fix(transit): a held vehicle's snapshot height is its drawn marker's while the frame pass places it

Near the ground the marker follows the terrain sampled between two reports,
which a straight blend of the reports' floors misses over uneven ground.

docs/CURRENT-STATE.md

@@ -2277,8 +2277,8 @@ offers the same for a vehicle (`getSelectedVehicle`, `getCockpitSubject`,
 out of the camera's view, and its sprite, 3D model, card, trail and detection
 box are hidden; released, it is out of play until a visibility sweep finds it in
 view. Its snapshot's `position.groundHeightM` is the floor where playback draws
-it (its segment's two reports' floors at the playback fraction), not the newest
-report's, and null while that floor is unknown. Rail's `path.direction` is the
+it (the drawn marker's height while it is placed, else its segment's two
+reports' floors at the playback fraction), not the newest report's, and null while that floor is unknown. Rail's `path.direction` is the
 way the drawn train runs: once playback has placed it, a report that runs back
 turns it only when playback reaches that report. `docs/APPLICATION.md` (Cockpit subjects) shows a minimal provider. `src/ui/cockpitSubjectView.test.mjs`,
 `src/cockpitSubjectProviders.test.mjs`, `src/cockpitProgress.test.mjs`,

src/layers/transit/cockpitSubject.js

@@ -1,3 +1,4 @@
+import * as Cesium from 'cesium';
 import {
   getRegisteredTransitFeed,
   transitRouteLabel,
@@ -77,9 +78,12 @@ export function createTransitCockpitSubject({ state, parts, services }) {
     return reuse(previous, next);
   }
 
+  const scratchCartographic = new Cesium.Cartographic();
   /**
-   * The floor under the vehicle where playback draws it: its segment's two
-   * reports' floors at the playback fraction, without the shared lift. Null
+   * The floor under the vehicle where playback draws it, without the shared
+   * lift: the drawn marker's height while the frame pass places it (in view,
+   * or held), which near the ground follows the terrain between reports;
+   * else its segment's two reports' floors at the playback fraction. Null
    * while either floor is unknown, as on cold terrain, so a view waits for
    * the surface. Not `entry.heightM`, the floor at the newest report, which
    * playback has not reached yet.
@@ -87,6 +91,19 @@ export function createTransitCockpitSubject({ state, parts, services }) {
   function drawnFloorOf(entry) {
     const segment = entry.segment;
     if (!segment || entry.heightPending) return null;
+    const placed =
+      entry.surfaceReady === true &&
+      entry.hasRendered &&
+      entry.marker?.position &&
+      (state._visible.has(entry) || entry.key === state._cockpitSubjectKey);
+    if (placed) {
+      const drawn = Cesium.Cartographic.fromCartesian(
+        entry.marker.position,
+        undefined,
+        scratchCartographic,
+      );
+      if (drawn) return drawn.height - liftM;
+    }
     const from = segment.from?.h;
     const to = segment.to?.h;
     if (!Number.isFinite(from) || !Number.isFinite(to)) return null;

src/layers/transit/lifecycle.test.mjs

@@ -4247,7 +4247,10 @@ test('the selected vehicle reads as a snapshot of its place, motion and source',
   assert.equal(view.provenance.positionSource, 'gps');
   assert.deepEqual(view.stop, { stopId: 'place-1', status: 'IN_TRANSIT_TO' });
   const entry = app.vehicles()[0];
-  assert.equal(view.position.groundHeightM, entry.heightM - 1.5);
+  // Where it is drawn, without the shared lift (read back from the marker).
+  assert.ok(
+    Math.abs(view.position.groundHeightM - (entry.heightM - 1.5)) < 1e-3,
+  );
   assert.ok(Number.isFinite(view.position.latitude));
 });