diff --git a/src/layers/vessels/edgeCases.test.mjs b/src/layers/vessels/edgeCases.test.mjs
new file mode 100644
index 000000000..4e40f7656
--- /dev/null
+++ b/src/layers/vessels/edgeCases.test.mjs
@@ -0,0 +1,241 @@
+// Edge case for ais-live-vessels from docs/migration/planet-separation/layers.md:
+// a vessel crosses into the next area cell while AIS is interrupted. There is
+// never a second request in flight and the card comes back by MMSI.
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { createFakeEngine } from '../../globe/testing/fakeEngine.js';
+import * as selection from '../../data/selectionDispatcher.js';
+import * as picking from '../../data/pickRegistry.js';
+import { createVesselLayer } from './index.js';
+
+const noop = () => {};
+const RAD = Math.PI / 180;
+
+function recordingRenderer(engine) {
+  const sprites = new Set();
+  const moveEnd = new Set();
+  const camera = {
+    positionWC: { x: 0, y: 0, z: 6_500_000 },
+    // 60 km up: the layer asks for a 60 km area around the camera.
+    positionCartographic: {
+      latitude: 40.6 * RAD,
+      longitude: -73.9 * RAD,
+      height: 60_000,
+    },
+    rightWC: { x: 1, y: 0, z: 0 },
+    upWC: { x: 0, y: 1, z: 0 },
+    heading: 0,
+    pitch: 0,
+    roll: 0,
+  };
+  let attached = false;
+  return {
+    sprites,
+    camera,
+    moveTo(lat, lon) {
+      camera.positionCartographic = {
+        latitude: lat * RAD,
+        longitude: lon * RAD,
+        height: 60_000,
+      };
+      for (const listener of moveEnd) listener();
+    },
+    renderer: {
+      kind: 'plain',
+      engine,
+      view: {
+        camera,
+        scene: { canvas: { clientWidth: 0, clientHeight: 0 } },
+        horizon: () => ({ isPointVisible: () => true }),
+        worldToWindow: () => undefined,
+        canvasSize: () => ({ width: 1280, height: 800 }),
+        requestRender: noop,
+        on(event, listener) {
+          if (event !== 'moveEnd') return noop;
+          moveEnd.add(listener);
+          return () => moveEnd.delete(listener);
+        },
+      },
+      fleet: {
+        collection: null,
+        attached: () => attached,
+        attach() {
+          attached = true;
+        },
+        setVisible: noop,
+        add(spec) {
+          const sprite = { ...spec };
+          sprites.add(sprite);
+          return sprite;
+        },
+        remove: (sprite) => sprites.delete(sprite),
+        detach() {
+          attached = false;
+        },
+      },
+      pickedRecord: (picked) =>
+        picked?.id && typeof picked.id === 'object' ? picked.id : null,
+      follow: { onChanged: () => noop, active: () => false },
+      trails: { create: () => null },
+      destroy: noop,
+    },
+  };
+}
+
+function services(renderer) {
+  return {
+    context: {
+      clearSelectedEntityContextForLayer: noop,
+      registerEntityContext: () => null,
+      selectEntityContext: noop,
+    },
+    trails: { createTrail: null },
+    labels: {},
+    picking,
+    overlay: {
+      setOverlayEntries: noop,
+      setOverlaySourceVisible: noop,
+      clearOverlaySource: noop,
+      hitTestWorldOverlay: () => null,
+    },
+    geoid: { ensureGeoidReady: () => new Promise(noop), geoidHeight: () => 0 },
+    sprites: {
+      registerSpriteCollection: noop,
+      restoreSpriteOrder: noop,
+      restoreSpriteOrderOnEnable: noop,
+    },
+    focus: {
+      focusNowMs: (now) => now,
+      getFocusTarget: () => null,
+      focusPassIsNeeded: () => false,
+      forgetSpriteFocus: noop,
+    },
+    worldFocus: { requestWorldFocus: noop },
+    render: {
+      holdContinuousRender: noop,
+      releaseContinuousRender: noop,
+      governorRequestRender: noop,
+    },
+    selection,
+    renderer: { create: () => renderer },
+  };
+}
+
+const row = (mmsi, latitude, longitude, observedAtMs) => ({
+  id: mmsi,
+  reference: mmsi,
+  latitude,
+  longitude,
+  name: `SHIP ${mmsi}`,
+  type: 'Cargo',
+  observedAtMs,
+  speedMps: 6,
+  headingDeg: 90,
+  courseDeg: 90,
+});
+
+test('edge:ais-live-vessels: a vessel crossing into the next cell during an AIS gap keeps one request and its card by MMSI', async (t) => {
+  const engine = createFakeEngine();
+  const recorder = recordingRenderer(engine);
+  const previousDocument = globalThis.document;
+  globalThis.document = Object.assign(new EventTarget(), {
+    getElementById: () => null,
+  });
+  t.after(() => {
+    globalThis.document = previousDocument;
+  });
+  const requests = [];
+  let inFlight = 0;
+  let maxInFlight = 0;
+  const responses = [
+    // Cell A: the vessel is in view.
+    {
+      records: [row('111', 40.6, -73.9, 1_000)],
+      complete: true,
+      transportStatus: 'live',
+      rawRowCount: 1,
+      observedAtMs: 1_000,
+      freshness: 'current',
+    },
+    // AIS gap: the upstream socket is reconnecting and nothing arrives.
+    {
+      records: [],
+      complete: false,
+      transportStatus: 'reconnecting',
+      rawRowCount: 0,
+      observedAtMs: null,
+      freshness: 'stale',
+      stale: true,
+      reconnectAttempt: 1,
+    },
+    // Cell B, after the gap: the same MMSI, now across the boundary.
+    {
+      records: [row('111', 41.2, -73.0, 90_000)],
+      complete: true,
+      transportStatus: 'live',
+      rawRowCount: 1,
+      observedAtMs: 90_000,
+      freshness: 'current',
+    },
+  ];
+  let release = null;
+  const layer = createVesselLayer({
+    services: services(recorder.renderer),
+    source: {
+      async getSnapshot(query) {
+        requests.push(query.area ?? null);
+        inFlight += 1;
+        maxInFlight = Math.max(maxInFlight, inFlight);
+        try {
+          // The gap response is held until the camera has moved on.
+          if (requests.length === 2)
+            await new Promise((resolve) => {
+              release = resolve;
+            });
+          return {
+            source: 'AIS fixture',
+            ...responses[Math.min(requests.length, responses.length) - 1],
+          };
+        } finally {
+          inFlight -= 1;
+        }
+      },
+      getTrack: async () => ({ records: [] }),
+    },
+  });
+  t.after(() => layer.destroy());
+  layer.init(null, { engine });
+  await layer.enable();
+  assert.ok(layer.selectById('111'));
+  const record = [...recorder.sprites].find(({ id }) => id?.mmsi === '111')?.id;
+  assert.ok(record, 'the vessel is drawn in cell A');
+
+  const gap = layer.update();
+  // The followed vessel leaves cell A while the gap request is still open:
+  // the move must not open a second request beside it.
+  recorder.moveTo(41.2, -73.0);
+  assert.equal(requests.length, 2);
+  release();
+  await gap;
+  assert.equal(
+    layer.getSelectedInfo()?.mmsi,
+    '111',
+    'the card survives the gap',
+  );
+
+  // The next poll asks for cell B and finds the vessel there.
+  await layer.update();
+  assert.equal(requests.length, 3);
+  assert.equal(maxInFlight, 1, 'never two requests at once');
+  assert.ok(requests.at(-1), 'the request names an area');
+  assert.ok(
+    Math.abs(requests.at(-1).lat - 41.2) < 1e-6,
+    'the request asks for the new cell',
+  );
+  const info = layer.getSelectedInfo();
+  assert.equal(info?.mmsi, '111', 'the card is restored by MMSI');
+  assert.equal(info.latitude, 41.2);
+  const drawn = [...recorder.sprites].filter(({ id }) => id?.mmsi === '111');
+  assert.equal(drawn.length, 1, 'one chevron for the vessel');
+  assert.equal(drawn[0].id, record, 'the same record, not a new contact');
+});