fix(camera): keep the globe in view from orbit

From the whole-globe view the camera could end up facing empty space with
only stars on screen. Cesium's free look (Shift+drag) turns the camera in
place, and from 18,000 km the planet's edge is only about 15 degrees from
nadir, so a few drags turned it out of frame. The rolled pose was then
saved into the share hash, so a reload or shared link opened on stars too.

- Allow free look only below 1,000 km. Orbit, tilt and zoom are unchanged.
- Measure how much of the frame shows the planet's surface by casting a
  32x18 grid of view rays at the WGS84 ellipsoid. While the scope mask is
  on (the default) only the rays inside its keyhole count, since the mask
  hides or dims the rest of the frame.
- When a camera move ends above 1,000 km with no ray on the surface, turn
  straight down at the same position and heading. Limb glow and a sliver
  up to about one grid cell thick can count as out of view. The end of a
  move comes from Cesium's moveEnd and from a check of its own that
  samples the pose every 250 ms after moveStart and acts once it has held
  for half a second: at some of these poses the camera drifts by a
  rounding error every frame, so Cesium never sees it stop and never
  raises moveEnd. The zoom-out guard from #1522 keeps wheel and pinch
  zoom-outs in view; this catches the other ways a move can end looking
  away, such as a saved or shared view that did, or a
  drag that Cesium turns into an in-place look below its 7,500 km
  trackball height (one that starts on the stars, or a tilt drag with the
  screen centre off the planet). Views with more surface, tracked or
  locked cameras and the 2D and Columbus modes are left alone.
- Voice tilt and in-place turns of a free 3D globe camera above 1,000 km
  stop when about a tenth of the frame still shows the planet (or about as
  much as it showed when it started, two grid cells of sampling noise
  allowed), never below about 1% of the visible frame; a view that already
  shows less only keeps or gains what it has. A turn that cannot go half a
  degree further without crossing that limit is refused with a reason
  instead of reported as done, and a turn it accepts turns at least that
  half degree. A turn that keeps or gains surface is not refused, even
  after the window has been resized, except that near the limit, where the
  planet's edge crosses the sample grid, sampling wobble can occasionally
  refuse one (most often above about 15,000 km and on phone or ultrawide
  frames); nothing then turns away from the planet. A turn that starts
  where the last voice turn left the camera keeps that turn's limit, or a
  higher one if the planet now fills more, so repeating the request does
  not take any more; once anything else has moved or turned the camera, a
  turn is measured from what is on screen. A request made while the camera
  is tracked is judged by the pose voice's release of the follow camera
  will leave, before that release, so a refused turn keeps the tracking.

The frame measure and the look-step limit live in src/globeInView.js,
shared by the viewer and the voice camera verbs.

src/app/application.test.mjs

@@ -2,7 +2,20 @@ import test from 'node:test';
 import assert from 'node:assert/strict';
 import { spawnSync } from 'node:child_process';
 import { createApplication } from './application.js';
-import { installTrackpadPinchZoom } from './viewer.js';
+import { KEYHOLE_OUTER_RADIUS } from '../celestialRing.js';
+import * as Cesium from 'cesium';
+import {
+  installFreeLookHeightLimit,
+  installGlobeInViewRecovery,
+  installTrackpadPinchZoom,
+} from './viewer.js';
+import {
+  FREE_LOOK_MAX_HEIGHT_METERS,
+  globeFrameShare,
+  globeOutOfFrame,
+  lookAngleWithinFrame,
+  lookStepLosesGlobe,
+} from '../globeInView.js';
 
 const phases = ['Scene', 'Controls', 'Data', 'Tools'];
 function fixture(overrides = {}) {
@@ -343,3 +356,419 @@ test('trackpad pinch falls back to the original event when relay construction fa
   assert.equal(event.propagationStopped, false);
   assert.equal(f.relayed.length, 0);
 });
+
+function freeLookFixture({ enableLook = true, height = 0 } = {}) {
+  const listeners = new Set();
+  const controller = { enableLook };
+  const camera = { positionCartographic: { height } };
+  const viewer = {
+    camera,
+    scene: {
+      screenSpaceCameraController: controller,
+      preUpdate: {
+        addEventListener(listener) {
+          listeners.add(listener);
+          return () => listeners.delete(listener);
+        },
+      },
+    },
+  };
+  const frame = (nextHeight) => {
+    camera.positionCartographic = { height: nextHeight };
+    for (const listener of listeners) listener();
+  };
+  return { viewer, controller, listeners, frame };
+}
+
+test('free look follows camera height and is restored on dispose', () => {
+  const f = freeLookFixture({ height: 500 });
+  const dispose = installFreeLookHeightLimit(f.viewer);
+  assert.equal(f.controller.enableLook, true);
+
+  f.frame(FREE_LOOK_MAX_HEIGHT_METERS + 1);
+  assert.equal(f.controller.enableLook, false);
+  f.frame(FREE_LOOK_MAX_HEIGHT_METERS);
+  assert.equal(f.controller.enableLook, true);
+  f.frame(Number.NaN);
+  assert.equal(f.controller.enableLook, true);
+
+  f.frame(FREE_LOOK_MAX_HEIGHT_METERS * 20);
+  dispose();
+  dispose();
+  assert.equal(f.controller.enableLook, true);
+  assert.equal(f.listeners.size, 0);
+});
+
+test('free look stays off when the viewer had it disabled', () => {
+  const f = freeLookFixture({ enableLook: false, height: 500 });
+  installFreeLookHeightLimit(f.viewer);
+  f.frame(10);
+  assert.equal(f.controller.enableLook, false);
+});
+
+// A real Cesium camera on the smallest scene its constructor reads, so the
+// frustum test runs on Cesium's own view math rather than a stub.
+function realCamera({ width = 1600, height = 900, mode } = {}) {
+  const scene = {
+    canvas: { clientWidth: width, clientHeight: height },
+    drawingBufferWidth: width,
+    drawingBufferHeight: height,
+    mapProjection: new Cesium.GeographicProjection(),
+    mode: mode ?? Cesium.SceneMode.SCENE3D,
+    pixelRatio: 1,
+    ellipsoid: Cesium.Ellipsoid.WGS84,
+    mapMode2D: Cesium.MapMode2D.INFINITE_SCROLL,
+    renders: 0,
+    requestRender() {
+      this.renders += 1;
+    },
+  };
+  const camera = new Cesium.Camera(scene);
+  camera.frustum.aspectRatio = width / height;
+  return { camera, scene, viewer: { camera, scene } };
+}
+
+function aim(
+  camera,
+  { lon = -94.69, lat = 18.87, alt, heading = 0, pitch, roll = 0 },
+) {
+  camera.setView({
+    destination: Cesium.Cartesian3.fromDegrees(lon, lat, alt),
+    orientation: {
+      heading: Cesium.Math.toRadians(heading),
+      pitch: Cesium.Math.toRadians(pitch),
+      roll: Cesium.Math.toRadians(roll),
+    },
+  });
+}
+
+const reportedStuckView = {
+  alt: 17_996_811,
+  heading: 180,
+  pitch: -42,
+  roll: 180,
+};
+// Oblique poses that still show part of the planet: Director recipe shots
+// (Global Flights Radar, Omniscience Pullback) and a voice tilt from orbit.
+const partialGlobeViews = [
+  { lon: -30, lat: 20, alt: 19_000_000, pitch: -65 },
+  { lon: 2, lat: 46, alt: 8_000_000, pitch: -52 },
+  { lon: 110, lat: 20, alt: 9_500_000, pitch: -66 },
+  { lon: 0, lat: 0, alt: 2_000_000, pitch: -30 },
+];
+
+// Stars and a few pixels of limb, no surface: where a frame limit measured
+// against a sphere of the planet's largest radius used to stop a voice tilt.
+const limbOnlyView = { alt: 18_000_000, pitch: -56.75 };
+
+/**
+ * An oracle independent of the code under test: the share of an 80×45 grid of
+ * Cesium pick rays across the canvas that hit the ellipsoid surface.
+ */
+function surfacePixelShare(camera, width, height, { keyhole = false } = {}) {
+  const radius = (KEYHOLE_OUTER_RADIUS * height) / 2;
+  let hits = 0;
+  let counted = 0;
+  for (let row = 0; row < 45; row += 1) {
+    for (let column = 0; column < 80; column += 1) {
+      const x = ((column + 0.5) * width) / 80;
+      const y = ((row + 0.5) * height) / 45;
+      if (keyhole && Math.hypot(x - width / 2, y - height / 2) > radius)
+        continue;
+      counted += 1;
+      const ray = camera.getPickRay(new Cesium.Cartesian2(x, y));
+      if (Cesium.IntersectionTests.rayEllipsoid(ray, Cesium.Ellipsoid.WGS84))
+        hits += 1;
+    }
+  }
+  return hits / counted;
+}
+
+test('the globe is out of frame only when no surface is on screen', () => {
+  const cases = [
+    [reportedStuckView, 1225, 923, true],
+    [limbOnlyView, 1600, 900, true],
+    [{ alt: 17_996_811, pitch: -90 }, 1225, 923, false],
+    ...partialGlobeViews.map((view) => [view, 1600, 900, false]),
+  ];
+  for (const [view, width, height, out] of cases) {
+    const { camera } = realCamera({ width, height });
+    aim(camera, view);
+    const label = JSON.stringify(view);
+    assert.equal(globeOutOfFrame(camera), out, label);
+    assert.equal(surfacePixelShare(camera, width, height) === 0, out, label);
+    assert.ok(
+      Math.abs(
+        globeFrameShare(camera) - surfacePixelShare(camera, width, height),
+      ) < 0.03,
+      `${label}: the frame share matches the pick-ray oracle`,
+    );
+  }
+});
+
+test('a move that ends with the globe out of frame turns back to nadir', () => {
+  const { camera, scene, viewer } = realCamera({ width: 1225, height: 923 });
+  const dispose = installGlobeInViewRecovery(viewer);
+  aim(camera, reportedStuckView);
+  const position = Cesium.Cartesian3.clone(camera.positionWC);
+  const heading = camera.heading;
+  camera.moveEnd.raiseEvent();
+  assert.ok(Cesium.Math.equalsEpsilon(camera.pitch, -Math.PI / 2, 1e-6));
+  assert.ok(Cesium.Math.equalsEpsilon(camera.heading, heading, 1e-6));
+  assert.ok(
+    Cesium.Cartesian3.equalsEpsilon(camera.positionWC, position, 0, 1e-3),
+  );
+  assert.equal(globeOutOfFrame(camera), false);
+  assert.equal(scene.renders, 1);
+
+  // Looking up near the ground, then zoomed out past the limit.
+  aim(camera, { alt: 3_834_000, pitch: 10 });
+  camera.moveEnd.raiseEvent();
+  assert.ok(Cesium.Math.equalsEpsilon(camera.pitch, -Math.PI / 2, 1e-6));
+
+  // Only the limb on a 16:9 screen still counts as lost.
+  const wide = realCamera({ width: 1600, height: 900 });
+  const disposeWide = installGlobeInViewRecovery(wide.viewer);
+  aim(wide.camera, limbOnlyView);
+  wide.camera.moveEnd.raiseEvent();
+  assert.ok(Cesium.Math.equalsEpsilon(wide.camera.pitch, -Math.PI / 2, 1e-6));
+  disposeWide();
+
+  dispose();
+  dispose();
+  aim(camera, reportedStuckView);
+  camera.moveEnd.raiseEvent();
+  assert.equal(globeOutOfFrame(camera), true, 'disposed recovery is inert');
+});
+
+// True when a moveEnd leaves the camera's view direction as it was.
+function moveEndKeepsView(camera) {
+  const direction = Cesium.Cartesian3.clone(camera.directionWC);
+  camera.moveEnd.raiseEvent();
+  return Cesium.Cartesian3.equalsEpsilon(camera.directionWC, direction, 1e-9);
+}
+
+test('globe recovery leaves partial, low, locked and 2D views alone', () => {
+  for (const view of partialGlobeViews) {
+    const { camera, viewer } = realCamera();
+    installGlobeInViewRecovery(viewer);
+    aim(camera, view);
+    assert.ok(moveEndKeepsView(camera), JSON.stringify(view));
+  }
+
+  const low = realCamera();
+  installGlobeInViewRecovery(low.viewer);
+  aim(low.camera, { alt: 900_000, pitch: 85 });
+  assert.equal(globeOutOfFrame(low.camera), true);
+  assert.ok(moveEndKeepsView(low.camera), 'below the limit');
+
+  // Locked on a point in space, between it and the planet, facing away.
+  const locked = realCamera();
+  installGlobeInViewRecovery(locked.viewer);
+  locked.camera.lookAtTransform(
+    Cesium.Matrix4.fromTranslation(new Cesium.Cartesian3(30_000_000, 0, 0)),
+    new Cesium.Cartesian3(-1_000_000, 0, 0),
+  );
+  assert.equal(globeOutOfFrame(locked.camera), true);
+  assert.ok(moveEndKeepsView(locked.camera), 'locked camera');
+
+  // Tracking an entity that has no position yet keeps an identity transform.
+  const tracked = realCamera();
+  tracked.viewer.trackedEntity = { id: 'pending-fix' };
+  installGlobeInViewRecovery(tracked.viewer);
+  aim(tracked.camera, reportedStuckView);
+  assert.ok(moveEndKeepsView(tracked.camera), 'tracked camera');
+
+  const flat = realCamera({ mode: Cesium.SceneMode.COLUMBUS_VIEW });
+  installGlobeInViewRecovery(flat.viewer);
+  flat.scene.mode = Cesium.SceneMode.SCENE3D;
+  aim(flat.camera, reportedStuckView);
+  flat.scene.mode = Cesium.SceneMode.COLUMBUS_VIEW;
+  assert.ok(moveEndKeepsView(flat.camera), 'Columbus view');
+});
+
+test('the look limit from orbit tolerates grid noise and never reaches zero', () => {
+  const radians = Cesium.Math.toRadians;
+  // About 2% of a 16:9 frame on the planet, below the 10% limit.
+  const sliver = { alt: 18_000_000, pitch: -60 };
+  const { camera } = realCamera();
+  aim(camera, sliver);
+  const start = surfacePixelShare(camera, 1600, 900);
+  assert.ok(start > 0 && start < 0.1, `starts below the limit: ${start}`);
+
+  // Sideways across the sliver: the share wobbles by a cell, the planet stays.
+  assert.equal(lookStepLosesGlobe(camera, 'lookLeft', radians(0.5)), false);
+  const startShare = globeFrameShare(camera);
+  for (let i = 0; i < 20; i += 1) {
+    const step = lookAngleWithinFrame(camera, 'lookLeft', radians(0.25), {
+      startShare,
+    });
+    assert.equal(step, radians(0.25), `sideways step ${i + 1}`);
+    camera.lookLeft(step);
+  }
+  assert.ok(surfacePixelShare(camera, 1600, 900) > 0);
+
+  // Toward the planet is always allowed.
+  aim(camera, sliver);
+  assert.equal(
+    lookAngleWithinFrame(camera, 'lookDown', radians(10)),
+    radians(10),
+  );
+
+  // Away from it stops short of losing it.
+  aim(camera, sliver);
+  const away = lookAngleWithinFrame(camera, 'lookUp', radians(30));
+  assert.ok(away < radians(30));
+  camera.lookUp(away);
+  assert.ok(globeFrameShare(camera) > 0);
+  assert.ok(surfacePixelShare(camera, 1600, 900) > 0);
+});
+
+test('the look limit leaves a view with no surface to the move-end recovery', () => {
+  const { camera } = realCamera({ width: 1225, height: 923 });
+  aim(camera, reportedStuckView);
+  assert.equal(
+    lookAngleWithinFrame(camera, 'lookUp', Cesium.Math.toRadians(5)),
+    Cesium.Math.toRadians(5),
+  );
+});
+
+// Surface only in the corners of a 16:9 frame, all of it outside the scope's
+// keyhole: the full frame shows some planet, the scope shows none.
+const cornerOnlyView = { alt: 18_000_000, pitch: -52, roll: 60 };
+
+test('with a scope keyhole, surface hidden in the masked corners counts as none', () => {
+  const { camera } = realCamera();
+  aim(camera, cornerOnlyView);
+  const keyhole = { keyholeRadius: KEYHOLE_OUTER_RADIUS };
+  assert.ok(surfacePixelShare(camera, 1600, 900) > 0);
+  assert.equal(surfacePixelShare(camera, 1600, 900, { keyhole: true }), 0);
+  assert.equal(globeOutOfFrame(camera), false);
+  assert.equal(globeOutOfFrame(camera, keyhole), true);
+  aim(camera, { alt: 18_000_000, pitch: -90 });
+  assert.ok(
+    Math.abs(
+      globeFrameShare(camera, keyhole) -
+        surfacePixelShare(camera, 1600, 900, { keyhole: true }),
+    ) < 0.03,
+    'the keyhole share matches the pick-ray oracle',
+  );
+});
+
+test('the recovery follows the scope: corner-only views are turned back only behind it', () => {
+  for (const [scopeOn, expectRecovery] of [
+    [true, true],
+    [false, false],
+  ]) {
+    const { camera, viewer } = realCamera();
+    installGlobeInViewRecovery(viewer, {
+      keyholeRadius: () => (scopeOn ? KEYHOLE_OUTER_RADIUS : 0),
+    });
+    aim(camera, cornerOnlyView);
+    camera.moveEnd.raiseEvent();
+    assert.equal(
+      Cesium.Math.equalsEpsilon(camera.pitch, -Math.PI / 2, 1e-6),
+      expectRecovery,
+      `scope ${scopeOn ? 'on' : 'off'}`,
+    );
+  }
+});
+
+function fakeTimers() {
+  let next = 1;
+  const pending = new Map();
+  return {
+    setTimeout: (callback) => {
+      const id = next++;
+      pending.set(id, callback);
+      return id;
+    },
+    clearTimeout: (id) => pending.delete(id),
+    pending: () => pending.size,
+    /** Run the earliest pending callback, as 250 ms passing would. */
+    tick() {
+      const [id, callback] = pending.entries().next().value ?? [];
+      if (id === undefined) return false;
+      pending.delete(id);
+      callback();
+      return true;
+    },
+  };
+}
+
+test('a stuck view is recovered once it has held still, without waiting for moveEnd', () => {
+  // At this pose the camera can drift by a rounding error every frame, so
+  // Cesium never sees it stop and never raises moveEnd for it.
+  const timers = fakeTimers();
+  const { camera, viewer } = realCamera({ width: 1225, height: 923 });
+  installGlobeInViewRecovery(viewer, timers);
+  aim(camera, reportedStuckView);
+  camera.moveStart.raiseEvent();
+  timers.tick();
+  assert.equal(globeOutOfFrame(camera), true, 'one quiet sample is not enough');
+  // A rounding-error drift between samples still counts as at rest.
+  camera.lookUp(1e-15);
+  timers.tick();
+  assert.ok(Cesium.Math.equalsEpsilon(camera.pitch, -Math.PI / 2, 1e-6));
+  // Under that drift Cesium raises no new moveStart, so a later move that
+  // ends looking away is caught by the same check.
+  aim(camera, reportedStuckView);
+  timers.tick();
+  timers.tick();
+  assert.equal(globeOutOfFrame(camera), true, 'not yet at rest');
+  timers.tick();
+  assert.ok(Cesium.Math.equalsEpsilon(camera.pitch, -Math.PI / 2, 1e-6));
+  camera.moveEnd.raiseEvent();
+  assert.equal(timers.pending(), 0, 'moveEnd ends the check');
+});
+
+test('the settle check follows a long move and gives up five minutes after the last one', () => {
+  // A drag of half a minute that stops at a stuck pose is still recovered.
+  const long = fakeTimers();
+  const dragged = realCamera({ width: 1225, height: 923 });
+  installGlobeInViewRecovery(dragged.viewer, long);
+  aim(dragged.camera, reportedStuckView);
+  dragged.camera.moveStart.raiseEvent();
+  for (let i = 0; i < 120; i += 1) {
+    dragged.camera.moveLeft(0.01);
+    long.tick();
+  }
+  assert.equal(globeOutOfFrame(dragged.camera), true, 'still moving');
+  while (long.tick());
+  assert.ok(
+    Cesium.Math.equalsEpsilon(dragged.camera.pitch, -Math.PI / 2, 1e-6),
+    'recovered once it stopped',
+  );
+
+  // A camera that keeps moving is followed, and left alone, for as long as it
+  // moves; once it rests the check gives up five minutes later.
+  const timers = fakeTimers();
+  const { camera, viewer } = realCamera({ width: 1225, height: 923 });
+  installGlobeInViewRecovery(viewer, timers);
+  aim(camera, reportedStuckView);
+  camera.moveStart.raiseEvent();
+  for (let i = 0; i < 2000; i += 1) {
+    camera.moveLeft(0.01);
+    timers.tick();
+  }
+  assert.equal(timers.pending(), 1, 'still following after eight minutes');
+  assert.equal(globeOutOfFrame(camera), true, 'a moving camera is left alone');
+  let idleSamples = 0;
+  while (timers.tick()) idleSamples += 1;
+  assert.ok(
+    Cesium.Math.equalsEpsilon(camera.pitch, -Math.PI / 2, 1e-6),
+    'recovered once it stopped',
+  );
+  assert.ok(
+    idleSamples >= 1200 && idleSamples <= 1205,
+    `gave up ${idleSamples} samples after it stopped`,
+  );
+
+  const disposed = fakeTimers();
+  const other = realCamera({ width: 1225, height: 923 });
+  const dispose = installGlobeInViewRecovery(other.viewer, disposed);
+  aim(other.camera, reportedStuckView);
+  other.camera.moveStart.raiseEvent();
+  dispose();
+  assert.equal(disposed.pending(), 0, 'dispose cancels the pending check');
+});