Create the application once per page and switch engines through the scene host

createStandaloneApplication opens its first globe, attaches the scene host and returns attachScene/detachScene: a switch hands the session off, parks the outgoing renderer and attaches the incoming one with its catalog to the parts the phases registered once. A failed candidate releases what it attached; the working engine attaches again on the same shell, voice, tools and stores. ?switch=legacy keeps the route that builds an application per engine for one release. scripts/qa-app-once.mjs now asserts identity, UI, tool and listener continuity, the rollback and the legacy route.

scripts/qa-app-once.mjs

@@ -80,6 +80,59 @@ async function prepareUi(page) {
   }, FIELD);
 }
 
+// The session's tools: a mark on the annotation board, an imported file and
+// the Draw shape, set on Cesium before the switches.
+async function prepareTools(page) {
+  return page.evaluate(async () => {
+    const annotated = await window.__gevAnnotations.annotate([
+      { type: 'pin', latitude: 42.3554, longitude: -71.0605, label: 'QA mark' },
+    ]);
+    const file = new File(
+      [
+        JSON.stringify({
+          type: 'FeatureCollection',
+          features: [
+            {
+              type: 'Feature',
+              properties: { name: 'QA area' },
+              geometry: {
+                type: 'Polygon',
+                coordinates: [
+                  [
+                    [-71.07, 42.35],
+                    [-71.05, 42.35],
+                    [-71.05, 42.36],
+                    [-71.07, 42.35],
+                  ],
+                ],
+              },
+            },
+          ],
+        }),
+      ],
+      'qa-area.geojson',
+      { type: 'application/geo+json' },
+    );
+    const imported = await window.__gevGeoImport.importFiles([file]);
+    window.__gevDrawTool.setShape('line');
+    return { annotated: annotated.drawn, imported: imported[0]?.ok };
+  });
+}
+
+const readTools = (page) =>
+  page.evaluate(() => ({
+    marks: window.__gevAnnotations?.count() ?? null,
+    marksDrawn: window.__gevAnnotations?.attached ?? null,
+    files: window.__gevGeoImport?.list().length ?? null,
+    filesDrawn: window.__gevGeoImport?.diagnostics().drawn ?? null,
+    drawShape: window.__gevDrawTool?.shape ?? null,
+  }));
+
+// Listeners whose difference is the parked renderer's: the Navara engine
+// keeps its own input, resize and attribution listeners while it is parked
+// for a switch back (scripts that are not the session's).
+const PARKED_RENDERER = /@navara-engine\.js$/;
+
 const browser = await launch();
 try {
   const { page, errors } = await openSessionPage(
@@ -90,8 +143,20 @@ try {
   // Let the first boot's deferred listeners land before counting.
   await new Promise((resolve) => setTimeout(resolve, 2000));
   const boot = await readSession(page);
-  const bootListeners = await listenerCounts(page);
+  const bootListeners = await listenerCounts(page, { bySource: true });
   report.boot = boot;
+  report.toolsPrepared = await prepareTools(page);
+  check(
+    report.toolsPrepared.annotated === 1 && report.toolsPrepared.imported,
+    'the tools could not be prepared',
+    report.toolsPrepared,
+  );
+  // Let the imported file finish drawing.
+  await page.waitForFunction(
+    () => window.__gevGeoImport.diagnostics().drawn === 1,
+  );
+  const toolsBefore = await readTools(page);
+  report.toolsBefore = toolsBefore;
 
   await prepareUi(page);
   const escapeBefore = await escapeEffect(page);
@@ -110,17 +175,32 @@ try {
       await page.evaluate(() => {
         window.__qaParkedNavara = window.__godsEyeView.engine;
       });
-    if (report.switches.length === 1)
-      report.firstRoundTripListeners = listenerDifference(
-        bootListeners,
-        await listenerCounts(page),
+    if (report.switches.length === 1) {
+      await new Promise((resolve) => setTimeout(resolve, 2000));
+      report.firstRoundTrip = await listenerCounts(page, { bySource: true });
+    }
+    if (kind === 'cesium')
+      await page.waitForFunction(
+        () => window.__gevGeoImport.diagnostics().drawn === 1,
+        { timeout: 30_000 },
       );
+    const tools = await readTools(page);
+    check(
+      tools.marks === toolsBefore.marks &&
+        tools.files === toolsBefore.files &&
+        tools.drawShape === toolsBefore.drawShape &&
+        tools.marksDrawn === (kind === 'cesium') &&
+        tools.filesDrawn === (kind === 'cesium' ? 1 : 0),
+      `the session tools changed on the switch to ${kind}`,
+      { toolsBefore, tools },
+    );
     report.switches.push({
       kind,
       ms: Date.now() - started,
       ids: idDifference(boot.ids, state.ids),
       counters: state.counters,
       ui,
+      tools,
     });
     check(
       Object.keys(idDifference(boot.ids, state.ids)).length === 0,
@@ -141,14 +221,30 @@ try {
   }
   await new Promise((resolve) => setTimeout(resolve, 2000));
   const after = await readSession(page);
-  const afterListeners = await listenerCounts(page);
+  const afterListeners = await listenerCounts(page, { bySource: true });
   report.after = after;
+  // After three round trips the page holds exactly the listeners it held
+  // after the first boot, apart from the parked Navara renderer's own (it
+  // stays alive for the switch back), which the first round trip added and
+  // the later ones did not grow.
   report.listeners = listenerDifference(bootListeners, afterListeners);
+  report.listenersSinceFirstRoundTrip = listenerDifference(
+    report.firstRoundTrip,
+    afterListeners,
+  );
+  const sessionListeners = Object.keys(report.listeners).filter(
+    (key) => !PARKED_RENDERER.test(key),
+  );
   check(
-    Object.keys(report.listeners).length === 0,
-    'window/document listeners changed after three round trips',
+    sessionListeners.length === 0,
+    'session listeners on window/document changed after three round trips',
     report.listeners,
   );
+  check(
+    Object.keys(report.listenersSinceFirstRoundTrip).length === 0,
+    'listeners on window/document grew after the first round trip',
+    report.listenersSinceFirstRoundTrip,
+  );
   check(
     after.counters?.attachments === 1 + sequence.length &&
       after.counters?.detaches === sequence.length,
@@ -227,6 +323,24 @@ try {
   }
   report.errors = errors;
   check(errors.length === 0, 'page errors', errors);
+
+  // The legacy route (?switch=legacy) still builds an application per
+  // engine, for one release.
+  const legacy = await openSessionPage(
+    browser,
+    sessionAddress(base, { engine: 'cesium', extra: { switch: 'legacy' } }),
+  );
+  await waitReady(legacy.page, 'cesium');
+  await switchEngine(legacy.page, 'navara');
+  await switchEngine(legacy.page, 'cesium');
+  report.legacy = await readSession(legacy.page);
+  check(
+    report.legacy.counters.applications === 3,
+    'the legacy route builds an application per engine',
+    report.legacy.counters,
+  );
+  check(legacy.errors.length === 0, 'legacy page errors', legacy.errors);
+  await legacy.page.close();
   console.log(JSON.stringify(report, null, 2));
   if (!reportOnly) console.log('PASS application created once; session kept');
 } finally {

scripts/qa/appSessionHarness.mjs

@@ -288,10 +288,22 @@ export function readSession(page) {
   }));
 }
 
-/** Live listeners on window and document, counted per event type. */
-export async function listenerCounts(page) {
+/**
+ * Live listeners on window and document, counted per event type. With
+ * `bySource`, each key also names the script that added the listener (its
+ * file name), so a listener held by the parked renderer can be told apart
+ * from one the session leaked.
+ */
+export async function listenerCounts(page, { bySource = false } = {}) {
   const client = await page.createCDPSession();
   try {
+    const scripts = new Map();
+    if (bySource) {
+      client.on('Debugger.scriptParsed', ({ scriptId, url }) =>
+        scripts.set(scriptId, url),
+      );
+      await client.send('Debugger.enable');
+    }
     const counts = {};
     for (const name of ['window', 'document']) {
       const { result } = await client.send('Runtime.evaluate', {
@@ -301,13 +313,20 @@ export async function listenerCounts(page) {
         objectId: result.objectId,
       });
       for (const listener of listeners) {
-        const key = `${name}:${listener.type}`;
+        const source = bySource
+          ? `@${(scripts.get(listener.scriptId) ?? '')
+              .split('/')
+              .pop()
+              .replace(/-[\w-]{8}\.js$/, '.js')}`
+          : '';
+        const key = `${name}:${listener.type}${source}`;
         counts[key] = (counts[key] ?? 0) + 1;
       }
       await client.send('Runtime.releaseObject', {
         objectId: result.objectId,
       });
     }
+    if (bySource) await client.send('Debugger.disable');
     return counts;
   } finally {
     await client.detach();

src/main.js

@@ -97,11 +97,19 @@ function liveComponents() {
     : null;
 }
 
+// The page's application is created once; a switch detaches and attaches
+// only its engine-bound parts. `?switch=legacy` keeps the previous route, one
+// application per engine, for one release.
+const legacySwitch =
+  new URLSearchParams(globalThis.location?.search ?? '').get('switch') ===
+  'legacy';
+
 // Hand the session off and release the running engine. Its renderer is
 // parked for a later switch back.
 async function leave(from) {
   const components = liveComponents();
-  if (components) {
+  const attached = Boolean(components?.scene.engine);
+  if (attached) {
     cover ??= keepGlobeFrame(components.scene.engine);
     cockpit = components.controls.styleManager.getCockpitState();
     handOffSession(layerSession, {
@@ -111,36 +119,68 @@ async function leave(from) {
     });
     renderers.retain(from);
   }
-  await application?.destroy();
+  if (legacySwitch) await application?.destroy();
+  else if (attached) await application.detachScene({ handoff: true });
   session.count('detaches');
 }
 
 // Start an engine on the retained session. Success means the engine runs
 // with the session attached and the address's view restored; its first frame
 // only decides when the cover goes (settle).
-async function enter(kind) {
+async function enter(kind, { recovery = false } = {}) {
+  if (legacySwitch) return enterLegacy(kind);
+  if (!application || application.getState().status !== 'ready') {
+    // The first engine never started: there is no application to attach to.
+    return enterLegacy(kind);
+  }
+  layerSession.restoringView = true;
+  try {
+    await application.attachScene(kind);
+    signal.throwIfAborted();
+    await resume(application.getComponents().controls.styleManager);
+    if (recovery) session.count('rollbacks');
+  } catch (error) {
+    // pagehide owns the teardown once the lifetime is aborted. A failed
+    // candidate is detached; the session stays for the working engine.
+    if (!signal.aborted)
+      await application
+        .detachScene({ handoff: true })
+        .catch((cleanupError) =>
+          console.error('A failed globe could not be released:', cleanupError),
+        );
+    throw error;
+  }
+}
+
+// The session continues on the engine that attached: the view is restored,
+// parked work resumes and Cockpit re-enters on the aircraft it had.
+async function resume(style) {
+  await style.initialRestorePromise;
+  signal.throwIfAborted();
+  layerSession.restoringView = false;
+  session.count('attachments');
+  resumeSession(layerSession);
+  // Only resume the aircraft that actually survived the handoff. A history
+  // entry may select a different subject, and a stale track may disappear.
+  const target = style.getAircraftTrackingTarget();
+  if (
+    cockpit?.active &&
+    target &&
+    target?.id === cockpit.subject?.id &&
+    target?.layerId === cockpit.subject?.layerId &&
+    style.controlCockpit('enter').ok
+  ) {
+    style.cockpitView.setVisionMode(cockpit.visionMode);
+  }
+}
+
+// The legacy route: a new application on the retained session.
+async function enterLegacy(kind) {
   const app = createApplication(kind, true);
   try {
     const { controls } = await app.start();
     signal.throwIfAborted();
-    const style = controls.styleManager;
-    await style.initialRestorePromise;
-    signal.throwIfAborted();
-    layerSession.restoringView = false;
-    session.count('attachments');
-    resumeSession(layerSession);
-    // Only resume the aircraft that actually survived the handoff. A history
-    // entry may select a different subject, and a stale track may disappear.
-    const target = style.getAircraftTrackingTarget();
-    if (
-      cockpit?.active &&
-      target &&
-      target?.id === cockpit.subject?.id &&
-      target?.layerId === cockpit.subject?.layerId &&
-      style.controlCockpit('enter').ok
-    ) {
-      style.cockpitView.setVisionMode(cockpit.visionMode);
-    }
+    await resume(controls.styleManager);
   } catch (error) {
     // pagehide owns the teardown once the lifetime is aborted.
     if (!signal.aborted)

src/mapStartup.test.mjs

@@ -1,6 +1,8 @@
 import test from 'node:test';
 import assert from 'node:assert/strict';
 import { readFileSync } from 'node:fs';
+import { createSceneHost } from './app/sceneHost.js';
+import { attachScenePart } from './app/sceneParts.js';
 import vm from 'node:vm';
 import { createGoogleTokenSource } from './maps/googleTokens.js';
 import {
@@ -85,11 +87,20 @@ async function standaloneStartup(Cesium, credentials, fetchImpl) {
       ...(await loadPhotorealisticTileset(Cesium, options)),
       operations: {},
     }),
+    createSceneHost,
+    attachScenePart,
+    AbortController,
+    AbortSignal,
   });
   vm.runInContext(source, context);
   const app = context.createStandaloneApplication(credentials);
-  // The application passes each constructor its signal and defer(cleanup).
-  return app.createScene({ signal: new AbortController().signal, defer() {} });
+  // The application passes each constructor its signal and defer(cleanup);
+  // the scene it returns reads the attached globe.
+  const scene = await app.createScene({
+    signal: new AbortController().signal,
+    defer() {},
+  });
+  return scene.current();
 }
 
 test('standalone falls back to ion when the server token route is unavailable', async () => {

src/standalone/application.js

@@ -16,6 +16,8 @@ import {
 import { mountApplicationViews } from '../ui/views/index.js';
 import { createGoogleTokenSource } from '../maps/googleTokens.js';
 import { resolveEngineKind } from '../app/engineChoice.js';
+import { createSceneHost } from '../app/sceneHost.js';
+import { attachScenePart } from '../app/sceneParts.js';
 
 // Only one engine may own the page at a time.
 let owner = null;
@@ -36,7 +38,14 @@ function sessionChrome(layerSession) {
   return (layerSession.chrome ??= {});
 }
 
-/** Compose one engine session. Destroy it before starting another. */
+/**
+ * Compose the page's application, created once per page. Its first globe is
+ * `engineKind`; `attachScene(kind)` and `detachScene()` switch engines by
+ * detaching and attaching only the engine-bound parts (src/app/sceneHost.js)
+ * while the shell, stores, voice, tools and layer models stay. Destroy it
+ * before creating another (the legacy route, ?switch=legacy, still builds
+ * one per engine).
+ */
 export function createStandaloneApplication({
   googleApiKey,
   cesiumToken,
@@ -66,9 +75,119 @@ export function createStandaloneApplication({
   const loadingScreen = document.getElementById('loading-screen');
   const loaderStatus = loadingScreen.querySelector('.loader-status');
   let placeSearch;
-  let catalog;
+  let qaSources = {};
+  // The engine-bound parts of every phase attach through this host.
+  const sceneHost = createSceneHost({
+    onError: (error, part) =>
+      console.error(`The ${part} scene part failed:`, error),
+  });
+  // The attached globe (one renderer attachment with its catalog), or null
+  // between engines.
+  let current = null;
+  let lifetime = null;
+  let sessionContext = null;
+
+  /** Acquire `kind`'s renderer and build its catalog: one attachment. */
+  async function openScene(kind) {
+    const controller = new AbortController();
+    const releases = [];
+    const release = async () => {
+      controller.abort();
+      const errors = [];
+      while (releases.length) {
+        try {
+          await releases.pop()();
+        } catch (error) {
+          errors.push(error);
+        }
+      }
+      if (errors.length)
+        throw new AggregateError(errors, 'The globe could not be released');
+    };
+    const signal = AbortSignal.any([controller.signal, lifetime]);
+    try {
+      const scene = await createStandaloneScene({
+        ...sessionContext,
+        signal,
+        defer: (cleanup) => releases.push(cleanup),
+        googleApiKey,
+        // Without a key, Google 3D can still load with tokens from the
+        // app's server when it offers them.
+        googleTokens: googleApiKey ? null : createGoogleTokenSource(),
+        cesiumToken,
+        loaderStatus,
+        engineKind: kind,
+        rendererCache,
+        layerSession,
+      });
+      signal.throwIfAborted();
+      // The catalog of this engine: on Navara only the layers with a Navara
+      // renderer (src/app/layers/renderers.js), the others' state kept for
+      // links. The layer models are the session's (layerSession.models).
+      const sceneCatalog = createStandaloneCatalog({
+        engineKind: kind,
+        layerSession,
+        sources: { ...sources, ...qaSources },
+        streetLevelProviders,
+        nepalBoundaryResolver: (resolverSignal) =>
+          scene.operations.annotationResolver.resolveRegionRingForQuery(
+            'Nepal',
+            resolverSignal,
+            placeSearch,
+            // The locator draws the border whenever it arrives.
+            { budgetMs: Infinity },
+          ),
+        signal,
+        surface: scene.operations.surface,
+      });
+      return Object.freeze({
+        ...scene,
+        kind,
+        catalog: sceneCatalog,
+        release,
+      });
+    } catch (error) {
+      await release().catch((cleanupError) =>
+        console.error('A failed globe could not be released:', cleanupError),
+      );
+      throw error;
+    }
+  }
+
+  // What the phases and the page read as `scene`: the attached globe's
+  // handles (null between engines), the session's operations and the host.
+  const sceneView = Object.freeze({
+    get kind() {
+      return current?.kind ?? null;
+    },
+    get viewer() {
+      return current?.viewer ?? null;
+    },
+    get engine() {
+      return current?.engine ?? null;
+    },
+    get tileset() {
+      return current?.tileset ?? null;
+    },
+    get mapStackController() {
+      return current?.mapStackController ?? null;
+    },
+    get packRenderer() {
+      return current?.packRenderer ?? null;
+    },
+    get catalog() {
+      return current?.catalog ?? null;
+    },
+    get operations() {
+      return layerSession?.operations ?? current?.operations ?? null;
+    },
+    current: () => current,
+    sceneHost,
+  });
+
   const application = createApplication({
     createScene: async (context) => {
+      lifetime = context.signal;
       // The page's language listener and place search belong to the page
       // session when there is one (an engine switch keeps them), else to
       // this application.
@@ -101,26 +220,11 @@ export function createStandaloneApplication({
             layerSession.lifetime.signal,
           ))
         : createPlaceSearch(context.signal);
-      const scene = await createStandaloneScene({
-        ...context,
-        googleApiKey,
-        // Without a key, Google 3D can still load with tokens from the
-        // app's server when it offers them.
-        googleTokens: googleApiKey ? null : createGoogleTokenSource(),
-        cesiumToken,
-        loaderStatus,
-        engineKind,
-        rendererCache,
-        layerSession,
-      });
-      // The catalog of this engine: on Navara only the layers with a Navara
-      // renderer (src/app/layers/renderers.js; none yet), the others' state
-      // kept for links.
       // QA fixture sources (?qaFeed=…) on the dev server only.
       const search = globalThis.location?.search ?? '';
       // import.meta.env.DEV at the import site, so production builds drop
       // the chunk instead of shipping it unreachable.
-      const qaSources =
+      qaSources =
         import.meta.env?.DEV &&
         allowQaRegistration &&
         /[?&]qaFeed=/.test(search)
@@ -129,23 +233,26 @@ export function createStandaloneApplication({
                 createQaSources(parseQaFeed(search)),
             )
           : {};
-      catalog = createStandaloneCatalog({
-        engineKind,
-        layerSession,
-        sources: { ...sources, ...qaSources },
-        streetLevelProviders,
-        nepalBoundaryResolver: (signal) =>
-          scene.operations.annotationResolver.resolveRegionRingForQuery(
-            'Nepal',
-            signal,
-            placeSearch,
-            // The locator draws the border whenever it arrives.
-            { budgetMs: Infinity },
+      // The session's own defer (its logo gaze and key when no layer
+      // session holds them) outlives every globe.
+      sessionContext = { defer: context.defer };
+      current = await openScene(engineKind);
+      // The first globe: the phases build on it and register their parts.
+      await sceneHost.attach(current);
+      // After every part detached (the tools phase detaches them first),
+      // the globe's renderer is parked or released.
+      context.defer(async () => {
+        await sceneHost.detach({
+          handoff: Boolean(
+            layerSession?.handoffLayers &&
+            !layerSession.lifetime.signal.aborted,
           ),
-        signal: context.signal,
-        surface: scene.operations.surface,
+        });
+        const scene = current;
+        current = null;
+        await scene?.release();
       });
-      return scene;
+      return sceneView;
     },
     createControls: (context) =>
       createStandaloneControls({
@@ -154,13 +261,13 @@ export function createStandaloneApplication({
         layerSession,
         loaderStatus,
         placeSearch,
-        catalog,
+        catalog: current.catalog,
       }),
     createData: (context) =>
       createStandaloneData({
         ...context,
         allowQaRegistration,
-        catalog,
+        catalog: current.catalog,
         layerSession,
       }),
     createTools: async (context) => {
@@ -173,20 +280,29 @@ export function createStandaloneApplication({
       });
       // The engine-neutral QA probe on the debug handle (src/app/qaProbe.js),
       // on the dev server only: import.meta.env.DEV at the import site, so
-      // production builds drop it.
+      // production builds drop it. It reads one engine: a part.
       const debug = globalThis.window?.__godsEyeView;
       if (import.meta.env?.DEV && debug) {
         const { createQaProbe } = await import('../app/qaProbe.js');
-        const probe = createQaProbe({
-          engine: context.scene.engine,
-          dataManager: context.data.dataManager,
-          availability: catalog.availability,
-        });
-        debug.layers = probe;
-        context.defer(() => {
-          probe.destroy();
-          if (debug.layers === probe) delete debug.layers;
-        });
+        await attachScenePart(
+          context.scene,
+          {
+            name: 'qa-probe',
+            attachScene(next, { defer }) {
+              const probe = createQaProbe({
+                engine: next.engine,
+                dataManager: context.data.dataManager,
+                availability: next.catalog.availability,
+              });
+              debug.layers = probe;
+              defer(() => {
+                probe.destroy();
+                if (debug.layers === probe) delete debug.layers;
+              });
+            },
+          },
+          { defer: context.defer },
+        );
       }
       return tools;
     },
@@ -199,5 +315,47 @@ export function createStandaloneApplication({
     )
       owner = null;
   });
-  return application;
+
+  /**
+   * Switch to `kind`: acquire its renderer and attach every part. A failure
+   * detaches what attached and releases the candidate; the session stays as
+   * it was, ready for the working engine to attach again.
+   */
+  async function attachScene(kind) {
+    if (application.getState().status !== 'ready')
+      throw new Error('The application is not running');
+    if (current) throw new Error('A globe is attached; detach it first');
+    const next = await openScene(kind);
+    try {
+      await sceneHost.attach(next);
+    } catch (error) {
+      await next
+        .release()
+        .catch((cleanupError) =>
+          console.error('A failed globe could not be released:', cleanupError),
+        );
+      throw error;
+    }
+    current = next;
+    return sceneView;
+  }
+
+  /** Detach every part from the attached globe and park or release it. */
+  async function detachScene(options = {}) {
+    const scene = current;
+    if (!scene) return;
+    try {
+      await sceneHost.detach(options);
+    } finally {
+      current = null;
+      await scene.release();
+    }
+  }
+
+  return Object.freeze({
+    ...application,
+    sceneHost,
+    attachScene,
+    detachScene,
+  });
 }

src/standalone/applicationSwitch.test.mjs

@@ -0,0 +1,110 @@
+// The standalone application is created once; an engine switch detaches and
+// attaches only its engine-bound parts. A candidate that fails releases its
+// renderer, and the working engine attaches again on the same parts.
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { readFileSync } from 'node:fs';
+import vm from 'node:vm';
+import { createSceneHost } from '../app/sceneHost.js';
+import { attachScenePart } from '../app/sceneParts.js';
+
+function standalone({ failOpen = null } = {}) {
+  const source = readFileSync(
+    new URL('./application.js', import.meta.url),
+    'utf8',
+  )
+    .replace(/^import [^;]*;\n/gm, '')
+    .replace(/^export /gm, '')
+    .replace(/import\.meta\.env/g, '({}).env');
+  const opened = [];
+  const released = [];
+  const context = vm.createContext({
+    document: {
+      getElementById: () => ({ querySelector: () => ({}) }),
+    },
+    console,
+    resolveLocale: () => 'en',
+    setLocale() {},
+    getLocale: () => 'en',
+    onLocaleChange: () => () => {},
+    mountApplicationViews() {},
+    applyDocumentLanguage() {},
+    applyDocumentTranslations() {},
+    CITY_POIS: [],
+    createApplication: (options) => ({
+      ...options,
+      subscribe() {},
+      getState: () => ({ status: 'ready' }),
+    }),
+    resolveEngineKind: () => 'cesium',
+    createStandalonePlaceSearch: () => ({ kind: 'search' }),
+    createStandaloneCatalog: ({ engineKind }) => ({ engineKind }),
+    createGoogleTokenSource: () => null,
+    createStandaloneScene: async ({ engineKind, defer }) => {
+      if (failOpen === engineKind) throw new Error(`${engineKind} refused`);
+      const engine = { kind: engineKind, id: opened.length };
+      opened.push(engine);
+      defer(() => released.push(engine));
+      return { viewer: null, engine, operations: { kind: 'operations' } };
+    },
+    createSceneHost,
+    attachScenePart,
+    AbortController,
+    AbortSignal,
+  });
+  vm.runInContext(source, context);
+  const app = context.createStandaloneApplication({});
+  return { app, opened, released };
+}
+
+test('the application switches engines on the parts it registered once', async () => {
+  const { app, opened, released } = standalone();
+  const scene = await app.createScene({
+    signal: new AbortController().signal,
+    defer() {},
+  });
+  assert.equal(scene.engine.kind, 'cesium');
+  const log = [];
+  await scene.sceneHost.register({
+    name: 'part',
+    attachScene: (next) => log.push(`attach ${next.kind}`),
+    detachScene: () => log.push('detach'),
+  });
+  await app.detachScene({ handoff: true });
+  assert.equal(scene.engine, null, 'no globe between engines');
+  assert.deepEqual(released, [opened[0]]);
+  await app.attachScene('navara');
+  assert.equal(scene.engine.kind, 'navara');
+  assert.equal(scene.catalog.engineKind, 'navara');
+  assert.deepEqual(log, ['attach cesium', 'detach', 'attach navara']);
+  await assert.rejects(app.attachScene('cesium'), /detach it first/);
+});
+
+test('a failed candidate releases what it attached and the working engine attaches again', async () => {
+  const { app, opened, released } = standalone();
+  const scene = await app.createScene({
+    signal: new AbortController().signal,
+    defer() {},
+  });
+  const part = {
+    name: 'part',
+    attached: [],
+    attachScene(next) {
+      if (next.kind === 'navara') throw new Error('navara part failed');
+      part.attached.push(next.engine);
+    },
+  };
+  await scene.sceneHost.register(part);
+  await app.detachScene({ handoff: true });
+  await assert.rejects(app.attachScene('navara'), /navara part failed/);
+  assert.equal(scene.engine, null);
+  assert.equal(released.length, 2, 'the candidate renderer was released');
+  assert.equal(released[1].kind, 'navara');
+  await app.attachScene('cesium');
+  assert.equal(scene.engine.kind, 'cesium');
+  assert.deepEqual(
+    part.attached.map(({ kind }) => kind),
+    ['cesium', 'cesium'],
+  );
+  assert.equal(opened.length, 3);
+});