diff --git a/src/data/bhoteKoshiLocator.edgeCases.test.mjs b/src/data/bhoteKoshiLocator.edgeCases.test.mjs
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+++ b/src/data/bhoteKoshiLocator.edgeCases.test.mjs
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+// Edge cases from docs/migration/planet-separation/layers.md
+// (bhote-koshi-locator): a boundary that resolves late, after its request was
+// cancelled, does not change the live scene; both engines show the same area
+// and the same target.
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import * as Cesium from 'cesium';
+import { createBhoteKoshiLocatorLayer } from './bhoteKoshiLocator.js';
+
+const STALE_RING = [
+  [10, 10],
+  [11, 10],
+  [11, 11],
+  [10, 11],
+];
+const LIVE_RING = [
+  [80, 26],
+  [88, 26],
+  [88, 30],
+  [80, 30],
+];
+
+function deferred() {
+  let resolve;
+  const promise = new Promise((done) => (resolve = done));
+  return { promise, resolve };
+}
+
+/** A locator on one engine; `shown()` is the data source the scene draws. */
+function locator(engine) {
+  const lookups = [];
+  const renders = [];
+  const publications = [];
+  const shown = new Set();
+  const overlayHost = {
+    setEntries: (sourceId, entries) => publications.push(entries),
+    clearSource() {},
+  };
+  const viewer = {
+    creditDisplay: { addStaticCredit() {}, removeStaticCredit() {} },
+    dataSources: {
+      add: (source) => shown.add(source),
+      remove: (source) => shown.delete(source),
+    },
+  };
+  const navara = {
+    kind: 'navara',
+    render: { request: (reason) => renders.push(reason) },
+    credits: { add() {}, remove() {} },
+  };
+  const layer = createBhoteKoshiLocatorLayer({
+    boundaryResolver: (signal) => {
+      const lookup = deferred();
+      lookups.push({ signal, ...lookup });
+      return lookup.promise;
+    },
+    createNativeRenderer: (_engine, dataSource) => {
+      shown.add(dataSource);
+      return { destroy: () => shown.delete(dataSource) };
+    },
+    requestRender: (reason) => renders.push(reason),
+    overlayHost,
+    scheduleFrame: () => 1,
+    cancelFrame() {},
+    scheduleDelay: () => 1,
+    cancelDelay() {},
+  });
+  const scene = engine === 'cesium' ? viewer : null;
+  layer.attachRenderer(scene, {
+    engine: engine === 'navara' ? navara : null,
+  });
+  return {
+    layer,
+    scene,
+    lookups,
+    renders,
+    publications,
+    shown: () => [...shown],
+  };
+}
+
+/** The drawn area as degree pairs, and the drawn targets. */
+function drawn(source) {
+  const entities = source.entities.values;
+  const fill = source.entities.getById('nepal-border-fill');
+  const area = fill
+    ? fill.polygon.hierarchy.getValue().positions.map((position) => {
+        const c = Cesium.Cartographic.fromCartesian(position);
+        return [
+          Math.round(Cesium.Math.toDegrees(c.longitude)),
+          Math.round(Cesium.Math.toDegrees(c.latitude)),
+        ];
+      })
+    : null;
+  return { area, ids: entities.map((entity) => entity.id).sort() };
+}
+
+for (const engine of ['cesium', 'navara']) {
+  test(`edge:bhote-koshi-locator: a boundary resolving after cancel leaves the live ${engine} scene alone`, async () => {
+    const h = locator(engine);
+    const cancelled = h.layer.enable(h.scene);
+    await h.layer.disable();
+    const live = h.layer.enable(h.scene);
+    assert.equal(h.lookups.length, 2);
+    h.lookups[1].resolve({ ring: LIVE_RING });
+    assert.equal(await live, true);
+
+    const [source] = h.shown();
+    assert.equal(h.shown().length, 1);
+    const before = drawn(source);
+    const publications = h.publications.length;
+    const renders = h.renders.length;
+
+    h.lookups[0].resolve({ ring: STALE_RING });
+    await assert.rejects(cancelled, (error) => error?.name === 'AbortError');
+
+    assert.deepEqual(h.shown(), [source], 'the live source stays drawn');
+    assert.deepEqual(drawn(source), before);
+    assert.deepEqual(before.area, LIVE_RING);
+    assert.equal(h.publications.length, publications, 'no late callouts');
+    assert.equal(h.renders.length, renders, 'no late frame request');
+    assert.equal(h.layer.getStats().status, 'nominal');
+    await h.layer.destroy();
+  });
+}
+
+test('edge:bhote-koshi-locator: both engines draw the same area and the same target', async () => {
+  const result = {};
+  for (const engine of ['cesium', 'navara']) {
+    const h = locator(engine);
+    const enabling = h.layer.enable(h.scene);
+    h.lookups[0].resolve({ ring: LIVE_RING });
+    await enabling;
+    const [source] = h.shown();
+    result[engine] = {
+      ...drawn(source),
+      callouts: h.publications.at(-1).map(({ id, position }) => ({
+        id,
+        position: Cesium.Cartesian3.clone(position),
+      })),
+    };
+    await h.layer.destroy();
+  }
+  assert.deepEqual(result.navara, result.cesium);
+  assert.deepEqual(result.cesium.area, LIVE_RING);
+  assert.ok(result.cesium.ids.includes('bhote-koshi-incident-halo'));
+  assert.deepEqual(
+    result.cesium.callouts.map(({ id }) => id),
+    ['nepal-context-callout', 'bhote-koshi-incident-callout'],
+  );
+});
+
+test('edge:bhote-koshi-locator: an engine swap while the boundary resolves lands it on the new engine only', async () => {
+  const shown = { cesium: new Set(), navara: new Set() };
+  const lookup = deferred();
+  const layer = createBhoteKoshiLocatorLayer({
+    boundaryResolver: () => lookup.promise,
+    createNativeRenderer: (_engine, dataSource) => {
+      shown.navara.add(dataSource);
+      return { destroy: () => shown.navara.delete(dataSource) };
+    },
+    requestRender() {},
+    overlayHost: { setEntries() {}, clearSource() {} },
+    scheduleFrame: () => 1,
+    cancelFrame() {},
+    scheduleDelay: () => 1,
+    cancelDelay() {},
+  });
+  const viewer = {
+    creditDisplay: { addStaticCredit() {}, removeStaticCredit() {} },
+    dataSources: {
+      add: (source) => shown.cesium.add(source),
+      remove: (source) => shown.cesium.delete(source),
+    },
+  };
+  layer.attachRenderer(viewer);
+  const enabling = layer.enable(viewer);
+  layer.detachRenderer();
+  layer.attachRenderer(null, {
+    engine: {
+      kind: 'navara',
+      render: { request() {} },
+      credits: { add() {}, remove() {} },
+    },
+  });
+  lookup.resolve({ ring: LIVE_RING });
+  assert.equal(await enabling, true);
+  assert.equal(shown.cesium.size, 0, 'nothing left on the released scene');
+  assert.equal(shown.navara.size, 1);
+  assert.deepEqual(drawn([...shown.navara][0]).area, LIVE_RING);
+  await layer.destroy();
+  assert.equal(shown.navara.size, 0);
+});