Keep the tracked aircraft through a failed engine switch on Cesium

Cesium follows a display's sprite through a proxy Entity that mirrors the
native's readout fields, and gevRestoring was not one of them. A follow
the flights layer resumed on attach therefore read as new navigation to
the shell's tracked-entity watcher, which made the share restore of the
same attach count itself superseded and clear the selection. That is why
the aircraft was lost after a failed switch's rollback onto Cesium, and
after the retry. The proxy now shows the restoring mark while its follow
starts.

The flights renderer test runs a detach, a failed attach to a candidate,
the rollback and a retry on headless Cesium engines, and checks that the
aircraft stays tracked and no navigation is stamped.

src/globe/cesium/follow.js

@@ -23,13 +23,18 @@ import { toCesium3 } from './convert.js';
 const MAX_FRAME_ATTEMPTS = 120;
 const entityName = (entity) => (entity ? entity.name || entity.id : undefined);
 // What viewer.trackedEntity readers take from a tracked Entity, read through
-// a proxy from the layer's own handle.
+// a proxy from the layer's own handle. gevRestoring marks a follow that
+// rebinds the session's own subject (a resumed renderer, a share restore)
+// while it starts: trackedEntityChanged readers (the shell's navigation
+// stamp) must see it on the proxy, or a resumed follow supersedes the
+// restore it belongs to.
 const READOUT_FIELDS = [
   'gevTrackedId',
   'gevLabelModel',
   'gevDisplayPosition',
   'gevVisualPosition',
   'gevSelectionOrigin',
+  'gevRestoring',
 ];
 const ZOOM_INERTIA_STATES = new WeakMap();
 

src/globe/cesium/follow.test.mjs

@@ -109,6 +109,26 @@ test('a native that is not an Entity is echoed and followed through an ENU proxy
   assert.equal(viewer.entities.contains(proxy), false);
 });
 
+test("the proxy shows a restoring native's mark while its follow starts", () => {
+  const { viewer, camera } = setup();
+  const native = { id: 'flights-tracked-1', gevTrackedId: 'flights:abc123' };
+  // What trackedEntityChanged readers see as the follow starts.
+  const seen = [];
+  viewer.trackedEntityChanged.addEventListener((entity) =>
+    seen.push(entity?.gevRestoring),
+  );
+  native.gevRestoring = true;
+  const handle = camera.follow({
+    native,
+    position: () => TARGET,
+    viewFrom: () => ({ x: 0, y: -500, z: 200 }),
+  });
+  delete native.gevRestoring;
+  assert.deepEqual(seen, [true]);
+  assert.equal(viewer.trackedEntity.gevRestoring, undefined, 'read through');
+  handle.stop();
+});
+
 test('a camera command or another tracked entity ends the follow', () => {
   const { viewer, camera, changes } = setup();
   const options = { position: () => TARGET, viewFrom: { x: 0, y: -500, z: 0 } };

src/layers/flights/renderer.test.mjs

@@ -293,3 +293,87 @@ test('the flights logic draws, tracks and follows on the Cesium engine through t
   assert.equal(engine.lines.list().length, 0, 'no line left behind');
   engine.destroy();
 });
+
+test('a failed engine switch, its rollback and the retry re-follow the tracked aircraft as a restore', async () => {
+  const { createCesiumMoversEngine } =
+    await import('../../testSupport/cesiumMoversEngine.mjs');
+  const { createFlightsDisplay } = await import('./render/display.js');
+  const { aircraftAtlasLayout } = await import('../aircraft/render/display.js');
+  const layout = aircraftAtlasLayout();
+  // The shell's navigation stamp (LayerBindings.observeCamera): a follow
+  // Cesium announces through trackedEntityChanged is new navigation unless
+  // it rebinds the session's own subject. A new navigation supersedes the
+  // switch's share restore, which then clears the selection.
+  const navigations = [];
+  const cesium = () => {
+    const made = createCesiumMoversEngine();
+    made.engine.camera.setView({
+      destination: geo.cartesian3.fromDegrees(-97.5, 30.3, 400_000),
+    });
+    made.viewer.trackedEntityChanged.addEventListener((entity) => {
+      if (entity && !entity.gevRestoring)
+        navigations.push(entity.gevTrackedId);
+    });
+    return made;
+  };
+  const working = cesium();
+  let display = null;
+  const layer = createApplicationFlights({
+    surface: defaultSurface,
+    source: {
+      label: 'Fixture aircraft',
+      getSnapshot: async () => snapshotOf(['abc123', 'def456']),
+    },
+    militaryRegistry,
+    renderer: {
+      create: ({ engine: given }) =>
+        (display = createFlightsDisplay(given, {
+          drawAtlas: async () => ({
+            canvas: { width: layout.width, height: layout.height },
+            cells: layout.cells,
+          }),
+        })),
+    },
+  });
+  layer.init(working.viewer, { engine: working.engine });
+  layer.enable();
+  const candidate = createFakeEngine();
+  let retried = null;
+  try {
+    await display.whenReady();
+    await layer.update(null);
+    await new Promise((resolve) => setTimeout(resolve, 0));
+    // The person's own selection is navigation.
+    assert.equal(layer.trackById('def456', { origin: 'user' }), true);
+    assert.deepEqual(navigations, ['flights:def456']);
+    navigations.length = 0;
+    // The switch: the working engine detaches; the candidate attaches the
+    // layer, then fails and releases what it attached.
+    layer.detachRenderer(working.viewer);
+    layer.attachRenderer(null, { engine: candidate });
+    assert.equal(candidate.camera.following()?.label?.id, 'flights:def456');
+    layer.detachRenderer(null);
+    // The rollback: the working engine attaches again and follows the same
+    // aircraft, announced as a restore.
+    layer.attachRenderer(working.viewer, { engine: working.engine });
+    assert.equal(layer.getTrackedInfo()?.icao24, 'def456');
+    assert.equal(working.viewer.trackedEntity?.gevTrackedId, 'flights:def456');
+    assert.deepEqual(navigations, [], 'the rollback is not new navigation');
+    // The retry onto a new Cesium engine.
+    layer.detachRenderer(working.viewer);
+    retried = cesium();
+    layer.attachRenderer(retried.viewer, { engine: retried.engine });
+    assert.equal(layer.getTrackedInfo()?.icao24, 'def456');
+    assert.equal(retried.viewer.trackedEntity?.gevTrackedId, 'flights:def456');
+    assert.equal(
+      retried.engine.camera.following()?.label?.id,
+      'flights:def456',
+    );
+    assert.deepEqual(navigations, [], 'the retry is not new navigation');
+  } finally {
+    layer.disable();
+    layer.destroy(retried?.viewer ?? working.viewer);
+  }
+  for (const engine of [working.engine, candidate, retried?.engine])
+    engine?.destroy();
+});