fix: aerial fog follows the camera on the tick, not on OrbitControls change

Drive, crow and aircraft follow-cameras write the pose directly each
frame and never fire `change`. Fog lived on that event, throttled at
2% of altitude, so a descent under a follow camera fell back to the
one-hertz clock and stepped the haze four times instead of easing it.
A 300 ms fog dip got at most one sample.

The recompute now runs on the city's tick, after `roadTraffic.tick`
and the actor/aircraft `setPose` writes — the same pump
`applyReliefRamp` already uses, for the same reason: a camera-dependent
term cannot live on an event the descent does not fire. The 2%
altitude throttle is unchanged, so a chapter flight is still a few
dozen recomputations rather than sixty a second. Orbit `change` still
drives handover and prefetch; those are overview-only.

`aerialFogMoved` is the throttle as a function of two altitudes.
`followCameraFog.test.ts` writes a Drive pose without `controls.update`,
asserts `change` does not fire, and still pulls the fog in from the
new pose.

TODO.md

@@ -54,40 +54,27 @@ things fall out of it that somebody has to pick up.
 ### Aerial perspective is wired, and the coarse seam is now a live defect
 
 Fog now follows camera altitude (`aerialReach` in `engine/atmosphere.ts`, §12.2).
-`main.ts` recomputes the rig on the controls' `change` event, throttled at 2% of
+`main.ts` recomputes the rig on the city's tick (`onCameraFrame`, after the
-the current altitude, which collapses a chapter flight to a few dozen
+follow cameras write the pose), throttled at 2% of the current altitude, which
-recomputations instead of sixty a second.
+collapses a chapter flight to a few dozen recomputations instead of sixty a
+second. Drive, crow and aircraft follow-cameras write the pose directly and do
+not fire OrbitControls `change`; they used to fall back to the one-hertz clock
+and a fast descent stepped the fog four times. That path is on the frame loop.
 
-What that does **not** cover is a camera moved by something other than
+The state-to-metro transition is a **fog dip** — collapse the outgoing board's
-`OrbitControls`: the drive, actor and aircraft follow-cameras set the pose
+`setAerialFog` toward the camera over ~0.3 s, swap in the obscured frame, lift
-directly each frame and do not fire `change`. Those poses fall back to the
+the incoming board's over ~0.5 s — chosen over a crossfade because a crossfade
-one-hertz clock tick, so a fast descent under a follow camera steps the fog up
+cannot afford the triangles (ca+sf is 2,640,307 against the Bay Area's
-to four times instead of easing it.
+2,600,000 cap) and because a dip is the only transition that hides the 4.2x
+vertical-exaggeration pop, the 4 km projection slide and the vanishing 2 km
+freeways at once, since all three happen at maximum obscuration.
 
-**Promoted from hypothetical to live, 2026-08-23.** This entry used to end "a
+That makes fog a **transition mechanism** rather than an ambience setting. The
-continuous state-to-city descent *would* need this on the frame loop rather than
+dip itself is already on the app pump (`stepFogDip`); aerial perspective under
-on an event". That descent is now the thing being built: the state-to-metro
+a follow camera is now on the same kind of pump. What is still true: the 2%
-transition is a **fog dip** — collapse the outgoing board's `setAerialFog`
+throttle is a threshold on *altitude*, and a dip changes near/far with the
-toward the camera over ~0.3 s, swap in the obscured frame, lift the incoming
+altitude held still — which is why the dip writes `setAerialFog` itself rather
-board's over ~0.5 s — chosen over a crossfade because a crossfade cannot afford
+than going through `refreshAerialFog`.
-the triangles (ca+sf is 2,640,307 against the Bay Area's 2,600,000 cap) and
-because a dip is the only transition that hides the 4.2x vertical-exaggeration
-pop, the 4 km projection slide and the vanishing 2 km freeways at once, since
-all three happen at maximum obscuration.
-
-That makes fog a **transition mechanism** rather than an ambience setting, and a
-mechanism driven by an event the descent does not fire is a mechanism that does
-not run. Two specific consequences:
-
-- A dip driven from a follow camera — leaving the corridor drive into the Bay
-  Area is the obvious case — gets the one-hertz tick, so a 300 ms collapse gets
-  at most one step. The dip either does not happen or happens as a single jump.
-- The 2%-of-altitude throttle is a threshold on *altitude*, and a dip changes
-  near/far with the altitude held still. Nothing in the current trigger notices
-  that at all.
-
-So this moves onto the frame loop, and it moves as part of the transition work
-rather than after it.
 
 ### The north is authored, and three things in it are thin
 

src/engine/atmosphere.ts

@@ -548,7 +548,7 @@ export interface AerialFog {
  * California's 1,919 m-wide freeway symbols all at once, because all three
  * happen at maximum obscuration — and it costs zero triangles, zero draw calls
  * and no render target, because `SceneHandle.setAerialFog` already exists and
- * `main.ts` already drives it on every controls `change`.
+ * `main.ts` already drives it from the frame loop.
  *
  * Out is shorter than in on purpose: leaving is an instruction the visitor just
  * gave and wants obeyed, arriving is a picture they are being shown.
@@ -680,12 +680,15 @@ export interface Atmosphere {
    *
    * Because the sun and the camera move at completely different rates and were
    * being served by one call. `main.ts` recomputes on the clock **once a
-   * minute**; it recomputes on the orbit controls' `change` event, throttled to
+   * minute**; it recomputes on the city's tick, throttled to 2% of altitude,
-   * 2% of altitude, which is a few dozen times during a single drag. Routing the
+   * which is a few dozen times during a single drag or a chapter flight. Routing
-   * second of those through `apply` + `Scene.setLighting` fans a camera gesture
+   * the second of those through `apply` + `Scene.setLighting` fans a camera
-   * out across six layer setters, a material `needsUpdate`, an instanced-mesh
+   * gesture out across six layer setters, a material `needsUpdate`, an
-   * rebuild and the environment rig's fingerprint — none of which have anything
+   * instanced-mesh rebuild and the environment rig's fingerprint — none of which
-   * to do with the camera having moved, because none of them read a distance.
+   * have anything to do with the camera having moved, because none of them read
+   * a distance. The tick, rather than OrbitControls `change`, is load-bearing:
+   * drive, crow and aircraft follow-cameras write the pose directly and never
+   * fire that event.
    *
    * ## Why it returns two numbers and not a `LightingState`
    *
@@ -1010,6 +1013,26 @@ export function aerialFog(options: {
   return { near: ceiling.near * reach, far: ceiling.far * reach };
 }
 
+/**
+ * Whether the camera has climbed or dropped far enough to recompute aerial fog.
+ *
+ * Two per cent of the last altitude, floored at a metre so a camera sitting on
+ * the ground does not recompute on every sample. The first sample — `last` not
+ * a finite non-negative number, which is how `main.ts` seeds a new board —
+ * always counts, because a board that just arrived has no previous altitude.
+ *
+ * This is a threshold on *altitude*, not on time and not on the fog pair. A
+ * chapter flight therefore collapses to a few dozen recomputations instead of
+ * sixty a second; a follow-camera descent still steps whenever height moves.
+ * A dip that changes near/far with the altitude held still is a different
+ * trigger and does not go through here.
+ */
+export function aerialFogMoved(altitudeMetres: number, lastAltitudeMetres: number): boolean {
+  if (!(lastAltitudeMetres >= 0)) return true;
+  if (!Number.isFinite(altitudeMetres)) return false;
+  return Math.abs(altitudeMetres - lastAltitudeMetres) >= Math.max(1, lastAltitudeMetres * 0.02);
+}
+
 // ---- The daylight table ---------------------------------------------------
 
 interface Rig {

src/engine/scene.ts

@@ -410,6 +410,18 @@ export interface SceneOptions {
    * `owner-decisions.md` and the note on `TrafficSource.detail`.
    */
   onAircraftPick?: (aircraft: Aircraft | null) => void;
+  /**
+   * After this tick's camera pose is committed.
+   *
+   * Drive, crow and aircraft follow-cameras write the pose directly and never
+   * fire OrbitControls `change`. Aerial fog has to see those writes, so it
+   * lives on this callback — the same pump `applyReliefRamp` already runs on —
+   * rather than on an event the descent does not fire. Called after the follow
+   * cameras have overwritten the pose and before the renderer paints, so a
+   * frame of follow-camera motion is a frame of fog. Absent, the tick is what
+   * it was.
+   */
+  onCameraFrame?: () => void;
   /**
    * The shared environment map, when the page has one.
    *
@@ -515,14 +527,16 @@ export interface SceneHandle {
    * Aerial perspective is the one term in the rig that depends on where the
    * camera is standing, and the camera moves in a completely different tempo
    * from the sky: `main.ts` recomputes the sun once a minute and recomputes the
-   * fog on every orbit step past a 2% altitude threshold — a few dozen times in
+   * fog on this board's tick past a 2% altitude threshold — a few dozen times in
-   * one drag. Before this seam existed both went through `setLighting`, so a
+   * one drag or a chapter flight, and every frame of a follow-camera descent
-   * gesture that changed two floats also re-pushed the sun, the hemisphere, the
+   * that actually moved. Before this seam existed both went through
-   * ambient, the sky dome, the moon and the fog colour into six layers, dirtied
+   * `setLighting`, so a gesture that changed two floats also re-pushed the sun,
-   * a `MeshLambertMaterial` in `ports.ts`, rebuilt the vessel wake instances,
+   * the hemisphere, the ambient, the sky dome, the moon and the fog colour into
-   * and re-fingerprinted the PMREM environment in `environmentRig.ts`. None of
+   * six layers, dirtied a `MeshLambertMaterial` in `ports.ts`, rebuilt the
-   * those read a distance. Measured over a dolly plus a drag, that path costs
+   * vessel wake instances, and re-fingerprinted the PMREM environment in
-   * 0.14-0.19 ms a step against 0.02 ms for this one.
+   * `environmentRig.ts`. None of those read a distance. Measured over a dolly
+   * plus a drag, that path costs 0.14-0.19 ms a step against 0.02 ms for this
+   * one.
    *
    * ## Why it carries no colour, which is the load-bearing half
    *
@@ -1687,6 +1701,17 @@ export async function createScene(
       if (ownership.aircraft && sceneAircraft) kit.setPose(sceneAircraft.followPose());
       realtimePeers?.tick(Date.now());
       roadTraffic?.tick(dt);
+      /**
+       * Aerial fog, after every camera owner has written this frame's pose.
+       *
+       * `applyReliefRamp` above is the model: a camera-dependent term that
+       * cannot live on OrbitControls `change`, because a follow camera never
+       * fires it. Drive writes `camera.position` inside `roadTraffic.tick`;
+       * crow and aircraft call `setPose` just above. Calling here means a
+       * descent under any of them is a fog step on the same frame, not a 1 Hz
+       * jump on the clock.
+       */
+      options.onCameraFrame?.();
       clouds.tick(dt);
       fireLayer?.tick(dt);
       vesselLayer?.tick(dt);

src/main.ts

@@ -30,6 +30,7 @@
  */
 
 import {
+  aerialFogMoved,
   collapsedFog,
   createAtmosphere,
   dipFog,
@@ -736,17 +737,25 @@ let cityFlights: TrafficSource | null = null;
  */
 let fireWatch: FireWatch | null = null;
 /**
- * Stops this board's camera listening to the fog, or `null` between boards.
+ * Stops this board's camera listening for handover and prefetch, or `null`
+ * between boards.
  *
- * The rig follows the clock at about one hertz, which was fine while every term
+ * Aerial fog used to live on the same OrbitControls `change` listener. Drive,
- * in it was a function of time. Aerial perspective is a function of where the
+ * crow and aircraft follow-cameras write the pose directly and never fire
- * camera is, and a camera can cross a board in a second — so a fog recomputed
+ * `change`, so a descent under those stepped the haze at one hertz. Fog now
- * only on the clock steps four times during a chapter flight and pops each
+ * lives on the city's tick — see `refreshAerialFog` — and these listeners are
- * time. `OrbitControls` fires `change` on every update that actually moved
+ * what remains: free-camera promotion and the background lane, both of which
- * something, including damping and including `setPose`, which is exactly the
+ * are overview-only and still need to know the orbit moved.
- * event this needs and is already the event the minimap redraws on.
  */
 let cameraFogWatch: (() => void) | null = null;
+/**
+ * Last altitude `refreshAerialFog` actually applied at, in metres.
+ *
+ * Seeded at `-1` so the first sample of a board always counts. Reset in
+ * `activateBoard` rather than in `refreshAerialFog`, because a board that has
+ * just arrived must not inherit the altitude of the one it replaced.
+ */
+let lastAerialFogAltitude = -1;
 /** The panel that says what the board is showing, and what it is not. */
 let firePanel: FirePanelHandle | null = null;
 /** The board's own rectangle, held so a poll landing later can be clipped to it. */
@@ -1227,11 +1236,12 @@ function houseLevelFor(solarElevationDeg: number): number {
  *
  * Two things move at completely different rates and were being served by one
  * call. The **sun** moves on the wall clock, which this file steps once a
- * minute. The **camera** moves on a gesture: `change` fires every frame under
+ * minute. The **camera** moves on the city's tick: follow cameras write a new
- * damping, and even throttled to 2% of altitude a single drag gets through a few
+ * pose every frame, damping writes one every frame of a drag, and even throttled
- * dozen times. This function is the second of those, and it now sends the only
+ * to 2% of altitude a single chapter flight gets through a few dozen times.
- * thing that is actually a fact about the camera — how far you can see from
+ * This function is the second of those, and it now sends the only thing that is
- * where it is standing.
+ * actually a fact about the camera — how far you can see from where it is
+ * standing.
  *
  * What it used to send was the whole rig, and the whole rig is a fan-out: six
  * layer setters, a dirtied `MeshLambertMaterial` in the port yard, a rebuild of
@@ -1272,6 +1282,24 @@ function applyCameraFog(): void {
   );
 }
 
+/**
+ * Aerial fog, on the city's tick, throttled at 2% of altitude.
+ *
+ * `applyReliefRamp` is the model: a camera-dependent term that has to see the
+ * pose *this frame*, including the pose a follow camera just wrote. Drive,
+ * crow and aircraft never fire OrbitControls `change`, so a listener on that
+ * event is a 1 Hz clock with extra steps. The throttle is what keeps a chapter
+ * flight from recomputing sixty times a second for a number that has not
+ * visibly moved.
+ */
+function refreshAerialFog(): void {
+  if (!city) return;
+  const altitude = city.cameraAltitudeMetres();
+  if (!aerialFogMoved(altitude, lastAerialFogAltitude)) return;
+  lastAerialFogAltitude = altitude;
+  applyCameraFog();
+}
+
 function updateSun() {
   const active = CITIES.find((c) => c.id === cityId)?.city ?? SAN_FRANCISCO;
 
@@ -2037,6 +2065,14 @@ async function buildBoard(
      * `showAircraftDetail` for where the provenance comes from.
      */
     onAircraftPick: (a) => showAircraftDetail(a),
+    /**
+     * Aerial fog, on the same pump as the follow cameras that write the pose.
+     *
+     * Drive, crow and aircraft never fire OrbitControls `change`. Putting the
+     * recompute here — after those writes, throttled at 2% of altitude — is
+     * what makes a descent under any of them a fog dip instead of a 1 Hz jump.
+     */
+    onCameraFrame: () => refreshAerialFog(),
     signal: mount.signal,
     /**
      * Built, not shown. `presentBoard` puts it on the stage once the fog has
@@ -2115,12 +2151,13 @@ async function buildBoard(
   /**
    * Recompute the **fog** when the camera moves, not only when the clock does.
    *
-   * Throttled on the altitude itself rather than on time: `change` fires every
+   * Throttled on the altitude itself rather than on time, and driven from the
-   * frame under damping and building a rig walks a keyframe table and a lunar
+   * city's tick (`onCameraFrame`) rather than from OrbitControls `change`.
-   * ephemeris, which is not work to do sixty times a second for a number that
+   * Follow cameras write the pose directly and never fire `change`; a chapter
-   * has not moved. Two per cent of the current altitude is under the threshold
+   * flight under damping would otherwise recompute sixty times a second for a
-   * at which the fog planes visibly shift, and it collapses a whole chapter
+   * number that has not moved. Two per cent of the current altitude is under
-   * flight to a few dozen recomputations.
+   * the threshold at which the fog planes visibly shift, and it collapses a
+   * whole chapter flight to a few dozen recomputations.
    *
    * `applyCameraFog` and not `updateSun`, and not `setLighting` either. The
    * clock tick ends in `renderChrome()`, a DOM pass with no business running
@@ -2587,17 +2624,18 @@ function activateBoard(record: MountedBoard): void {
   // where the sun's elevation comes from.
   if (record.carriesSky) void askSky();
 
+  lastAerialFogAltitude = -1;
   {
     const controls = record.handle.stageScene.controls;
     let lastAltitude = -1;
     const onCameraMoved = () => {
       const altitude = record.handle.cameraAltitudeMetres();
-      // Two per cent of the current altitude, floored at a metre so a camera
+      // Two per cent of the current altitude, same gate the fog uses. Fog
-      // resting exactly on the ground does not recompute on every event.
+      // itself no longer lives here — follow cameras never fire `change` —
-      const moved = Math.abs(altitude - lastAltitude);
+      // but handover and prefetch still should not run sixty times a second
-      if (lastAltitude >= 0 && moved < Math.max(1, lastAltitude * 0.02)) return;
+      // under damping.
+      if (!aerialFogMoved(altitude, lastAltitude)) return;
       lastAltitude = altitude;
-      applyCameraFog();
       maybeHandover(record);
       maybePrefetch(record);
     };

src/test/integration/sceneWiring.test.ts

@@ -708,10 +708,24 @@ test("a camera step never reaches the light, and so never reaches the environmen
       "sole light owner, so the app may not work a distance out for itself",
   );
   assert.ok(
-    /controls\.addEventListener\("change", onCameraMoved\)/.test(MAIN) &&
+    /onCameraFrame:\s*\(\)\s*=>\s*refreshAerialFog\(\)/.test(MAIN),
-      /applyCameraFog\(\)/.test(MAIN),
+    "aerial fog must be on the city's tick; OrbitControls `change` is not fired by " +
-    "the orbit `change` event must still drive the fog; without it a chase camera " +
+      "drive, crow or aircraft follow-cameras, so a descent under those used to " +
-      "on the state board flies through the haze the whole feature exists to draw",
+      "step the haze at 1 Hz",
+  );
+  assert.ok(
+    /function refreshAerialFog/.test(MAIN) &&
+      /aerialFogMoved\(altitude, lastAerialFogAltitude\)/.test(MAIN),
+    "the tick path must keep the 2% altitude throttle; without it a chapter flight " +
+      "recomputes sixty times a second for a number that has not moved",
+  );
+  const sceneTick = scene.indexOf("roadTraffic?.tick(dt);");
+  assert.ok(sceneTick > 0, "the drive follow camera has been renamed or deleted");
+  const afterFollow = scene.slice(sceneTick, sceneTick + 800);
+  assert.ok(
+    /options\.onCameraFrame\?\.\(\)/.test(afterFollow),
+    "onCameraFrame must run after the follow cameras write the pose, not before; " +
+      "a callback above roadTraffic.tick would still see last frame's drive camera",
   );
 
   const setter = scene.indexOf("setAerialFog: (");

src/test/render/aerialPerspective.test.ts

@@ -30,6 +30,7 @@ import { describe, it } from "node:test";
 
 import {
   aerialFog,
+  aerialFogMoved,
   aerialReach,
   createAtmosphere,
   observe,
@@ -455,3 +456,37 @@ describe("the camera moves the fog and nothing else", () => {
     }
   });
 });
+
+/**
+ * The 2% altitude throttle, as a property rather than as a comment in main.ts.
+ *
+ * A chapter flight would otherwise recompute sixty times a second for a number
+ * the eye cannot resolve, and a camera sitting on the ground would recompute
+ * on every sample of noise. Follow-camera descents still have to get through:
+ * the first sample always counts, and a drop past 2% of the last altitude
+ * counts, which is the whole of what putting fog on the tick is for.
+ */
+describe("aerial fog recomputes when altitude has actually moved", () => {
+  it("always fires on the first sample of a board", () => {
+    assert.equal(aerialFogMoved(12_000, -1), true);
+    assert.equal(aerialFogMoved(0, Number.NaN), true);
+  });
+
+  it("ignores a move under two per cent of the last altitude", () => {
+    assert.equal(aerialFogMoved(101, 100), false);
+    assert.equal(aerialFogMoved(10_199, 10_000), false);
+    assert.equal(aerialFogMoved(9_801, 10_000), false);
+  });
+
+  it("fires at two per cent and above", () => {
+    assert.equal(aerialFogMoved(102, 100), true);
+    assert.equal(aerialFogMoved(9_800, 10_000), true);
+    assert.equal(aerialFogMoved(1_200, 40_000), true);
+  });
+
+  it("floors the threshold at a metre, so a camera on the ground is still", () => {
+    assert.equal(aerialFogMoved(0.4, 0), false);
+    assert.equal(aerialFogMoved(1, 0), true);
+    assert.equal(aerialFogMoved(1.5, 0.5), true);
+  });
+});

src/test/render/followCameraFog.test.ts

@@ -0,0 +1,113 @@
+/**
+ * Aerial fog on a follow camera, which never fires OrbitControls `change`.
+ *
+ * Drive writes `camera.position` and `controls.target` then `lookAt`, with no
+ * `controls.update()`. That is structurally silent: the orbit `change` listener
+ * that used to own aerial fog never runs, and a fast descent stepped the haze
+ * at one hertz off the clock. Crow and aircraft call `setPose`, which does
+ * `update()`, but Drive is the case that cannot be rescued by that event.
+ *
+ * This file is that silence, and the frame-pump path that still moves the fog:
+ * read the pose, ask `aerialFog` for the pair, `setFogDistances`. `main.ts`
+ * wires the same three calls to the city's tick; the source test in
+ * `sceneWiring.test.ts` pins that the app actually does it.
+ */
+
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import * as THREE from "three";
+
+(globalThis as unknown as { window: unknown }).window = {
+  matchMedia: () => ({ matches: false, addEventListener() {}, removeEventListener() {} }),
+  innerWidth: 1_200,
+  innerHeight: 800,
+  devicePixelRatio: 1,
+  addEventListener() {},
+  removeEventListener() {},
+};
+
+const { createSceneKit } = await import("../../engine/scenekit.ts");
+const { aerialFog, aerialFogMoved } = await import("../../engine/atmosphere.ts");
+
+function fakeDom(): HTMLElement {
+  return {
+    style: {},
+    clientWidth: 1_200,
+    clientHeight: 800,
+    addEventListener() {},
+    removeEventListener() {},
+    setPointerCapture() {},
+    releasePointerCapture() {},
+    getBoundingClientRect: () => ({
+      left: 0, top: 0, width: 1_200, height: 800, right: 1_200, bottom: 800, x: 0, y: 0,
+    }),
+    getRootNode: () => ({ addEventListener() {}, removeEventListener() {} }),
+    ownerDocument: { addEventListener() {}, removeEventListener() {} },
+  } as unknown as HTMLElement;
+}
+
+/** A lighting rig that exists only to install `scene.fog`. */
+function foggedLight(near: number, far: number) {
+  return {
+    sun: { direction: [0, 1, 0] as [number, number, number], color: 0xffffff, intensity: 1 },
+    hemisphere: { sky: 0xffffff, ground: 0x888888, intensity: 0.4 },
+    ambient: { color: 0xffffff, intensity: 0.2 },
+    sky: { top: 0x88aaff, horizon: 0xaaccff },
+    fog: { color: 0xaaccff, near, far },
+  };
+}
+
+describe("a follow-camera pose that does not fire change still updates fog", () => {
+  it("Drive writes the pose without controls.update, and the frame pump still pulls the fog in", () => {
+    const scene = new THREE.Scene();
+    const kit = createSceneKit({ scene, dom: fakeDom() });
+    const metresPerUnit = 100;
+    const ceiling = { near: 1_150, far: 3_900 };
+
+    kit.setPose({
+      position: new THREE.Vector3(0, 400, 400),
+      target: new THREE.Vector3(0, 0, 0),
+    });
+    const highView = {
+      altitudeMetres: (kit.camera.position.y - kit.controls.target.y) * metresPerUnit,
+      standoffMetres: kit.camera.position.distanceTo(kit.controls.target) * metresPerUnit,
+    };
+    const highFog = aerialFog({ ceiling, metresPerUnit, ...highView });
+    kit.applyLighting(foggedLight(highFog.near, highFog.far));
+    assert.ok(scene.fog instanceof THREE.Fog);
+    const farAtHeight = scene.fog.far;
+
+    let changeFired = 0;
+    kit.controls.addEventListener("change", () => {
+      changeFired += 1;
+    });
+
+    // Drive follow: write the pose, look at the target, no controls.update().
+    kit.camera.position.set(0, 20, 20);
+    kit.controls.target.set(0, 0, 0);
+    kit.camera.lookAt(kit.controls.target);
+    assert.equal(changeFired, 0, "Drive follow must not fire OrbitControls change");
+
+    const lowView = {
+      altitudeMetres: (kit.camera.position.y - kit.controls.target.y) * metresPerUnit,
+      standoffMetres: kit.camera.position.distanceTo(kit.controls.target) * metresPerUnit,
+    };
+    assert.ok(
+      aerialFogMoved(lowView.altitudeMetres, highView.altitudeMetres),
+      "a drop this large is past the 2% throttle; the test is not measuring a no-op",
+    );
+    const lowFog = aerialFog({ ceiling, metresPerUnit, ...lowView });
+    kit.setFogDistances(lowFog.near, lowFog.far);
+
+    assert.ok(scene.fog instanceof THREE.Fog);
+    assert.equal(scene.fog.near, lowFog.near);
+    assert.equal(scene.fog.far, lowFog.far);
+    assert.ok(
+      scene.fog.far < farAtHeight,
+      `a lower camera must pull the fog in (${scene.fog.far} after ${farAtHeight})`,
+    );
+    assert.equal(changeFired, 0, "applying fog must not need a change event");
+
+    kit.dispose();
+  });
+});