask(SOCAL_MID, HANDOVER_STANDOFF_M.socal! * 2, full), "socal");
+  });
+});
+
+describe("the two multiples are safe against the real packs", () => {
+  /**
+   * The dead band. If the Bay's trigger were above California's finest rung the
+   * prefetch would fire while the visitor was still choosing a region; if it
+   * were below the first Bay rung it would fire too late to have helped.
+   */
+  it("puts the Bay trigger inside the ladder's authored dead band", () => {
+    const trigger = HANDOVER_STANDOFF_M.sf! * PREFETCH_HANDOVER_MULTIPLE;
+    assert.ok(trigger > HANDOVER_STANDOFF_M.sf!, "the trigger is below the handover");
+    assert.ok(trigger < 242_206, `the Bay trigger ${trigger} is above California's finest rung`);
+  });
+
+  /**
+   * The covering-set argument, re-derived. Two overlapping discs would mean a
+   * camera position from which both metros are candidates, and on a two-deep
+   * handheld cache that is one board too many.
+   */
+  it("gives the two metros discs that cannot overlap", () => {
+    const M = 111_320;
+    const reach = (b: PrefetchTier["bounds"]) => {
+      const mid = (b.minLat + b.maxLat) / 2;
+      const north = (b.maxLat - b.minLat) * M;
+      const east = (b.maxLng - b.minLng) * M * Math.cos((mid * Math.PI) / 180);
+      return (Math.hypot(north, east) / 2) * PREFETCH_REACH_MULTIPLE;
+    };
+    const gapM =
+      (SAN_FRANCISCO.bounds.minLat - SOCAL.bounds.maxLat) * M;
+    const together = reach(SAN_FRANCISCO.bounds) + reach(SOCAL.bounds);
+    assert.ok(
+      SAN_FRANCISCO.bounds.minLat > SOCAL.bounds.maxLat,
+      "the metro bounds now intersect — the whole residency argument rests on them not doing so",
+    );
+    assert.ok(
+      together < gapM,
+      `the prefetch discs overlap: ${Math.round(together / 1000)} km of reach across a ` +
+        `${Math.round(gapM / 1000)} km gap`,
+    );
+  });
+
+  it("keeps California inside every board's bounds, which is what makes it the pin", () => {
+    for (const metro of [SAN_FRANCISCO, SOCAL]) {
+      assert.ok(metro.bounds.minLat >= CALIFORNIA.bounds.minLat);
+      assert.ok(metro.bounds.maxLat <= CALIFORNIA.bounds.maxLat);
+      assert.ok(metro.bounds.minLng >= CALIFORNIA.bounds.minLng);
+      assert.ok(metro.bounds.maxLng <= CALIFORNIA.bounds.maxLng);
+    }
+  });
+});
+
+describe("admission never costs the board on screen", () => {
+  it("refuses when the only victim left is the visible board", () => {
+    // A handheld holding the pin plus the board being looked at is full, and the
+    // only thing it could drop is the thing the visitor is looking at.
+    assert.equal(
+      canAdmit({
+        id: "socal",
+        resident: ["california", "sf"],
+        visible: "sf",
+        capacity: HANDHELD,
+        pinned: ["california"],
+      }),
+      false,
+    );
+  });
+
+  it("admits when a board that is neither pinned nor visible can go", () => {
+    assert.equal(
+      canAdmit({
+        id: "socal",
+        resident: ["california", "sf"],
+        visible: "california",
+        capacity: HANDHELD,
+        pinned: ["california"],
+      }),
+      true,
+    );
+  });
+
+  it("admits freely below capacity, and is idempotent for a resident board", () => {
+    assert.equal(
+      canAdmit({ id: "sf", resident: ["california"], visible: "california", capacity: DESKTOP }),
+      true,
+    );
+    assert.equal(
+      canAdmit({ id: "sf", resident: ["california", "sf"], visible: "sf", capacity: 2 }),
+      true,
+    );
+  });
+
+  /** And the policy must be reflected in what `prefetchTarget` proposes. */
+  it("proposes nothing a full handheld cache could not take", () => {
+    assert.equal(
+      ask(SOCAL_MID, HANDOVER_STANDOFF_M.socal! * 2, {
+        resident: ["california", "sf"],
+        visible: "sf",
+        capacity: HANDHELD,
+        pinned: ["california"],
+      }),
+      null,
+    );
+  });
+});
diff --git a/src/test/reconcileConsumers.test.ts b/src/test/reconcileConsumers.test.ts
new file mode 100644
index 0000000..2bb407b
--- /dev/null
+++ b/src/test/reconcileConsumers.test.ts
@@ -0,0 +1,181 @@
+/**
+ * Everything that reads a pack must read the pack the World is drawing.
+ *
+ * ## The bug this is downstream of
+ *
+ * `World`'s constructor is `this.city = reconciledCity(city)` (`world.ts:116`),
+ * so the moment any reconciliation rule is on, there are **two** Californias in
+ * the process: the object `cities/california.ts` exported, and the rewritten one
+ * the engine is actually projecting, exaggerating and drawing roads from.
+ *
+ * Two consumers were reading the first one while the camera stood in the second:
+ *
+ *   - `buildLadder` (`main.ts`) derives every rung's stand-off from `focus`, so
+ *     a raw-pack ladder measures a world nobody is looking at — the Places rail
+ *     silently disagreeing with the camera about where it is.
+ *   - `createMinimap` was handed `entry.city` beside a reconciled `handle.world`,
+ *     so the plan view drew a different California than the frame beside it.
+ *
+ * Neither is visible with the flag off, because `reconciledCity` returns the
+ * pack **by identity** when no rule is on — which is exactly what made the bug
+ * survive: it is latent until the day somebody turns a rule on to take a
+ * measurement, and then it corrupts the measurement rather than announcing
+ * itself.
+ *
+ * ## What is asserted
+ *
+ * Not "main.ts calls the right function" — that is a source-text assertion and
+ * `integration/sceneWiring.test.ts` already owns that genre. This asserts the
+ * *property*: for every pack and every rule, the reconciled object differs from
+ * the raw one in the columns that rule owns, so a consumer holding the wrong one
+ * is holding provably different numbers. A future consumer wired to the raw pack
+ * is a bug this file describes even if it cannot name the call site.
+ */
+import assert from "node:assert/strict";
+import { after, describe, it } from "node:test";
+
+import CALIFORNIA from "../cities/california.ts";
+import SAN_FRANCISCO from "../cities/sf.ts";
+import SOCAL from "../cities/socal.ts";
+import { RULES, reconciledCity, setReconcile, setReconcileWarn } from "../cities/reconcile.ts";
+import type { Rule } from "../cities/reconcile.ts";
+import type { City } from "../engine/types.ts";
+
+const PACKS: readonly (readonly [string, City])[] = [
+  ["california", CALIFORNIA],
+  ["sf", SAN_FRANCISCO],
+  ["socal", SOCAL],
+];
+
+after(() => {
+  setReconcile(null);
+  setReconcileWarn(null);
+});
+
+/** The columns each rule owns, as the reader of a pack would see them. */
+const OWNED: Readonly<Record<Rule, (city: City) => unknown>> = {
+  projection: (city) => city.lngScale ?? null,
+  exaggeration: (city) => city.verticalExaggeration,
+  roads: (city) => city.roads.map((road) => road.width).join(","),
+  ground: (city) => `${city.coastFalloff}|${JSON.stringify(city.palette ?? null)}`,
+};
+
+describe("a reconciled pack is not the pack the module exported", () => {
+  it("returns the very same object when nothing is on", () => {
+    setReconcile(false);
+    for (const [id, pack] of PACKS) {
+      assert.equal(reconciledCity(pack), pack, `${id} was copied with the flag off`);
+    }
+  });
+
+  /**
+   * The load-bearing one. If a rule changes nothing for every pack then a
+   * consumer reading the raw pack is harmless and this whole file is theatre —
+   * so the file has to prove it is not.
+   */
+  it("changes the column its rule owns, for at least one pack, for every rule", () => {
+    for (const rule of RULES) {
+      setReconcile([rule]);
+      const moved = PACKS.filter(([, pack]) => {
+        const read = OWNED[rule];
+        return read(reconciledCity(pack)) !== read(pack);
+      });
+      assert.ok(
+        moved.length > 0,
+        `rule "${rule}" changed nothing on any pack — either the rule is dead ` +
+          "or OWNED is reading the wrong column",
+      );
+    }
+  });
+
+  it("never mutates the pack the module exported", () => {
+    const before = PACKS.map(([, pack]) => JSON.stringify(pack));
+    setReconcile(true);
+    for (const [, pack] of PACKS) reconciledCity(pack);
+    setReconcile(false);
+    for (const [index, [id]] of PACKS.entries()) {
+      assert.equal(JSON.stringify(PACKS[index]![1]), before[index], `${id} was mutated in place`);
+    }
+  });
+
+  /**
+   * The worker's half of the contract. `World` posts `this.city` to the terrain
+   * worker as a structured clone and the worker builds a second `World` from it;
+   * `reconciled: true` is what stops that second pass applying every rule again
+   * on top of itself. Without it the exaggeration rule would square.
+   */
+  it("is idempotent, so the worker's second pass is a no-op", () => {
+    setReconcile(true);
+    for (const [id, pack] of PACKS) {
+      const once = reconciledCity(pack);
+      assert.equal(once.reconciled, true, `${id} did not mark itself reconciled`);
+      assert.equal(reconciledCity(once), once, `${id} was reconciled twice`);
+    }
+  });
+});
+
+describe("the rule parser", () => {
+  it("takes exact rule names", () => {
+    setReconcile(null);
+    for (const rule of RULES) {
+      // Through the public surface: setReconcile with an explicit list is what
+      // a node script uses, and the query-string path shares `parse`.
+      setReconcile([rule]);
+      const read = OWNED[rule];
+      const moved = PACKS.some(([, pack]) => read(reconciledCity(pack)) !== read(pack));
+      assert.ok(moved, `"${rule}" did not select itself`);
+    }
+  });
+
+  /**
+   * The defect: `parse` prefix-matched, so `?reconcile=palette` selected nothing
+   * and produced an empty set — **identical to the flag being absent**. A
+   * photograph taken to judge a rule was a photograph of the raw board, and
+   * nothing in the picture could tell you which.
+   */
+  it("warns loudly rather than silently reading an unknown rule as off", async () => {
+    const said: string[] = [];
+    setReconcileWarn((message) => said.push(message));
+    setReconcile(null);
+    const { activeRules } = await import("../cities/reconcile.ts");
+    const search = { search: "?reconcile=palette" };
+    const globals = globalThis as { location?: unknown };
+    const had = "location" in globals;
+    const previous = globals.location;
+    globals.location = search;
+    try {
+      const on = activeRules();
+      assert.equal(on.size, 0, "an unknown rule must not select a real one");
+      assert.equal(said.length, 1, "an unknown rule must warn exactly once");
+      assert.match(said[0] ?? "", /unknown rule "palette"/);
+      assert.match(said[0] ?? "", /the flag is NOT off/);
+    } finally {
+      if (had) globals.location = previous;
+      else delete globals.location;
+      setReconcileWarn(null);
+    }
+  });
+
+  /**
+   * The other half of prefix-matching: one letter used to select a whole rule,
+   * which is fine until two rules share it. `p` must now select nothing.
+   */
+  it("does not accept an abbreviation", async () => {
+    const said: string[] = [];
+    setReconcileWarn((message) => said.push(message));
+    setReconcile(null);
+    const { activeRules } = await import("../cities/reconcile.ts");
+    const globals = globalThis as { location?: unknown };
+    const had = "location" in globals;
+    const previous = globals.location;
+    globals.location = { search: "?reconcile=p" };
+    try {
+      assert.equal(activeRules().size, 0, '"p" selected a rule by prefix');
+      assert.equal(said.length, 1);
+    } finally {
+      if (had) globals.location = previous;
+      else delete globals.location;
+      setReconcileWarn(null);
+    }
+  });
+});
diff --git a/src/test/render/environmentRig.test.ts b/src/test/render/environmentRig.test.ts
index 3816192..1bc2e4e 100644
--- a/src/test/render/environmentRig.test.ts
+++ b/src/test/render/environmentRig.test.ts
@@ -353,3 +353,145 @@ test("the rig constructs no light of any kind", () => {
   // hence matching the import statement rather than the word.)
   assert.doesNotMatch(source, /^\s*import[^\n]*RoomEnvironment/m);
 });
+
+// ---- The prefetch hazard ---------------------------------------------------
+
+/**
+ * A board being built off-stage must not relight the board on screen.
+ *
+ * ## The mechanism
+ *
+ * `createScene` calls `options.environment?.apply(scene, opening, "city")` at
+ * `scene.ts:811` — unconditionally, and two hundred lines above the
+ * `if (options.present !== false) stage.setScene(stageScene)` at `:1331` that
+ * decides whether anybody is going to look at this scene at all. `main.ts`
+ * builds one rig on one renderer for the life of the page, so an off-stage
+ * board and the visible one are the same rig.
+ *
+ * The rig caches exactly one PMREM target per kind, keyed on a fingerprint of
+ * the lighting. On a key **miss** it does three things, in this order: renders
+ * and convolves a new probe on the shared renderer, `dispose()`s the target the
+ * visible board is currently using, and then walks `applied` reassigning every
+ * scene of that kind to the new texture.
+ *
+ * That is correct and necessary when the miss comes from time passing. It is a
+ * defect when the miss comes from a board **nobody is looking at**: a board
+ * prefetched for another part of the state is observed at its own centre, which
+ * across California is about forty minutes of apparent solar time, so its sun
+ * direction — and therefore its key — *will* differ. The visible board's sky
+ * would change because something invisible was loaded behind it.
+ *
+ * It is harmless today only because every build is followed within about
+ * 800 ms by the swap that makes it the visible board. It stops being harmless
+ * the moment anything builds a board it does not intend to show.
+ *
+ * ## What this asserts
+ *
+ * The property, at the rig's own boundary: applying to a scene while telling the
+ * rig it is off-stage must not disturb any scene already applied. Written before
+ * the fix, and it failed before the fix, which is the only way to know a
+ * regression test is testing anything.
+ */
+test("an off-stage apply does not relight the scene already on screen", () => {
+  const fake = fakeRenderer();
+  const rig = createEnvironmentRig(fake.as());
+
+  const visible = new THREE.Scene();
+  rig.apply(visible, lightingState(), "city");
+  const wasShowing = visible.environment;
+  const wasTarget = targetOf(fake, visible);
+  assert.ok(wasShowing, "the visible board never got an environment");
+
+  // A board prefetched for elsewhere in the state: same kind, different sun.
+  // This is the key miss that does the damage.
+  const offstage = new THREE.Scene();
+  rig.apply(offstage, lightingState({ sun: { direction: [0.62, 0.51, -0.6], color: 0xffd9a8, intensity: 1.7 } }), "city", {
+    offstage: true,
+  });
+
+  assert.equal(
+    visible.environment,
+    wasShowing,
+    "building a board off-stage reassigned the visible board's environment",
+  );
+  assert.equal(
+    wasTarget.texture,
+    wasShowing,
+    "building a board off-stage disposed the texture the visible board is using",
+  );
+
+  rig.dispose();
+});
+
+/**
+ * The other half, and the reason the fix is a deferral rather than a refusal:
+ * the off-stage board must still end up correctly lit when it is presented. A
+ * fix that simply skipped the apply forever would give the incoming board no
+ * environment at all, which is the "plastic" look this rig exists to cure.
+ *
+ * Two cases, and they are different on purpose. With nothing cached there is
+ * nothing to borrow, so the build happens at presentation — which is correct,
+ * and is the same one build the board would have paid anyway. With a probe
+ * already cached on the same key, the off-stage board borrows it and
+ * presentation costs no convolution at all, which is the case that actually
+ * matters: it is what a prefetched board hits when the visitor arrives.
+ */
+test("an off-stage board is lit when it is finally presented", () => {
+  const fake = fakeRenderer();
+  const rig = createEnvironmentRig(fake.as());
+
+  const offstage = new THREE.Scene();
+  rig.apply(offstage, lightingState(), "city", { offstage: true });
+  rig.apply(offstage, lightingState(), "city");
+  assert.ok(offstage.environment, "a presented board has no environment");
+
+  rig.dispose();
+});
+
+test("a prefetched board borrows the cached probe and presents for free", () => {
+  const fake = fakeRenderer();
+  const rig = createEnvironmentRig(fake.as());
+
+  const visible = new THREE.Scene();
+  rig.apply(visible, lightingState(), "city");
+  const built = fake.renders;
+  assert.ok(built > 0, "the first apply did not build anything");
+
+  // The board built behind it, at the same hour, is the common case.
+  const prefetched = new THREE.Scene();
+  rig.apply(prefetched, lightingState(), "city", { offstage: true });
+  assert.equal(prefetched.environment, visible.environment, "the probe was not borrowed");
+  assert.equal(fake.renders, built, "an off-stage apply convolved a new probe");
+
+  // And arriving costs nothing.
+  rig.apply(prefetched, lightingState(), "city");
+  assert.equal(fake.renders, built, "presenting a cache hit convolved a new probe");
+  assert.ok(prefetched.environment, "the presented board lost its environment");
+
+  rig.dispose();
+});
+
+/**
+ * The failure mode the fix must not introduce: an off-stage board whose sun
+ * differs must not go dark. It borrows the cached probe — slightly wrong for its
+ * own hour, and invisible, and corrected the instant it is shown.
+ */
+test("an off-stage board at a different hour borrows rather than going unlit", () => {
+  const fake = fakeRenderer();
+  const rig = createEnvironmentRig(fake.as());
+
+  const visible = new THREE.Scene();
+  rig.apply(visible, lightingState(), "city");
+
+  const elsewhere = new THREE.Scene();
+  rig.apply(
+    elsewhere,
+    lightingState({ sun: { direction: [0.62, 0.51, -0.6], color: 0xffd9a8, intensity: 1.7 } }),
+    "city",
+    { offstage: true },
+  );
+  assert.ok(elsewhere.environment, "an off-stage board at another hour was left with no environment");
+  assert.equal(elsewhere.environment, visible.environment, "it did not borrow the cached probe");
+
+  rig.dispose();
+});