Merge pull request #1513 from bilawalsidhu/sameh/world-timeline-launches

Move launches with the timeline, choose its speed, and wait for data while it plays

changelog.d/1513-timeline-launches-speed-buffering.md

@@ -0,0 +1,7 @@
+- The timeline brings launches up as the time passes them, offers play
+  speeds from real time to a week a second, and plays smoothly, holding only
+  when the next weather frame has yet to load, with a spinner by the time
+  while it waits. The time rides in a bubble over the handle. The track zooms
+  with a scroll or a pinch, drags along, and goes to the time under a tap, on
+  a phone as on a desktop. Zooming out on the globe stops while the globe is
+  still in view instead of leaving it a speck (samehkhamis, #1513).

src/app/viewer.js

@@ -7,6 +7,48 @@ import {
   isFreeGlobeCamera,
 } from '../globeInView.js';
 
+/**
+ * At the farthest a hand can zoom out, the globe is this share of the
+ * screen's height across. Cesium sets no limit, so zooming on left the globe
+ * a speck among the stars.
+ */
+const MIN_GLOBE_SHARE = 0.3;
+
+/**
+ * The farthest a hand can zoom out, in metres above the ellipsoid, for a
+ * vertical field of view. It follows the screen's shape, as the app's own
+ * whole-globe view does, so that view always lies within it.
+ * @param {number} fovy Vertical field of view, radians.
+ */
+export function zoomOutLimit(fovy) {
+  const radius = Cesium.Ellipsoid.WGS84.maximumRadius;
+  const angular = Math.atan(MIN_GLOBE_SHARE * Math.tan(fovy / 2));
+  return radius / Math.sin(angular) - radius;
+}
+
+/**
+ * Hold a hand's zoom out to `zoomOutLimit` on the globe, each frame, since the
+ * field of view follows the screen's shape. The limit never sits below the
+ * camera: one already past it (a flight, a screen turned) stops there rather
+ * than jumping in at the next zoom. Flat maps keep Cesium's own, unlimited.
+ * @returns {() => void} Stop limiting.
+ */
+export function limitZoomOut(viewer) {
+  const { scene, camera } = viewer;
+  return scene.preRender.addEventListener(() => {
+    const controller = scene.screenSpaceCameraController;
+    const fovy = camera.frustum.fovy;
+    if (scene.mode !== Cesium.SceneMode.SCENE3D || !Number.isFinite(fovy)) {
+      controller.maximumZoomDistance = Number.POSITIVE_INFINITY;
+      return;
+    }
+    controller.maximumZoomDistance = Math.max(
+      zoomOutLimit(fovy),
+      camera.positionCartographic?.height ?? 0,
+    );
+  });
+}
+
 const PINCH_ZOOM_MULTIPLIER = 8;
 const MAX_PINCH_PIXEL_DELTA = 120;
 
@@ -372,6 +414,7 @@ export function createApplicationViewer({ container, creditContainer }) {
     // required on touch-first screens. Mouse/trackpad sensitivity stays native.
     if (globalThis.matchMedia?.('(pointer: coarse)').matches)
       viewer.scene.screenSpaceCameraController.zoomFactor *= 3;
+    limitZoomOut(viewer);
     // Before any tile builds a draw command: Cesium's per-vertex model
     // atmosphere fails to LINK on Apple's Metal backend and kills the
     // render loop. See app/atmosphereCompat.js.

src/app/viewer.test.mjs

@@ -0,0 +1,73 @@
+// A hand zooming out stops while the globe is still a fair share of the
+// screen, on any screen's shape, rather than leaving it a speck.
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import * as Cesium from 'cesium';
+import { limitZoomOut, zoomOutLimit } from './viewer.js';
+
+// The share of the screen's height the globe spans from `altitude`.
+function globeShare(altitude, fovy) {
+  const radius = Cesium.Ellipsoid.WGS84.maximumRadius;
+  const angular = Math.asin(radius / (radius + altitude));
+  return Math.tan(angular) / Math.tan(fovy / 2);
+}
+
+test("zooming out stops with the globe 30% of the screen's height across", () => {
+  for (const fovy of [Math.PI / 3, 0.4, 0.2]) {
+    const limit = zoomOutLimit(fovy);
+    assert.ok(Math.abs(globeShare(limit, fovy) - 0.3) < 1e-9, `fovy ${fovy}`);
+  }
+});
+
+test('a wide screen, whose view is narrower, may zoom out farther', () => {
+  assert.ok(zoomOutLimit(0.4) > zoomOutLimit(Math.PI / 3));
+  assert.ok(zoomOutLimit(Math.PI / 3) > 20_000_000, 'past the views a link opens');
+});
+
+function fakeViewer({ fovy = Math.PI / 3, height = 10_000_000 } = {}) {
+  const scene = {
+    mode: Cesium.SceneMode.SCENE3D,
+    preRender: new Cesium.Event(),
+    screenSpaceCameraController: { maximumZoomDistance: Infinity },
+  };
+  const camera = {
+    frustum: { fovy },
+    positionCartographic: { height },
+  };
+  return { scene, camera };
+}
+
+test('the limit follows the screen, and holds a camera already past it', () => {
+  const viewer = fakeViewer();
+  const controller = viewer.scene.screenSpaceCameraController;
+  const stop = limitZoomOut(viewer);
+  viewer.scene.preRender.raiseEvent();
+  assert.equal(controller.maximumZoomDistance, zoomOutLimit(Math.PI / 3));
+
+  // A wider screen narrows the view, and the globe may go farther.
+  viewer.camera.frustum.fovy = 0.4;
+  viewer.scene.preRender.raiseEvent();
+  assert.equal(controller.maximumZoomDistance, zoomOutLimit(0.4));
+
+  // Flown past it, the next zoom out stops there instead of jumping in.
+  viewer.camera.positionCartographic.height = zoomOutLimit(0.4) * 2;
+  viewer.scene.preRender.raiseEvent();
+  assert.equal(controller.maximumZoomDistance, zoomOutLimit(0.4) * 2);
+
+  stop();
+  viewer.camera.positionCartographic.height = 0;
+  viewer.scene.preRender.raiseEvent();
+  assert.equal(controller.maximumZoomDistance, zoomOutLimit(0.4) * 2);
+});
+
+test('flat maps keep zooming out unlimited', () => {
+  const viewer = fakeViewer();
+  const controller = viewer.scene.screenSpaceCameraController;
+  limitZoomOut(viewer);
+  viewer.scene.preRender.raiseEvent();
+  assert.ok(Number.isFinite(controller.maximumZoomDistance));
+  viewer.scene.mode = Cesium.SceneMode.SCENE2D;
+  viewer.camera.frustum = {};
+  viewer.scene.preRender.raiseEvent();
+  assert.equal(controller.maximumZoomDistance, Infinity);
+});

src/celestialRing.js

@@ -32,6 +32,9 @@ export const KEYHOLE_LABEL_FEATHER_MAX_RATIO = 0.4;
  */
 export const KEYHOLE_OUTSIDE_OPACITY_DEFAULT = 0.01;
 
+// How far the world time moves before the sun and moon move with it: a
+// minute moves the sun a quarter of a degree.
+const EPHEMERIS_WORLD_STEP_MS = 60_000;
 const RING_INSET_PX = 11;
 const MARKER_INSET_PX = 36;
 const COLLIDING_MARKER_EXTRA_INSET_PX = 32;
@@ -452,7 +455,15 @@ export class CelestialRing {
     // A jump in the world time moves the sun and moon at once.
     this._skyDate = new Date();
     this._skyTime = new Cesium.JulianDate();
-    this._unsubscribeWorldTime = worldClock.subscribe(() => {
+    // Played through, the sky moves on each minute of world time it passes.
+    this._ephemerisWorldMs = null;
+    this._unsubscribeWorldTime = worldClock.subscribe(({ timeMs }) => {
+      if (
+        this._ephemerisWorldMs !== null &&
+        Math.abs(timeMs - this._ephemerisWorldMs) < EPHEMERIS_WORLD_STEP_MS
+      )
+        return;
+      this._ephemerisWorldMs = timeMs;
       this._ephemerisDirty = true;
     });
     this.setEnabled(this.enabled);

src/creditAttribution.test.mjs

@@ -241,7 +241,7 @@ const VETTED_VARS = new Set([
 // Custom properties CSS sets to a fixed length besides their 0 default. The
 // clearance checks run at each length CSS gives them, not only at 0.
 const FIXED_LENGTH_VARS = new Set(['--world-timeline-lift']);
-const LIFTS_PX = [0, 50, 80];
+const LIFTS_PX = [0, 66, 80];
 let liftPx = 0;
 function eachLift(check) {
   for (const lift of LIFTS_PX) {

src/i18n/locales/ar.json

@@ -3487,5 +3487,11 @@
   "Video plays from Québec 511, which sees your IP address.": "يُشغَّل الفيديو من Québec 511، الذي يرى عنوان IP الخاص بك.",
   "World time": "وقت العالم",
   "Back to live": "العودة إلى البث المباشر",
-  "Live": "مباشر"
+  "Live": "مباشر",
+  "1 h/s": "ساعة/ث",
+  "1 d/s": "يوم/ث",
+  "1 wk/s": "أسبوع/ث",
+  "Play speed": "سرعة التشغيل",
+  "Scroll or pinch to zoom, drag to move along · double-click to see it all": "مرّر أو اقرص للتكبير، واسحب للتنقل · انقر نقرًا مزدوجًا لرؤية الكل",
+  "1 min/s": "دقيقة/ث"
 }

src/i18n/locales/de.json

@@ -3187,5 +3187,11 @@
   "Video plays from Québec 511, which sees your IP address.": "Das Video wird von Québec 511 abgespielt, das Ihre IP-Adresse sieht.",
   "World time": "Weltzeit",
   "Back to live": "Zurück zu live",
-  "Live": "Live"
+  "Live": "Live",
+  "1 h/s": "1 Std./s",
+  "1 d/s": "1 Tag/s",
+  "1 wk/s": "1 Wo./s",
+  "Play speed": "Wiedergabegeschwindigkeit",
+  "Scroll or pinch to zoom, drag to move along · double-click to see it all": "Scrollen oder mit zwei Fingern zoomen, ziehen zum Verschieben · Doppelklick zeigt alles",
+  "1 min/s": "1 Min./s"
 }

src/i18n/locales/es.json

@@ -3262,5 +3262,11 @@
   "Video plays from Québec 511, which sees your IP address.": "El vídeo se reproduce desde Québec 511, que ve tu dirección IP.",
   "World time": "Hora mundial",
   "Back to live": "Volver al directo",
-  "Live": "En directo"
+  "Live": "En directo",
+  "1 h/s": "1 h/s",
+  "1 d/s": "1 d/s",
+  "1 wk/s": "1 sem/s",
+  "Play speed": "Velocidad de reproducción",
+  "Scroll or pinch to zoom, drag to move along · double-click to see it all": "Desplaza o pellizca para ampliar, arrastra para moverte · doble clic para verlo todo",
+  "1 min/s": "1 min/s"
 }

src/i18n/locales/fr.json

@@ -3262,5 +3262,11 @@
   "Video plays from Québec 511, which sees your IP address.": "La vidéo est lue depuis Québec 511, qui voit votre adresse IP.",
   "World time": "Heure du monde",
   "Back to live": "Revenir au direct",
-  "Live": "Direct"
+  "Live": "Direct",
+  "1 h/s": "1 h/s",
+  "1 d/s": "1 j/s",
+  "1 wk/s": "1 sem/s",
+  "Play speed": "Vitesse de lecture",
+  "Scroll or pinch to zoom, drag to move along · double-click to see it all": "Faites défiler ou pincez pour zoomer, glissez pour vous déplacer · double-clic pour tout voir",
+  "1 min/s": "1 min/s"
 }

src/i18n/locales/it.json

@@ -3262,5 +3262,11 @@
   "Video plays from Québec 511, which sees your IP address.": "Il video viene riprodotto da Québec 511, che vede il tuo indirizzo IP.",
   "World time": "Ora del mondo",
   "Back to live": "Torna alla diretta",
-  "Live": "Diretta"
+  "Live": "Diretta",
+  "1 h/s": "1 h/s",
+  "1 d/s": "1 g/s",
+  "1 wk/s": "1 sett/s",
+  "Play speed": "Velocità di riproduzione",
+  "Scroll or pinch to zoom, drag to move along · double-click to see it all": "Scorri o pizzica per lo zoom, trascina per spostarti · doppio clic per vedere tutto",
+  "1 min/s": "1 min/s"
 }

src/i18n/locales/ja.json

@@ -3112,5 +3112,11 @@
   "Video plays from Québec 511, which sees your IP address.": "動画は Québec 511 から再生され、Québec 511 にはあなたの IP アドレスが見えます。",
   "World time": "世界時刻",
   "Back to live": "ライブに戻る",
-  "Live": "ライブ"
+  "Live": "ライブ",
+  "1 h/s": "1時間/秒",
+  "1 d/s": "1日/秒",
+  "1 wk/s": "1週間/秒",
+  "Play speed": "再生速度",
+  "Scroll or pinch to zoom, drag to move along · double-click to see it all": "スクロールまたはピンチでズーム、ドラッグで移動 · ダブルクリックで全体表示",
+  "1 min/s": "1分/秒"
 }

src/i18n/locales/pt.json

@@ -3262,5 +3262,11 @@
   "Video plays from Québec 511, which sees your IP address.": "O vídeo é reproduzido a partir do Québec 511, que vê o seu endereço IP.",
   "World time": "Hora do mundo",
   "Back to live": "Voltar ao vivo",
-  "Live": "Ao vivo"
+  "Live": "Ao vivo",
+  "1 h/s": "1 h/s",
+  "1 d/s": "1 d/s",
+  "1 wk/s": "1 sem/s",
+  "Play speed": "Velocidade de reprodução",
+  "Scroll or pinch to zoom, drag to move along · double-click to see it all": "Role ou faça pinça para ampliar, arraste para se mover · clique duplo para ver tudo",
+  "1 min/s": "1 min/s"
 }

src/i18n/locales/ru.json

@@ -3337,5 +3337,11 @@
   "Video plays from Québec 511, which sees your IP address.": "Видео воспроизводится с Québec 511, который видит ваш IP-адрес.",
   "World time": "Мировое время",
   "Back to live": "Вернуться к прямому эфиру",
-  "Live": "Сейчас"
+  "Live": "Сейчас",
+  "1 h/s": "1 ч/с",
+  "1 d/s": "1 дн/с",
+  "1 wk/s": "1 нед/с",
+  "Play speed": "Скорость воспроизведения",
+  "Scroll or pinch to zoom, drag to move along · double-click to see it all": "Прокрутите или сведите пальцы для масштаба, перетащите для перемещения · двойной щелчок — показать всё",
+  "1 min/s": "1 мин/с"
 }

src/i18n/locales/uk.json

@@ -3337,5 +3337,11 @@
   "Video plays from Québec 511, which sees your IP address.": "Відео відтворюється з Québec 511, який бачить вашу IP-адресу.",
   "World time": "Світовий час",
   "Back to live": "Повернутися до прямого ефіру",
-  "Live": "Зараз"
+  "Live": "Зараз",
+  "1 h/s": "1 год/с",
+  "1 d/s": "1 дн/с",
+  "1 wk/s": "1 тиж/с",
+  "Play speed": "Швидкість відтворення",
+  "Scroll or pinch to zoom, drag to move along · double-click to see it all": "Прокрутіть або зведіть пальці для масштабу, перетягніть для переміщення · подвійне клацання — показати все",
+  "1 min/s": "1 хв/с"
 }

src/i18n/locales/zh-CN.json

@@ -3112,5 +3112,11 @@
   "Video plays from Québec 511, which sees your IP address.": "视频从 Québec 511 播放，Québec 511 会看到你的 IP 地址。",
   "World time": "世界时间",
   "Back to live": "返回实时",
-  "Live": "实时"
+  "Live": "实时",
+  "1 h/s": "1 小时/秒",
+  "1 d/s": "1 天/秒",
+  "1 wk/s": "1 周/秒",
+  "Play speed": "播放速度",
+  "Scroll or pinch to zoom, drag to move along · double-click to see it all": "滚动或双指缩放，拖动以移动 · 双击查看全部",
+  "1 min/s": "1 分钟/秒"
 }

src/i18n/locales/zh-Hant.json

@@ -3112,5 +3112,11 @@
   "Video plays from Québec 511, which sees your IP address.": "影片從 Québec 511 播放，Québec 511 會看到你的 IP 位址。",
   "World time": "世界時間",
   "Back to live": "返回即時",
-  "Live": "即時"
+  "Live": "即時",
+  "1 h/s": "1 小時/秒",
+  "1 d/s": "1 天/秒",
+  "1 wk/s": "1 週/秒",
+  "Play speed": "播放速度",
+  "Scroll or pinch to zoom, drag to move along · double-click to see it all": "捲動或雙指縮放，拖曳以移動 · 按兩下查看全部",
+  "1 min/s": "1 分鐘/秒"
 }

src/i18n/same-as-english.json

@@ -216,6 +216,7 @@
     "▲ {frp} MW"
   ],
   "es": [
+    "1 min/s",
     "1090 MHz",
     "978 MHz UAT",
     "AIS: --",
@@ -355,6 +356,7 @@
     "iNaturalist"
   ],
   "fr": [
+    "1 min/s",
     "1090 MHz",
     "978 MHz UAT",
     "ALTITUDE",
@@ -549,6 +551,7 @@
     "▲ {frp} MW"
   ],
   "it": [
+    "1 min/s",
     "1090 MHz",
     "978 MHz UAT",
     "AFRICA",
@@ -784,6 +787,7 @@
     "▲ {frp} MW"
   ],
   "pt": [
+    "1 min/s",
     "1090 MHz",
     "978 MHz UAT",
     "AIS: --",

src/layers/launches/orbitRendering.js

@@ -67,6 +67,9 @@ export function createOrbitRendering({
     return Boolean(
       layerState._enabled &&
       layerState._dataSource?.show &&
+      !layerState.launchHidden(
+        layerState._missionOverlayRecords.get(launchId)?.launch,
+      ) &&
       (!layerState._selectedLaunchId ||
         layerState._selectedLaunchId === launchId),
     );

src/layers/launches/overlays.js

@@ -411,8 +411,11 @@ export function createOverlays({ state: layerState, services, parts, source }) {
       false,
     );
     layerState._selectedMissionOverlayTimeText = null;
+    const records = [...layerState._missionOverlayRecords.values()].filter(
+      (record) => !layerState.launchHidden(record.launch),
+    );
     const entries = selectRocketMissionMarkerOverlayCohort(
-      Array.from(layerState._missionOverlayRecords.values(), (record) => {
+      records.map((record) => {
         const entry = createRocketMissionMarkerOverlayEntry(
           record.launch,
           record.anchorPosition,

src/layers/launches/rendering.js

@@ -22,6 +22,25 @@ export function createRendering({
     if (current !== next) graphic.show = next;
   }
 
+  // Away from live, a launch shows from its lift-off on, so scrubbing the
+  // world time forward brings the launches up in the order they flew. The
+  // selected mission stays, whenever it flies.
+  function syncPendingLaunches() {
+    const pending = new Set();
+    for (const { launch } of layerState._missionOverlayRecords.values()) {
+      if (layerState.launchHidden(launch)) pending.add(launch.id);
+    }
+    const previous = layerState._pendingLaunchIds;
+    if (
+      pending.size === previous.size &&
+      [...pending].every((id) => previous.has(id))
+    )
+      return;
+    layerState._pendingLaunchIds = pending;
+    parts.overlays.syncMissionOverlayEntries();
+    parts.orbitRendering.syncMissionOrbitPrimitiveVisibility();
+  }
+
   function updateMissionFrame() {
     if (
       !layerState._enabled ||
@@ -31,6 +50,7 @@ export function createRendering({
       parts.overlays.hideReplayVehicleOverlay();
       return;
     }
+    syncPendingLaunches();
     parts.panel.updateMissionTelemetry();
     const time = Cesium.JulianDate.now(layerState._declutterTime);
     layerState._declutterOccluder.cameraPosition =
@@ -39,21 +59,28 @@ export function createRendering({
     parts.overlays.refreshSelectedMissionOverlayText();
     for (const entity of layerState._dataSource.entities.values) {
       if (!entity.show) continue;
-      if (!entity.position) continue;
       const id = parts.selection.entityLaunchId(entity);
       if (!id) continue;
+      const pending = layerState._pendingLaunchIds.has(id);
+      if (entity.polyline)
+        setGraphicVisibility(entity.polyline, !pending, time);
+      if (!entity.position) continue;
       const position = entity.position.getValue(time);
-      let horizonVisible = Boolean(
+      let horizonVisible =
-        position && layerState._declutterOccluder.isPointVisible(position),
+        !pending &&
-      );
+        Boolean(
+          position && layerState._declutterOccluder.isPointVisible(position),
+        );
       const launchAnchor = entity.id.startsWith('rocket-launch:');
       if (launchAnchor) {
-        horizonVisible = parts.overlays.missionAnchorVisible(
+        horizonVisible =
-          layerState._viewer.scene.camera.positionWC,
+          !pending &&
-          position,
+          parts.overlays.missionAnchorVisible(
-          id,
+            layerState._viewer.scene.camera.positionWC,
-          layerState._selectedLaunchId,
+            position,
-        );
+            id,
+            layerState._selectedLaunchId,
+          );
         // Keep entity.show reserved for mission-selection isolation. Horizon
         // state is applied to the individual graphics below so a rear-side
         // anchor remains eligible for reevaluation as the camera moves.

src/layers/launches/state.js

@@ -169,6 +169,26 @@ export function createState({ services }) {
 
   state.coverageChanged = () => worldClock.coverageChanged();
 
+  /**
+   * Whether a launch has yet to lift off at the world time. While live none
+   * has, so the layer shows every launch in its window as it always did.
+   */
+
+  state.launchPending = (launch) => {
+    if (worldClock.isLive()) return false;
+    const launchMs = Date.parse(launch?.launchTime);
+    return Number.isFinite(launchMs) && launchMs > state.now();
+  };
+
+  /** Whether a launch is hidden for lifting off after the world time. */
+
+  state.launchHidden = (launch) =>
+    launch?.id !== state._selectedLaunchId && state.launchPending(launch);
+
+  /** The launches hidden at the last frame, by id, to spot a change. */
+
+  state._pendingLaunchIds = new Set();
+
   state._missionOrbitPatternRegistered = false;
 
   state._pathDistanceCache = new WeakMap();

src/layers/launches/worldTime.test.mjs

@@ -0,0 +1,102 @@
+// Away from live, a launch shows from its lift-off on, so scrubbing the world
+// time brings the launches up in the order they flew.
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import * as Cesium from 'cesium';
+import { worldClock } from '../../time/worldClock.js';
+import { createState } from './state.js';
+import { createRendering } from './rendering.js';
+import { createSelection } from './selection.js';
+
+const noop = () => {};
+const LAUNCH_MS = Date.UTC(2026, 9, 20, 12);
+
+function scene(t) {
+  t.after(() => worldClock.goLive());
+  const state = createState({
+    services: {
+      overlays: {
+        setOverlayEntries: noop,
+        setOverlaySourceVisible: noop,
+        clearOverlaySource: noop,
+      },
+    },
+  });
+  const launch = {
+    id: 'ceres-1',
+    launchTime: new Date(LAUNCH_MS).toISOString(),
+  };
+  state._enabled = true;
+  state._dataSource = new Cesium.CustomDataSource('launches');
+  state._viewer = {
+    scene: { camera: { positionWC: new Cesium.Cartesian3(2e7, 0, 0) } },
+  };
+  state._missionOverlayRecords.set(launch.id, { launch });
+  const position = Cesium.Cartesian3.fromDegrees(0, 0);
+  const anchor = state._dataSource.entities.add({
+    id: `rocket-launch:${launch.id}`,
+    position,
+    point: { pixelSize: 5 },
+  });
+  const path = state._dataSource.entities.add({
+    id: `rocket-transfer:${launch.id}`,
+    polyline: { positions: [position, Cesium.Cartesian3.fromDegrees(1, 1)] },
+  });
+  const syncs = { labels: 0, orbits: 0 };
+  const parts = {
+    panel: { updateMissionTelemetry: noop },
+    overlays: {
+      hideReplayVehicleOverlay: noop,
+      updateReplayVehicleOverlay: noop,
+      refreshSelectedMissionOverlayText: noop,
+      missionAnchorVisible: () => true,
+      syncMissionOverlayEntries: () => syncs.labels++,
+    },
+    orbitRendering: {
+      syncMissionOrbitPrimitiveVisibility: () => syncs.orbits++,
+    },
+  };
+  const services = {
+    geometry: { getKeyholeGeometry: noop },
+    satellites: {},
+  };
+  parts.selection = createSelection({ state, services, parts });
+  const rendering = createRendering({ state, services, parts });
+  const shown = () => {
+    rendering.updateMissionFrame();
+    const time = Cesium.JulianDate.now();
+    return {
+      marker: anchor.point.show.getValue(time),
+      path: path.polyline.show?.getValue(time) ?? true,
+    };
+  };
+  return { state, launch, syncs, shown };
+}
+
+test('live shows every launch in the window, lifted off or not', (t) => {
+  const { state, launch, shown } = scene(t);
+  assert.equal(state.launchPending(launch), false);
+  assert.deepEqual(shown(), { marker: true, path: true });
+});
+
+test('away from live a launch shows from its lift-off on', (t) => {
+  const { state, launch, syncs, shown } = scene(t);
+  worldClock.seek(LAUNCH_MS - 60_000);
+  assert.equal(state.launchPending(launch), true);
+  assert.deepEqual(shown(), { marker: false, path: false });
+  assert.deepEqual(syncs, { labels: 1, orbits: 1 }, 'its label and orbit go');
+  shown();
+  assert.deepEqual(syncs, { labels: 1, orbits: 1 }, 'once, not each frame');
+
+  worldClock.seek(LAUNCH_MS);
+  assert.equal(state.launchPending(launch), false);
+  assert.deepEqual(shown(), { marker: true, path: true });
+  assert.deepEqual(syncs, { labels: 2, orbits: 2 });
+});
+
+test('the selected mission stays whenever it flies', (t) => {
+  const { state, launch, shown } = scene(t);
+  state._selectedLaunchId = launch.id;
+  worldClock.seek(LAUNCH_MS - 60_000);
+  assert.deepEqual(shown(), { marker: true, path: true });
+});

src/layers/weather/clock.js

@@ -4,6 +4,15 @@ export const RADAR_MAX_GAP_MS = 30 * 60_000;
 export const REGIONAL_INFRARED_MAX_GAP_MS = 30 * 60_000;
 export const GLOBAL_INFRARED_MAX_GAP_MS = 3 * 60 * 60_000;
 export const LIGHTNING_MAX_GAP_MS = 30 * 60_000;
+/** The longest play waits on a frame before it goes on without it. */
+export const PLAY_WAIT_MS = 8_000;
+// Playing on its own, the world does not wait on the frame it is at: that
+// frame loads as play goes on. Play holds only at the next frame while the
+// one before it still loads, then goes on to it once it shows, so every frame
+// shows, play stops only for a load that is slow, and a frame loaded ahead
+// costs play nothing.
+
+let clocks = 0;
 
 /**
  * Transient observed time. Products must honor the signal before installing a
@@ -33,6 +42,34 @@ export function createWeatherClock({
   let generation = 0;
   let timer = null;
   let pending = null;
+  // This clock's name on the world, for its holds and its gate.
+  const holdId = `weather:${++clocks}`;
+  // While the world plays, the hold on it at the next frame as the one before
+  // it loads, and the timer that lets play go on should that frame never
+  // come; and whether the move to the next frame waits on it.
+  let waiting = null;
+  let waitTimer = null;
+  let deferred = false;
+  const stopWaiting = () => {
+    const release = waiting;
+    waiting = null;
+    if (waitTimer !== null) clearTimeout(waitTimer);
+    waitTimer = null;
+    release?.();
+  };
+  const startWaiting = () => {
+    if (waiting || destroyed || !registry.size) return;
+    waiting = world.wait(holdId);
+    waitTimer = setTimeout(() => {
+      waitTimer = null;
+      stopWaiting();
+      // The frame that never came gives way to the one play is at.
+      if (deferred) void transition();
+    }, PLAY_WAIT_MS);
+  };
+  // Playing on its own, the world waits for frames as they load.
+  const worldPlaysOn = (state = world.getState()) =>
+    state.playing && !state.live && !state.stepping;
   let destroyed = false;
   const timesFor = (product) =>
     [...new Set(product.getTimes())]
@@ -65,6 +102,9 @@ export function createWeatherClock({
     mode,
     target,
     playing,
+    // Frames come on one after another: this clock replays them, or the world
+    // plays on through them, so the next is worth loading ahead.
+    advancing: playing || worldPlaysOn(),
     timeline: getTimeline(),
     products: [...registry.values()].map(({ product }) => ({
       id: product.id,
@@ -130,7 +170,10 @@ export function createWeatherClock({
     cancelTimer();
     const owner = ++generation;
     if (!canPlay()) stopPlaying();
+    // A scrub drops what loads and moves on; play goes on as this loads.
     applied = selections();
+    deferred = false;
+    stopWaiting();
     for (const entry of registry.values()) entry.controller?.abort();
     // Start all products together; a rejection must not skip the settle barrier.
     const tasks = [...registry.values()].map((entry) => {
@@ -144,10 +187,13 @@ export function createWeatherClock({
     });
     const result = Promise.allSettled(tasks).then(() => {
       if (destroyed || owner !== generation) return false;
+      stopWaiting();
       pending = null;
       if (!canPlay()) stopPlaying();
       notify();
       schedule();
+      // Play held at the next frame for this one goes on to it.
+      if (deferred) void transition();
       return true;
     });
     pending = result;
@@ -269,8 +315,10 @@ export function createWeatherClock({
     },
     refresh() {
       if (destroyed) return Promise.resolve(false);
-      // New frames, or products added or dropped.
+      // New frames, or products added or dropped, and with them the next.
       world.coverageChanged();
+      gated = undefined;
+      gateNext(world.getState());
       if (!canPlay()) {
         stopPlaying();
         cancelTimer();
@@ -289,6 +337,8 @@ export function createWeatherClock({
       stopPlaying();
       destroyed = true;
       ++generation;
+      stopWaiting();
+      world.gate(holdId, null);
       cancelTimer();
       unfollowWorld();
       // The last weather clock leaves the world live, as a new one expects.
@@ -306,18 +356,58 @@ export function createWeatherClock({
   // changes the mode or a product's frame.
   function followWorld(state) {
     if (destroyed) return;
+    gateNext(state);
+    // Play starting or stopping changes whether the next frame loads ahead.
+    const playsOn = worldPlaysOn(state);
+    const playChanged = playsOn !== playedOn;
+    playedOn = playsOn;
     const nextMode = state.live ? 'latest' : 'history';
     const nextTarget = state.live
       ? null
       : new Date(Math.floor(state.timeMs)).toISOString();
     const key = nextTarget ?? 'latest';
-    if (key === seen) return;
+    if (key === seen) {
+      if (playChanged) notify();
+      return;
+    }
     seen = key;
     const modeChanged = nextMode !== mode;
     mode = nextMode;
     target = nextTarget;
-    if (moving || modeChanged || selections() !== applied) void transition();
+    if (!moving && !modeChanged && selections() === applied) {
-    else notify();
+      // Back at the frame that loads, play need not hold for it.
+      if (deferred) {
+        deferred = false;
+        stopWaiting();
+      }
+      // Played through, the time moves each frame with nothing new to show:
+      // listeners hear of the frames as they change.
+      if (!playsOn || playChanged) notify();
+      return;
+    }
+    // Playing on into the next frame while the one before it loads, the
+    // world holds here until that one shows.
+    if (!moving && !modeChanged && pending && playsOn) {
+      deferred = true;
+      startWaiting();
+      notify();
+      return;
+    }
+    void transition();
+  }
+  // Playing on its own, the world stops at each next frame to load it rather
+  // than run past frames between its ticks.
+  let gated = null;
+  let playedOn = false;
+  function gateNext(state) {
+    const next = worldPlaysOn(state)
+      ? (getTimeline()
+          .map((time) => Date.parse(time))
+          .find((time) => time > Math.floor(state.timeMs)) ?? null)
+      : null;
+    if (next === gated) return;
+    gated = next;
+    world.gate(holdId, next);
   }
   const unfollowWorld = world.subscribe(followWorld);
   driver = world.attachDriver({

src/layers/weather/clock.test.mjs

@@ -6,6 +6,7 @@ import {
   REGIONAL_INFRARED_MAX_GAP_MS,
   GLOBAL_INFRARED_MAX_GAP_MS,
   LIGHTNING_MAX_GAP_MS,
+  PLAY_WAIT_MS,
 } from './clock.js';
 import { createWorldClock } from '../../time/worldClock.js';
 
@@ -493,3 +494,216 @@ test('a clock destroyed while it registers a product lets go of it', (t) => {
   assert.equal(world.coverage(), null);
   assert.equal(world.getState().supported, false);
 });
+
+// A radar whose frames load only when the test says.
+function slowRadar(minutes) {
+  const radar = product('radar', minutes);
+  radar.loads = [];
+  radar.apply = (selected, { signal }) =>
+    new Promise((resolve) => {
+      radar.loads.push({
+        selected,
+        signal,
+        done() {
+          radar.shown = selected;
+          resolve(true);
+        },
+      });
+    });
+  return radar;
+}
+
+test('play stops at the next frame, not past it, until it shows', async (t) => {
+  const { clock, world, tick } = harness(t);
+  const radar = slowRadar([0, 10, 20]);
+  clock.register(radar);
+  world.seek(Date.parse(time(1)));
+  await flush();
+  radar.loads.at(-1).done();
+  await flush();
+  world.play({ rate: 3_600, continuous: true });
+  world.coverageChanged();
+  tick(1_000);
+  assert.equal(
+    world.now(),
+    Date.parse(time(10)),
+    'an hour on, but held at the frame',
+  );
+});
+
+test('playing, a frame loads as play goes on, and play holds at the next until it shows', async (t) => {
+  const { clock, world, tick } = harness(t);
+  const radar = slowRadar([0, 10, 20, 30]);
+  clock.register(radar);
+  world.seek(Date.parse(time(9)));
+  await flush();
+  radar.loads.at(-1).done();
+  await flush();
+  world.play({ rate: 60, continuous: true });
+  // Into the next frame (the page's frames tell of it): it loads as play goes on.
+  tick(2_000);
+  world.coverageChanged();
+  await flush();
+  assert.equal(radar.loads.at(-1).selected, time(10));
+  assert.equal(
+    world.getState().buffering,
+    false,
+    'no hold for the frame play is at',
+  );
+  // At the frame after while that one still loads, play holds there.
+  tick(10_000);
+  world.coverageChanged();
+  await flush();
+  assert.equal(world.now(), Date.parse(time(20)));
+  assert.equal(world.getState().buffering, true);
+  assert.equal(
+    radar.loads.at(-1).selected,
+    time(10),
+    'the frame before still loads',
+  );
+  tick(5_000);
+  assert.equal(world.now(), Date.parse(time(20)), 'held while it loads');
+  radar.loads.at(-1).done();
+  await flush();
+  assert.equal(
+    radar.loads.at(-1).selected,
+    time(20),
+    'and on to the next once it shows',
+  );
+  assert.equal(world.getState().buffering, false);
+  tick(1_000);
+  assert.equal(world.now(), Date.parse(time(20)) + 60_000);
+
+  // A scrub aborts what loads and loads where the hand is, with no wait.
+  world.pause();
+  world.seek(Date.parse(time(30)));
+  await flush();
+  const scrubbed = radar.loads.at(-1);
+  world.seek(Date.parse(time(0)));
+  await flush();
+  assert.equal(scrubbed.signal.aborted, true);
+  assert.equal(radar.loads.at(-1).selected, time(0));
+  assert.equal(world.getState().buffering, false);
+});
+
+test('a frame that never comes holds play only so long', async (t) => {
+  const { clock, world, tick } = harness(t);
+  const radar = slowRadar([0, 10, 20]);
+  clock.register(radar);
+  world.seek(Date.parse(time(9)));
+  await flush();
+  radar.loads.at(-1).done();
+  await flush();
+  world.play({ rate: 60, continuous: true });
+  tick(2_000);
+  world.coverageChanged();
+  await flush();
+  tick(10_000);
+  world.coverageChanged();
+  await flush();
+  assert.equal(world.getState().buffering, true);
+  const stuck = radar.loads.at(-1);
+  tick(PLAY_WAIT_MS);
+  await flush();
+  assert.equal(world.getState().buffering, false, 'play goes on without it');
+  assert.equal(stuck.signal.aborted, true);
+  assert.equal(radar.loads.at(-1).selected, time(20), 'loading where play is');
+});
+
+test('held at the next frame, a scrub back to the one loading lets play go', async (t) => {
+  const { clock, world, tick } = harness(t);
+  const radar = slowRadar([0, 10, 20]);
+  clock.register(radar);
+  world.seek(Date.parse(time(9)));
+  await flush();
+  radar.loads.at(-1).done();
+  await flush();
+  world.play({ rate: 60, continuous: true });
+  tick(2_000);
+  world.coverageChanged();
+  await flush();
+  tick(10_000);
+  world.coverageChanged();
+  await flush();
+  assert.equal(world.getState().buffering, true);
+  world.pause();
+  world.seek(Date.parse(time(11)));
+  await flush();
+  world.play({ rate: 60, continuous: true });
+  assert.equal(
+    world.getState().buffering,
+    false,
+    'no hold for the frame it is at',
+  );
+  const at = world.now();
+  tick(1_000);
+  assert.equal(world.now(), at + 60_000);
+});
+
+test('a frame in flight as play starts loads as it goes on; a refresh does not hold', async (t) => {
+  const { clock, world } = harness(t);
+  const radar = slowRadar([0, 10, 20]);
+  clock.register(radar);
+  world.seek(Date.parse(time(5)));
+  await flush();
+  world.play({ continuous: true });
+  assert.equal(world.getState().buffering, false);
+  radar.loads.at(-1).done();
+  await flush();
+  void clock.refresh();
+  await flush();
+  assert.equal(world.getState().buffering, false, 'the same frames, no hold');
+  radar.loads.at(-1).done();
+});
+
+test('weather clocks sharing a world gate it each on their own', async (t) => {
+  let mono = 0;
+  const world = createWorldClock({
+    now: () => Date.parse(time(30)),
+    monotonicNow: () => mono,
+    setInterval: () => 0,
+    clearInterval() {},
+  });
+  const a = createWeatherClock({ world, now: () => Date.parse(time(30)) });
+  const b = createWeatherClock({ world, now: () => Date.parse(time(30)) });
+  t.after(() => {
+    a.destroy();
+    b.destroy();
+  });
+  a.register(product('radar', [0, 10]));
+  b.register(product('clouds', [0, 15]));
+  world.seek(Date.parse(time(1)));
+  await flush();
+  world.play({ rate: 3_600, continuous: true });
+  world.coverageChanged();
+  b.destroy();
+  world.coverageChanged();
+  mono += 1_000;
+  assert.equal(
+    world.now(),
+    Date.parse(time(10)),
+    'the other clock’s gate stays',
+  );
+});
+
+test('played through, listeners hear of frames as they change, and of play stopping', async (t) => {
+  const { clock, world, tick } = harness(t);
+  const radar = product('radar', [0, 10, 20]);
+  clock.register(radar);
+  world.seek(Date.parse(time(1)));
+  await flush();
+  world.play({ rate: 60, continuous: true });
+  await flush();
+  const heard = [];
+  clock.subscribe((state) => heard.push(state));
+  // Seconds of world time within one frame: nothing new to show.
+  for (let i = 0; i < 5; i++) {
+    tick(100);
+    world.coverageChanged();
+  }
+  await flush();
+  assert.equal(heard.length, 0);
+  world.pause();
+  await flush();
+  assert.equal(heard.at(-1)?.advancing, false, 'heard play stop');
+});

src/layers/weather/index.js

@@ -109,7 +109,8 @@ export function createWeatherLayer({
   const notify = () => listener?.();
   const shownTime = () => (noFrame ? null : rendering?.getDiagnostics().time);
   const unsubscribeClock = clock?.subscribe(() => {
-    if (!clock.getState().playing || suspended()) rendering?.cancelPrefetch?.();
+    if (!clock.getState().advancing || suspended())
+      rendering?.cancelPrefetch?.();
     notify();
   });
   const observationDelayed = () =>
@@ -211,7 +212,7 @@ export function createWeatherLayer({
     if (
       !enabled ||
       suspended() ||
-      !(clock ? clock.getState().playing : playing)
+      !(clock ? clock.getState().advancing : playing)
     )
       return;
     const times = manifest?.times ?? [];

src/time/worldClock.js

@@ -24,15 +24,21 @@ const MAX_DATE_MS = 8.64e15;
 export const isWorldTime = (ms) =>
   Number.isFinite(ms) && Math.abs(ms) <= MAX_DATE_MS;
 
-/** How often subscribers hear of a continuously playing cursor. */
+/**
+ * How often subscribers hear of a continuously playing cursor with no
+ * animation frames to go by; with them, they hear of it every frame.
+ */
 export const WORLD_CLOCK_TICK_MS = 250;
 
 /**
  * @param {{now?: () => number, monotonicNow?: () => number,
  *   setInterval?: typeof globalThis.setInterval,
  *   clearInterval?: typeof globalThis.clearInterval,
+ *   requestFrame?: typeof globalThis.requestAnimationFrame | null,
+ *   cancelFrame?: typeof globalThis.cancelAnimationFrame | null,
  *   follow?: Iterable<string>}} [options] `follow` lists the layers that
- *   move with the cursor from the start.
+ *   move with the cursor from the start. Playing, the clock tells its
+ *   listeners of the time each animation frame, or each tick without frames.
  */
 export function createWorldClock({
   // Read at each call, so a stand-in for Date.now (a test's, a harness's)
@@ -41,6 +47,8 @@ export function createWorldClock({
   monotonicNow = () => globalThis.performance.now(),
   setInterval = globalThis.setInterval,
   clearInterval = globalThis.clearInterval,
+  requestFrame = globalThis.requestAnimationFrame?.bind(globalThis) ?? null,
+  cancelFrame = globalThis.cancelAnimationFrame?.bind(globalThis) ?? null,
   follow = [],
 } = {}) {
   const listeners = new Set();
@@ -60,8 +68,13 @@ export function createWorldClock({
   let active = null;
   let ticker = null;
   let notifying = 0;
+  // Layers loading what the cursor needs; continuous play holds while any is.
+  const waits = new Map();
+  // Times continuous play may not pass yet, by layer: each one's next frame.
+  const gates = new Map();
 
   const continuous = () => playing && !active;
+  const buffering = () => waits.size > 0;
   const isFollowed = (id) => following.has(id) || (holds.get(id) ?? 0) > 0;
   const followedIds = () => [
     ...new Set([...following, ...[...holds.keys()].filter(isFollowed)]),
@@ -87,8 +100,11 @@ export function createWorldClock({
   };
   const time = () => {
     if (live) return now();
-    if (continuous()) return cursorMs + (monotonicNow() - anchorMono) * rate;
+    if (!continuous() || buffering()) return cursorMs;
-    return cursorMs;
+    let at = cursorMs + (monotonicNow() - anchorMono) * rate;
+    for (const gate of gates.values())
+      if (gate >= cursorMs) at = Math.min(at, gate);
+    return at;
   };
   const getState = () =>
     Object.freeze({
@@ -96,6 +112,10 @@ export function createWorldClock({
       timeMs: time(),
       playing,
       rate,
+      // A driver steps the clock, at its own pace rather than the rate.
+      stepping: Boolean(active),
+      // Playing, but held while a layer loads what the cursor needs.
+      buffering: continuous() && buffering(),
       following: followedIds(),
       supported: isSupported(),
     });
@@ -116,11 +136,23 @@ export function createWorldClock({
       }
     }
   };
+  // Playing, the time moves on smoothly: each frame, so the timeline and the
+  // globe move with it and a layer's next frame is met as it comes, or each
+  // tick where there are no frames.
+  const frames = Boolean(requestFrame && cancelFrame);
+  const frame = () => {
+    ticker = requestFrame(frame);
+    notify();
+  };
   const syncTicker = () => {
     if (continuous() && listeners.size) {
-      ticker ??= setInterval(notify, WORLD_CLOCK_TICK_MS);
+      if (ticker === null)
+        ticker = frames
+          ? requestFrame(frame)
+          : setInterval(notify, WORLD_CLOCK_TICK_MS);
     } else if (ticker !== null) {
-      clearInterval(ticker);
+      if (frames) cancelFrame(ticker);
+      else clearInterval(ticker);
       ticker = null;
     }
   };
@@ -152,23 +184,27 @@ export function createWorldClock({
       anchorMono = monotonicNow();
       changed();
     },
-    /** Back to the wall clock; playback stops. */
+    /** Back to the wall clock; playback stops and its rate goes back to 1. */
     goLive() {
-      if (live && !playing) return;
+      if (live && !playing && rate === 1) return;
       const stepping = active;
       live = true;
       cursorMs = null;
       playing = false;
       active = null;
+      rate = 1;
       // Its driven(false) finds nothing left to stop.
       stepping?.driver.pause();
       changed();
     },
-    /** Play from the cursor (from now, when live). */
+    /**
-    play({ rate: nextRate } = {}) {
+     * Play from the cursor (from now, when live). A driver steps it unless
+     * `continuous`, which runs the cursor at the rate past every driver.
+     */
+    play({ rate: nextRate, continuous: runOn = false } = {}) {
       if (nextRate !== undefined) api.setRate(nextRate);
       if (playing) return;
-      for (let index = drivers.length - 1; index >= 0; index -= 1) {
+      for (let index = drivers.length - 1; !runOn && index >= 0; index -= 1) {
         if (drivers[index].driver.play() !== false) return;
       }
       if (live) {
@@ -192,6 +228,40 @@ export function createWorldClock({
     togglePlay() {
       return playing ? api.pause() : api.play();
     },
+    /**
+     * Hold continuous play where it is while layer `id` loads what the cursor
+     * needs, as a video waits on its buffer; play goes on once every hold is
+     * released. A seek still moves the cursor. Returns the release. Neither
+     * tells listeners (play's own ticks do), so a layer can take and release a
+     * hold mid-change without anyone moving the clock under it.
+     */
+    wait(id) {
+      settle();
+      waits.set(id, (waits.get(id) ?? 0) + 1);
+      let held = true;
+      return () => {
+        if (!held) return;
+        held = false;
+        const count = waits.get(id) - 1;
+        if (count > 0) {
+          waits.set(id, count);
+          return;
+        }
+        waits.delete(id);
+        // Play goes on from where it held, not from where it would have been.
+        if (!buffering()) anchorMono = monotonicNow();
+      };
+    },
+    /**
+     * Keep continuous play from passing `ms` (null lifts it): the time layer
+     * `id` shows next, which it has yet to load. Play stops there, to wait on
+     * the load as it starts, rather than rush past frames between ticks.
+     */
+    gate(id, ms) {
+      settle();
+      if (ms === null || ms === undefined) gates.delete(id);
+      else if (isWorldTime(ms)) gates.set(id, ms);
+    },
     /** Simulated ms per real ms for continuous playback. */
     setRate(nextRate) {
       if (!Number.isFinite(nextRate) || nextRate <= 0)

src/time/worldClock.test.mjs

@@ -305,3 +305,85 @@ test('a coverage that throws counts as none', (t) => {
   assert.equal(clock.getState().supported, false);
   assert.equal(clock.coverage(), null);
 });
+
+test('continuous play runs past every driver, and live resets the rate', () => {
+  const t = fakeTime();
+  const clock = createWorldClock(t.options);
+  const calls = [];
+  clock.attachDriver({ play: () => calls.push('play'), pause() {} });
+  clock.seek(t.wall - 60_000);
+  clock.play({ rate: 30, continuous: true });
+  assert.deepEqual(calls, []);
+  t.advance(1_000);
+  assert.equal(clock.now(), t.wall - 61_000 + 30_000);
+  clock.goLive();
+  assert.equal(clock.getRate(), 1);
+});
+
+test('continuous play holds while a layer waits, and goes on from there', () => {
+  const t = fakeTime();
+  const clock = createWorldClock(t.options);
+  const start = t.wall - 600_000;
+  clock.seek(start);
+  clock.play({ rate: 60, continuous: true });
+  t.advance(1_000);
+  assert.equal(clock.now(), start + 60_000);
+  const release = clock.wait('radar');
+  const other = clock.wait('clouds');
+  assert.equal(clock.getState().buffering, true);
+  t.advance(5_000);
+  assert.equal(clock.now(), start + 60_000, 'held while loading');
+  release();
+  release();
+  t.advance(1_000);
+  assert.equal(clock.now(), start + 60_000, 'held until every layer is ready');
+  other();
+  assert.equal(clock.getState().buffering, false);
+  t.advance(1_000);
+  assert.equal(clock.now(), start + 120_000, 'on from where it held');
+});
+
+test('a seek still moves a waiting clock', () => {
+  const t = fakeTime();
+  const clock = createWorldClock(t.options);
+  clock.seek(t.wall - 600_000);
+  clock.play({ continuous: true });
+  const release = clock.wait('radar');
+  clock.seek(t.wall - 60_000);
+  assert.equal(clock.now(), t.wall - 60_000);
+  release();
+});
+
+test('playing, listeners hear of the time every animation frame', () => {
+  const t = fakeTime();
+  const frames = new Map();
+  let nextFrame = 1;
+  const clock = createWorldClock({
+    ...t.options,
+    requestFrame: (fn) => {
+      frames.set(nextFrame, fn);
+      return nextFrame++;
+    },
+    cancelFrame: (id) => frames.delete(id),
+  });
+  const heard = [];
+  clock.subscribe((state) => heard.push(state.timeMs));
+  const start = t.wall - 60_000;
+  clock.seek(start);
+  clock.play({ rate: 60 });
+  assert.equal(t.timers.size, 0, 'no ticks with frames to go by');
+  assert.equal(frames.size, 1);
+  const runFrame = () => {
+    const [[id, fn]] = frames;
+    frames.delete(id);
+    fn();
+  };
+  heard.length = 0;
+  for (let i = 0; i < 3; i++) {
+    t.advance(16);
+    runFrame();
+  }
+  assert.deepEqual(heard, [start + 960, start + 1_920, start + 2_880]);
+  clock.pause();
+  assert.equal(frames.size, 0);
+});

src/ui/stores/worldTime.js

@@ -8,10 +8,18 @@ const LIVE_TICK_MS = 1000;
 const LIVE_EDGE = 0.005;
 /** The least that counts as at now, past the readout's own second. */
 const LIVE_EDGE_MS = 5000;
+/** The speeds play offers, in world ms per real ms. */
+export const WORLD_TIME_SPEEDS = Object.freeze({
+  real: 1,
+  minute: 60,
+  hour: 3_600,
+  day: 86_400,
+  week: 604_800,
+});
 
 /**
- * The world clock as the timeline shows it: `{live, playing, timeMs,
+ * The world clock as the timeline shows it: `{live, playing, stepping,
- * supported, span}`, where `supported` says a layer that moves with the clock
+ * buffering, timeMs, supported, span}`, where `supported` says a layer that moves with the clock
  * has time to show and `span` is the stretch the timeline runs over: the
  * time those layers cover, reaching to now and to the world time, with their
  * marks. It follows the page's world clock, which tells it of every move, and
@@ -20,26 +28,68 @@ const LIVE_EDGE_MS = 5000;
  * track's end keep up with now.
  */
 export const useWorldTimeStore = defineStore('gev-world-time', () => {
-  const spanOf = (state) => {
+  // The time the layers cover, reaching to now.
-    if (!state.supported) return null;
+  const reach = () => {
     const coverage = worldClock.coverage();
     if (!coverage) return null;
     const now = worldClock.wallNow();
-    return Object.freeze({
+    return {
-      start: Math.min(coverage.start, now, state.timeMs),
+      start: Math.min(coverage.start, now),
-      end: Math.max(coverage.end, now, state.timeMs),
+      end: Math.max(coverage.end, now),
       marks: coverage.marks,
+    };
+  };
+  const spanOf = (state) => {
+    if (!state.supported) return null;
+    const covered = reach();
+    if (!covered) return null;
+    return Object.freeze({
+      start: Math.min(covered.start, state.timeMs),
+      end: Math.max(covered.end, state.timeMs),
+      marks: covered.marks,
     });
   };
+  const nearNow = (ms, length) =>
+    Math.abs(ms - worldClock.wallNow()) <=
+    Math.max(LIVE_EDGE_MS, length * LIVE_EDGE);
+  // Whether the track ends at now (and moves on with it), not past it.
+  const endsAtNow = (covered) =>
+    covered.end - worldClock.wallNow() <= LIVE_EDGE_MS;
+  // Play stops at the end of the track: at now it goes live, past now (a
+  // satellite's future) it holds there.
+  let stopping = false;
+  const stopAtEnd = (state) => {
+    if (stopping || !state.playing || state.live || !state.supported)
+      return false;
+    const covered = reach();
+    if (!covered || state.timeMs < covered.end) return false;
+    stopping = true;
+    try {
+      if (endsAtNow(covered)) worldClock.goLive();
+      else {
+        // Back to the end first, so nothing hears the overshoot as paused.
+        worldClock.seek(covered.end);
+        worldClock.pause();
+        worldClock.seek(covered.end);
+      }
+    } finally {
+      stopping = false;
+    }
+    return true;
+  };
   const read = (state = worldClock.getState()) =>
     Object.freeze({
       live: state.live,
       playing: state.playing,
+      stepping: state.stepping,
+      buffering: state.buffering,
       timeMs: state.timeMs,
       supported: state.supported,
       span: spanOf(state),
     });
   const time = shallowRef(read());
+  const speed = shallowRef('real');
+  const rate = () => WORLD_TIME_SPEEDS[speed.value];
   let ticker = null;
   const syncTicker = () => {
     // Playing, the clock tells of the time itself.
@@ -54,6 +104,7 @@ export const useWorldTimeStore = defineStore('gev-world-time', () => {
     }
   };
   const unsubscribe = worldClock.subscribe((state) => {
+    if (stopAtEnd(state)) return;
     time.value = read(state);
     syncTicker();
   });
@@ -66,13 +117,45 @@ export const useWorldTimeStore = defineStore('gev-world-time', () => {
 
   return {
     time,
-    togglePlay: () => worldClock.togglePlay(),
+    /**
+     * Play runs the track at the speed chosen, from the start when it is at
+     * its end (at live, say), as a video plays again from the top. It runs
+     * the clock itself, past weather's own replay: weather follows the time
+     * frame by frame like every other layer the track covers.
+     */
+    togglePlay() {
+      const { playing, live, timeMs, span } = time.value;
+      if (playing) return worldClock.pause();
+      const covered = reach();
+      if (span && covered && (live || timeMs >= covered.end))
+        worldClock.seek(span.start);
+      worldClock.play({ rate: rate(), continuous: true });
+    },
+    speed,
+    /**
+     * Choose how fast play runs; playing, it changes at once. Weather's own
+     * replay steps at its own pace, so a speed chosen while it plays takes
+     * playback over from where it is.
+     */
+    setSpeed(next) {
+      if (!Object.hasOwn(WORLD_TIME_SPEEDS, next)) return;
+      speed.value = next;
+      const { playing, stepping } = worldClock.getState();
+      if (!playing) return;
+      if (!stepping) return worldClock.setRate(rate());
+      worldClock.pause();
+      worldClock.play({ rate: rate(), continuous: true });
+    },
     goLive: () => worldClock.goLive(),
-    /** Scrub to `ms`: playback stops where the hand puts it; at now, live. */
+    /**
-    scrub(ms) {
+     * Scrub to `ms`: playback stops where the hand puts it; at now, live.
+     * How near now counts as now goes by `length`, the stretch the track
+     * shows, all of the span unless it is zoomed.
+     */
+    scrub(ms, length) {
       const span = time.value.span;
-      const edge = span ? (span.end - span.start) * LIVE_EDGE : 0;
+      length ??= span ? span.end - span.start : 0;
-      if (Math.abs(ms - worldClock.wallNow()) <= Math.max(LIVE_EDGE_MS, edge)) {
+      if (nearNow(ms, length)) {
         worldClock.goLive();
         return;
       }

src/ui/styles/world-timeline.css

@@ -5,8 +5,9 @@
   --world-timeline-lift: 0px;
 }
 
+/* The bar and the time's bubble over its handle. */
 body:has(#world-timeline) {
-  --world-timeline-lift: 50px;
+  --world-timeline-lift: 66px;
 }
 
 /* Clean view, recording and the cockpit put the dock away; the bar goes too. */
@@ -22,7 +23,10 @@ body.cockpit-mode #world-timeline {
   display: none;
 }
 
+/* Time runs left to right in every language, so the handle, the fill, the
+   ticks and the bubble agree under a right-to-left page too. */
 #world-timeline {
+  direction: ltr;
   position: fixed;
   bottom: 2vh;
   left: 50%;
@@ -77,32 +81,83 @@ body.cockpit-mode #world-timeline {
   text-transform: uppercase;
 }
 
-#world-timeline .world-timeline-time {
+#world-timeline .world-timeline-speed {
-  overflow: hidden;
+  flex: none;
-  font-family: var(--font-mono);
-  font-size: 13px;
-  font-variant-numeric: tabular-nums;
-  text-overflow: ellipsis;
-  white-space: nowrap;
 }
 
-#world-timeline .world-timeline-edge {
+/* The time rides over the handle. It slides across itself as the handle
-  flex: none;
+   crosses the track (its start at the handle at the track's start, its end at
-  color: var(--text-secondary);
+   the end), so however wide its date it stays within the track. */
+#world-timeline .world-timeline-time {
+  --inset: 0px;
+  position: absolute;
+  bottom: calc(100% + 6px);
+  left: calc(var(--inset) + (100% - var(--inset)) * var(--at));
+  padding: 2px 8px;
+  border: 1px solid rgba(0, 212, 255, 0.4);
+  border-radius: 6px;
+  background: var(--glass-bg);
+  color: var(--text-primary);
   font-family: var(--font-mono);
   font-size: 11px;
+  font-variant-numeric: tabular-nums;
   white-space: nowrap;
+  transform: translateX(calc(var(--at) * -100%));
+  pointer-events: none;
+}
+
+/* Playing, held while a layer loads the time's data: a spinner by the time. */
+#world-timeline .world-timeline-time.is-waiting {
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+}
+
+#world-timeline .world-timeline-spinner {
+  width: 10px;
+  height: 10px;
+  border: 2px solid rgba(255, 255, 255, 0.2);
+  border-top-color: var(--accent);
+  border-radius: 50%;
+  animation: world-timeline-spin 0.9s linear infinite;
+}
+
+@keyframes world-timeline-spin {
+  to {
+    transform: rotate(360deg);
+  }
+}
+
+@media (prefers-reduced-motion: reduce) {
+  #world-timeline .world-timeline-spinner {
+    animation: none;
+    border-color: var(--accent);
+  }
 }
 
 /* The track: ticks for each launch or frame, the elapsed fill, and a native
    range input on top for dragging and the keyboard. */
 #world-timeline .world-timeline-track {
   position: relative;
+  /* Two fingers pinch the track, not the page. */
+  touch-action: none;
   flex: 1 1 auto;
+  cursor: pointer;
   min-width: 80px;
   height: 24px;
 }
 
+/* Zoomed, the track's ends fade, as it goes on past them. */
+#world-timeline .world-timeline-track.is-zoomed::before {
+  background: linear-gradient(
+    90deg,
+    transparent,
+    rgba(123, 189, 211, 0.22) 12px,
+    rgba(123, 189, 211, 0.22) calc(100% - 12px),
+    transparent
+  );
+}
+
 #world-timeline .world-timeline-track::before {
   position: absolute;
   top: 50%;
@@ -139,6 +194,17 @@ body.cockpit-mode #world-timeline {
   pointer-events: none;
 }
 
+/* Zoomed, the track drags along. */
+#world-timeline .world-timeline-track.is-zoomed {
+  cursor: grab;
+}
+
+#world-timeline .world-timeline-track.is-zoomed:active {
+  cursor: grabbing;
+}
+
+/* Only the handle takes the pointer as a slider; the track around it takes
+   taps, drags and pinches itself. */
 #world-timeline .world-timeline-scrub {
   position: absolute;
   inset: 0;
@@ -146,8 +212,29 @@ body.cockpit-mode #world-timeline {
   height: 100%;
   margin: 0;
   background: transparent;
-  cursor: pointer;
   appearance: none;
+  pointer-events: none;
+}
+
+#world-timeline .world-timeline-scrub::-webkit-slider-thumb {
+  cursor: ew-resize;
+  pointer-events: auto;
+}
+
+#world-timeline .world-timeline-scrub::-moz-range-thumb {
+  cursor: ew-resize;
+  pointer-events: auto;
+}
+
+/* The time dragged off the track: the handle waits at its edge, unseen. */
+#world-timeline .world-timeline-scrub.is-off-view::-webkit-slider-thumb {
+  opacity: 0;
+  pointer-events: none;
+}
+
+#world-timeline .world-timeline-scrub.is-off-view::-moz-range-thumb {
+  opacity: 0;
+  pointer-events: none;
 }
 
 #world-timeline .world-timeline-scrub::-webkit-slider-runnable-track {
@@ -178,6 +265,20 @@ body.cockpit-mode #world-timeline {
   background: #031018;
 }
 
+/* A finger needs a bigger handle. */
+@media (pointer: coarse) {
+  #world-timeline .world-timeline-scrub::-webkit-slider-thumb {
+    width: 22px;
+    height: 22px;
+    margin-top: 1px;
+  }
+
+  #world-timeline .world-timeline-scrub::-moz-range-thumb {
+    width: 20px;
+    height: 20px;
+  }
+}
+
 #world-timeline .world-timeline-scrub:focus-visible {
   outline: 1px solid var(--accent);
   outline-offset: 2px;
@@ -190,7 +291,7 @@ body.cockpit-mode #world-timeline {
 }
 
 /* On phones the bar sits just above the tab bar, under the voice dock, with
-   the track on a row of its own beneath play, the time and live. */
+   the track and the speed on a row of their own beneath play and live. */
 body.mobile-shell:has(#world-timeline) {
   --world-timeline-lift: 80px;
 }
@@ -202,21 +303,23 @@ body.mobile-shell #world-timeline {
   height: auto;
 }
 
+/* The bubble rises into the first row, clear of play on its left. */
 body.mobile-shell #world-timeline .world-timeline-time {
-  flex: 1 1 0;
+  --inset: 48px;
 }
 
+/* The speed sits on the track's row, a full-size target beside it. */
 body.mobile-shell #world-timeline .world-timeline-track {
   order: 1;
-  flex-basis: 100%;
+  flex: 1 1 calc(100% - 96px);
 }
 
-@media (max-width: 640px) {
+body.mobile-shell #world-timeline .world-timeline-live {
-  #world-timeline .world-timeline-edge {
+  margin-inline-start: auto;
-    display: none;
-  }
-
-  #world-timeline .world-timeline-time {
-    font-size: 11px;
-  }
 }
+
+body.mobile-shell #world-timeline .world-timeline-speed {
+  order: 2;
+  height: 32px;
+}
+

src/ui/views/panels/WorldTimeline.test.mjs

@@ -148,3 +148,401 @@ test('the track runs over the time the layers show, to now, and scrubs', async (
   assert.equal(worldClock.isLive(), false, 'a scrub past now stays there');
   assert.equal(worldClock.now(), ahead);
 });
+
+test('play runs in real time, from the start of the track when at live', async (t) => {
+  const bar = mount(t);
+  const now = Date.now();
+  const start = now - 10 * 86_400_000;
+  const release = worldClock.hold('test-launches');
+  const unregister = worldClock.register({
+    id: 'test-launches',
+    coverage: () => ({ start, end: now - 86_400_000 }),
+  });
+  t.after(() => {
+    unregister();
+    release();
+  });
+  await nextTick();
+  // Weather's replay stepping its own frames does not take the track's play.
+  const stepped = [];
+  const driver = worldClock.attachDriver({
+    play: () => stepped.push('play'),
+    pause: () => stepped.push('pause'),
+  });
+  t.after(() => driver.detach());
+  bar().querySelector('.world-timeline-play').click();
+  worldClock.pause();
+  await nextTick();
+  assert.deepEqual(stepped, []);
+  const rate = worldClock.getRate();
+  assert.ok(worldClock.now() - start < rate * 1_000, 'from the start of the track');
+  assert.equal(rate, 1, 'real time to start with');
+  worldClock.play({ continuous: true });
+
+  // Playing on to now goes live.
+  worldClock.seek(Date.now() + 1_000);
+  await nextTick();
+  assert.equal(worldClock.isLive(), true);
+  assert.equal(worldClock.isPlaying(), false);
+  assert.equal(worldClock.getRate(), 1, 'live plays at the rate of the world');
+});
+
+test('play holds at the end of a track that reaches past now', async (t) => {
+  const bar = mount(t);
+  const now = Date.now();
+  // An hour ahead is within the track's share of now, but still ahead of it.
+  const end = now + 3_600_000;
+  const release = worldClock.hold('test-ahead');
+  const unregister = worldClock.register({
+    id: 'test-ahead',
+    coverage: () => ({ start: now - 10 * 86_400_000, end }),
+  });
+  t.after(() => {
+    unregister();
+    release();
+  });
+  await nextTick();
+  // Paused just past now, play plays on rather than from the top.
+  worldClock.seek(Date.now() + 1_000);
+  await nextTick();
+  const from = worldClock.now();
+  bar().querySelector('.world-timeline-play').click();
+  worldClock.pause();
+  assert.ok(worldClock.now() >= from, 'plays on from where it was');
+  worldClock.play();
+  worldClock.seek(end + 60_000);
+  await nextTick();
+  assert.equal(worldClock.isPlaying(), false);
+  assert.equal(worldClock.isLive(), false);
+  assert.equal(worldClock.now(), end);
+});
+
+test('the speed chosen sets how fast play runs, at once while playing', async (t) => {
+  const bar = mount(t);
+  const now = Date.now();
+  const release = worldClock.hold('test-launches');
+  const unregister = worldClock.register({
+    id: 'test-launches',
+    coverage: () => ({ start: now - 10 * 86_400_000, end: now - 86_400_000 }),
+  });
+  t.after(() => {
+    unregister();
+    release();
+  });
+  await nextTick();
+  const select = bar().querySelector('select.world-timeline-speed');
+  assert.equal(select.value, 'real');
+  assert.deepEqual(
+    [...select.options].map((option) => option.value),
+    ['real', 'minute', 'hour', 'day', 'week'],
+  );
+  select.value = 'week';
+  select.dispatchEvent(new Event('change'));
+  await nextTick();
+  bar().querySelector('.world-timeline-play').click();
+  assert.equal(worldClock.getRate(), 604_800, 'a week a second');
+  select.value = 'hour';
+  select.dispatchEvent(new Event('change'));
+  await nextTick();
+  assert.equal(worldClock.isPlaying(), true);
+  assert.equal(worldClock.getRate(), 3_600, 'changed while playing');
+  select.value = 'minute';
+  select.dispatchEvent(new Event('change'));
+  await nextTick();
+  assert.equal(worldClock.getRate(), 60, 'a minute a second');
+  select.value = 'real';
+  select.dispatchEvent(new Event('change'));
+  await nextTick();
+  assert.equal(worldClock.getRate(), 1, 'the world as it goes');
+});
+
+test('a speed chosen while weather replay steps takes playback over', async (t) => {
+  const bar = mount(t);
+  const at = Date.now() - 3_600_000;
+  supportingLayer(t, at);
+  const calls = [];
+  const driver = worldClock.attachDriver({
+    play() {
+      calls.push('play');
+      driver.driven(true, at - 600_000);
+    },
+    pause() {
+      calls.push('pause');
+      driver.driven(false);
+    },
+  });
+  t.after(() => driver.detach());
+  worldClock.play();
+  await nextTick();
+  assert.equal(worldClock.getState().stepping, true);
+  const select = bar().querySelector('select.world-timeline-speed');
+  select.value = 'hour';
+  select.dispatchEvent(new Event('change'));
+  await nextTick();
+  assert.deepEqual(calls, ['play', 'pause']);
+  assert.equal(worldClock.getState().stepping, false);
+  assert.equal(worldClock.isPlaying(), true);
+  assert.equal(worldClock.getRate(), 3_600);
+  assert.ok(worldClock.now() - (at - 600_000) < 3_600 * 1_000, 'on from where it was');
+  worldClock.pause();
+});
+
+test('paused just before the end, play carries on rather than restarting', async (t) => {
+  const bar = mount(t);
+  const now = Date.now();
+  const end = now + 86_400_000;
+  const release = worldClock.hold('test-ahead');
+  const unregister = worldClock.register({
+    id: 'test-ahead',
+    coverage: () => ({ start: now - 86_400_000, end }),
+  });
+  t.after(() => {
+    unregister();
+    release();
+  });
+  await nextTick();
+  worldClock.seek(end - 4_000);
+  await nextTick();
+  bar().querySelector('.world-timeline-play').click();
+  worldClock.pause();
+  assert.ok(worldClock.now() >= end - 4_000, 'not back to the start');
+});
+
+test('the track zooms around the handle, follows it, and shows it all again', async (t) => {
+  const bar = mount(t);
+  const now = Date.now();
+  const start = now - 26 * 86_400_000;
+  const release = worldClock.hold('test-launches');
+  const unregister = worldClock.register({
+    id: 'test-launches',
+    // A month of launches and three hours of lightning, sharing one track.
+    coverage: () => ({
+      start,
+      end: now,
+      marks: [start, now - 3 * 3_600_000, now - 2 * 3_600_000, now - 3_600_000],
+    }),
+  });
+  t.after(() => {
+    unregister();
+    release();
+  });
+  await nextTick();
+  const track = bar().querySelector('.world-timeline-track');
+  const input = bar().querySelector('input[type=range]');
+  const length = () => Number(input.max) - Number(input.min);
+  const full = length();
+  assert.equal(bar().querySelectorAll('.world-timeline-tick').length, 4);
+
+  // Live, the handle is at the end: zooming in keeps the view on now.
+  for (let i = 0; i < 10; i++) {
+    track.dispatchEvent(new WheelEvent('wheel', { deltaY: -200, cancelable: true }));
+    await nextTick();
+  }
+  assert.ok(length() < full / 100, 'zoomed in');
+  assert.ok(Number(input.max) >= now, 'still ending at now');
+  assert.equal(track.classList.contains('is-zoomed'), true);
+  assert.equal(bar().querySelectorAll('.world-timeline-tick').length, 3, 'only the ticks in view');
+
+  // A seek off the view takes the view along.
+  worldClock.seek(start + 86_400_000);
+  await nextTick();
+  assert.ok(Number(input.min) <= start + 86_400_000 && Number(input.max) >= start + 86_400_000);
+  assert.ok(length() < full / 100, 'at the same zoom');
+
+  // Minus widens, zero and a double-click show it all.
+  input.dispatchEvent(new KeyboardEvent('keydown', { key: '-' }));
+  await nextTick();
+  input.dispatchEvent(new KeyboardEvent('keydown', { key: '0' }));
+  await nextTick();
+  assert.ok(Math.abs(length() - full) < 1_000, 'all of it');
+  input.dispatchEvent(new KeyboardEvent('keydown', { key: '+' }));
+  await nextTick();
+  assert.ok(length() < full);
+  track.dispatchEvent(new MouseEvent('dblclick'));
+  await nextTick();
+  assert.equal(track.classList.contains('is-zoomed'), false);
+});
+
+test('zoomed, a scrub near now stays off live, and the view keeps a still handle', async (t) => {
+  const bar = mount(t);
+  const now = Date.now();
+  const start = now - 26 * 86_400_000;
+  let end = now;
+  const release = worldClock.hold('test-launches');
+  const unregister = worldClock.register({
+    id: 'test-launches',
+    coverage: () => ({ start, end, marks: [start, now - 3_600_000] }),
+  });
+  t.after(() => {
+    unregister();
+    release();
+  });
+  await nextTick();
+  const track = bar().querySelector('.world-timeline-track');
+  const input = bar().querySelector('input[type=range]');
+  // Ten notches of a line-mode wheel, as Firefox sends them.
+  for (let i = 0; i < 10; i++) {
+    track.dispatchEvent(new WheelEvent('wheel', { deltaY: -12, deltaMode: 1, cancelable: true }));
+    await nextTick();
+  }
+  assert.ok(Number(input.max) - Number(input.min) < 6 * 3_600_000, 'lines zoom like pixels');
+
+  // Twenty minutes back is near now on a month, not on a few hours.
+  input.value = String(now - 20 * 60_000);
+  input.dispatchEvent(new Event('input'));
+  await nextTick();
+  assert.equal(worldClock.isLive(), false);
+  assert.ok(Math.abs(worldClock.now() - (now - 20 * 60_000)) < 1_000);
+
+  // The span running on past the paused handle takes the view back to it.
+  end = now + 86_400_000;
+  worldClock.coverageChanged();
+  await nextTick();
+  const at = worldClock.now();
+  assert.ok(Number(input.min) <= at && Number(input.max) >= at, 'handle in view');
+
+  // With a modifier, plus is the browser's zoom.
+  const browserZoom = new KeyboardEvent('keydown', { key: '+', ctrlKey: true, cancelable: true });
+  input.dispatchEvent(browserZoom);
+  assert.equal(browserZoom.defaultPrevented, false);
+
+  // A double-click puts back the time its first press moved.
+  track.dispatchEvent(new PointerEvent('pointerdown', { bubbles: true, pointerId: 1 }));
+  track.dispatchEvent(new PointerEvent('pointerup', { bubbles: true, pointerId: 1 }));
+  input.value = String(at - 3_600_000);
+  input.dispatchEvent(new Event('input'));
+  track.dispatchEvent(new MouseEvent('dblclick', { bubbles: true }));
+  await nextTick();
+  assert.equal(track.classList.contains('is-zoomed'), false);
+  assert.ok(Math.abs(worldClock.now() - at) < 1_000, 'time as it was');
+});
+
+test('play held for a layer to load shows a spinner once the hold lasts', async (t) => {
+  const bar = mount(t);
+  const at = Date.UTC(2026, 9, 30, 6, 15, 42);
+  supportingLayer(t, at);
+  worldClock.seek(at - 1_800_000);
+  worldClock.play({ continuous: true });
+  await nextTick();
+  const bubble = () => bar().querySelector('.world-timeline-time');
+  const release = worldClock.wait('test-radar');
+  t.after(release);
+  // A clock tick (a quarter second) tells of the hold; the spinner waits on.
+  await new Promise((resolve) => setTimeout(resolve, 250));
+  await nextTick();
+  assert.equal(bubble().querySelector('.world-timeline-spinner'), null, 'not for a quick frame');
+  // The clock tells of the hold on its next tick, and the bar shows it after.
+  const until = async (shown) => {
+    for (let waited = 0; waited < 3_000; waited += 50) {
+      if (Boolean(bubble().querySelector('.world-timeline-spinner')) === shown) return true;
+      await new Promise((resolve) => setTimeout(resolve, 50));
+      await nextTick();
+    }
+    return false;
+  };
+  assert.ok(await until(true), 'spinner once the hold lasts');
+  assert.match(bubble().textContent, /UTC$/, 'the time stays');
+  release();
+  assert.ok(await until(false), 'gone once play goes on');
+});
+
+test('the track drags along, a tap goes to its time, and the handle waits off it', async (t) => {
+  const bar = mount(t);
+  const now = Date.now();
+  const start = now - 26 * 86_400_000;
+  const release = worldClock.hold('test-launches');
+  const unregister = worldClock.register({
+    id: 'test-launches',
+    coverage: () => ({ start, end: now, marks: [start] }),
+  });
+  t.after(() => {
+    unregister();
+    release();
+  });
+  await nextTick();
+  const track = bar().querySelector('.world-timeline-track');
+  const input = bar().querySelector('input[type=range]');
+  // A 1000 px track from x = 0.
+  Object.defineProperty(track, 'clientWidth', { value: 1_000 });
+  track.getBoundingClientRect = () => ({ left: 0, width: 1_000, top: 0, height: 24 });
+  const press = (type, clientX, pointerId = 1) =>
+    track.dispatchEvent(new PointerEvent(type, { bubbles: true, clientX, pointerId }));
+  const view = () => ({ min: Number(input.min), max: Number(input.max) });
+
+  // All of it shown, a tap goes to the time under it.
+  press('pointerdown', 500);
+  press('pointerup', 500);
+  await nextTick();
+  const middle = start + (now - start) / 2;
+  assert.ok(Math.abs(worldClock.now() - middle) < 60_000, 'went to the middle');
+
+  // Zoomed to a day around it, dragging right by half the track goes back half a day.
+  for (let i = 0; i < 6; i++) {
+    input.dispatchEvent(new KeyboardEvent('keydown', { key: '+' }));
+    await nextTick();
+  }
+  const before = view();
+  const day = before.max - before.min;
+  press('pointerdown', 600, 2);
+  press('pointermove', 700, 2);
+  press('pointermove', 1_100, 2);
+  press('pointerup', 1_100, 2);
+  await nextTick();
+  assert.ok(Math.abs(view().max - (before.max - day / 2)) < 1_000, 'dragged half a view');
+  assert.ok(Math.abs(worldClock.now() - middle) < 60_000, 'a drag leaves the time');
+
+  // Dragged off the handle, it waits unseen at the edge; a sideways swipe drags too.
+  track.dispatchEvent(new WheelEvent('wheel', { deltaX: -2_000, deltaY: 0, cancelable: true }));
+  await nextTick();
+  assert.ok(view().max < worldClock.now(), 'the handle is past the view');
+  assert.equal(input.classList.contains('is-off-view'), true);
+  // Moving the time takes the view back to it.
+  input.dispatchEvent(new KeyboardEvent('keydown', { key: 'PageUp' }));
+  await nextTick();
+  assert.equal(input.classList.contains('is-off-view'), false);
+});
+
+test('a pinch measures from where the fingers are, and a lifted finger ends it', async (t) => {
+  const bar = mount(t);
+  const now = Date.now();
+  const start = now - 26 * 86_400_000;
+  const release = worldClock.hold('test-launches');
+  const unregister = worldClock.register({
+    id: 'test-launches',
+    coverage: () => ({ start, end: now, marks: [start] }),
+  });
+  t.after(() => {
+    unregister();
+    release();
+  });
+  await nextTick();
+  const track = bar().querySelector('.world-timeline-track');
+  const input = bar().querySelector('input[type=range]');
+  Object.defineProperty(track, 'clientWidth', { value: 1_000 });
+  track.getBoundingClientRect = () => ({ left: 0, width: 1_000, top: 0, height: 24 });
+  const on = (target, type, clientX, pointerId) =>
+    target.dispatchEvent(new PointerEvent(type, { bubbles: true, clientX, pointerId }));
+  const length = () => Number(input.max) - Number(input.min);
+  for (let i = 0; i < 4; i++) {
+    input.dispatchEvent(new KeyboardEvent('keydown', { key: '+' }));
+    await nextTick();
+  }
+  const before = length();
+  // A finger on the handle moves before the second comes down.
+  on(input, 'pointerdown', 300, 1);
+  on(input, 'pointermove', 500, 1);
+  on(track, 'pointerdown', 700, 2);
+  on(input, 'pointermove', 500, 1);
+  await nextTick();
+  assert.ok(Math.abs(length() - before) < 1_000, 'no zoom from a stale finger');
+  on(track, 'pointermove', 900, 2);
+  await nextTick();
+  assert.ok(length() < before, 'spread apart, it zooms in');
+  // One finger up: the other moving does nothing, and throws nothing.
+  const zoomedTo = length();
+  on(track, 'pointerup', 900, 2);
+  on(input, 'pointermove', 100, 1);
+  await nextTick();
+  assert.ok(Math.abs(length() - zoomedTo) < 1_000);
+  on(input, 'pointercancel', 100, 1);
+});

src/ui/views/panels/WorldTimeline.vue

@@ -1,58 +1,316 @@
 <script setup>
-import { computed } from 'vue';
+import { computed, onBeforeUnmount, shallowRef, watch } from 'vue';
 import { storeToRefs } from 'pinia';
 import { formatDate, t } from '../../../i18n/index.js';
 import { useWorldTimeStore } from '../../stores/worldTime.js';
 import UiButton from '../../kit/UiButton.vue';
 import UiIcon from '../../kit/UiIcon.vue';
+import UiSelect from '../../kit/UiSelect.js';
 
-// The world timeline: play and pause on the left, the time, a track across
+// The world timeline: play and pause and the speed on the left, a track
-// the stretch the layers on can show (each launch or frame a tick) with a
+// across the stretch the layers on can show (each launch or frame a tick)
-// handle to scrub, and the way back to live on the right, lit while live. It
+// with a handle to scrub and the time in a bubble over it, and the way back
-// shows while a layer with a time of its own (weather, satellites, launches,
+// to live on the right, lit while live. Scrolling or pinching the track
+// zooms it around the handle, dragging it moves along it, and a double-click
+// shows it all again. It shows
+// while a layer with a time of its own (weather, satellites, launches,
 // or one a host follows) is on with time to show; with only live feeds such
 // as flights on, the bar stays hidden.
 const store = useWorldTimeStore();
-const { time } = storeToRefs(store);
+const { time, speed } = storeToRefs(store);
+const speeds = computed(() => [
+  { value: 'real', label: '1×' },
+  { value: 'minute', label: t('1 min/s') },
+  { value: 'hour', label: t('1 h/s') },
+  { value: 'day', label: t('1 d/s') },
+  { value: 'week', label: t('1 wk/s') },
+]);
 
 const DATE_TIME = Object.freeze({
   dateStyle: 'medium',
   timeStyle: 'medium',
   timeZone: 'UTC',
 });
-const EDGE_DATE = Object.freeze({
+// The bubble over the handle: the date and the minute, kept short.
+const BUBBLE_DATE = Object.freeze({
   month: 'short',
   day: 'numeric',
   timeZone: 'UTC',
 });
+const BUBBLE_TIME = Object.freeze({
+  hour: '2-digit',
+  minute: '2-digit',
+  hourCycle: 'h23',
+  timeZone: 'UTC',
+});
 // The keyboard's fallback step, as a share of the track, with no mark to go to.
 const KEY_STEP = 0.01;
 const PAGE_STEP = 0.1;
+// Zooming: the shortest stretch the track narrows to, the step a key or a
+// notch of the wheel takes, and how far a wheel's pixel of scroll zooms.
+const MIN_VIEW_MS = 5 * 60_000;
+const ZOOM_STEP = 1.5;
+const WHEEL_ZOOM = 0.0025;
+// Two presses this close are a double-click.
+const DOUBLE_PRESS_MS = 500;
 
 const readout = computed(() =>
   Number.isFinite(time.value.timeMs)
     ? `${formatDate(time.value.timeMs, DATE_TIME)} UTC`
     : '',
 );
+const bubble = computed(() =>
+  Number.isFinite(time.value.timeMs)
+    ? `${formatDate(time.value.timeMs, BUBBLE_DATE)} · ${formatDate(time.value.timeMs, BUBBLE_TIME)} UTC`
+    : '',
+);
+// Play held while a layer loads the time's data shows a spinner by the time,
+// once the hold lasts long enough to notice, so a quick frame does not flash
+// it, and until play has run a while, so a run of slow frames keeps it still.
+const WAIT_SHOWN_MS = 300;
+const WAIT_HIDDEN_MS = 500;
+const waiting = shallowRef(false);
+let waitTimer = null;
+watch(
+  () => time.value.buffering,
+  (buffering) => {
+    clearTimeout(waitTimer);
+    waitTimer = null;
+    if (buffering === waiting.value) return;
+    waitTimer = setTimeout(
+      () => {
+        waitTimer = null;
+        waiting.value = buffering;
+      },
+      buffering ? WAIT_SHOWN_MS : WAIT_HIDDEN_MS,
+    );
+  },
+);
+onBeforeUnmount(() => clearTimeout(waitTimer));
 const span = computed(() => time.value.span);
+// Where the handle is, or null with no track.
 const at = computed(() => {
+  if (!span.value) return null;
   const { start, end } = span.value;
   return Math.min(end, Math.max(start, time.value.timeMs));
 });
-const place = (ms) => {
+
+// The track shows the whole span, or zoomed, a stretch of it: `length` long,
+// ending at `end`, or at the span's end (and so at now) while `end` is null.
+// Everything still plays together; zooming only changes what the track shows.
+const zoom = shallowRef(null);
+const view = computed(() => {
+  if (!span.value) return null;
   const { start, end } = span.value;
+  const length = zoom.value?.length;
+  if (!length || length >= end - start) return { start, end };
+  const viewEnd = Math.min(end, Math.max(start + length, zoom.value.end ?? end));
+  return { start: viewEnd - length, end: viewEnd };
+});
+const zoomed = computed(() => zoom.value !== null);
+const viewLength = () => view.value.end - view.value.start;
+// Show `length` of the span ending at `viewEnd`, held to the span.
+function showView(length, viewEnd) {
+  const { start, end } = span.value;
+  if (length >= end - start) {
+    showWhole();
+    return;
+  }
+  const clamped = Math.min(end, Math.max(start + length, viewEnd));
+  zoom.value = { length, end: clamped >= end ? null : clamped };
+}
+// Narrow or widen the view to `length`, keeping the handle where it is on it,
+// or with the handle off it, the middle.
+function zoomTo(length) {
+  const { start: from, end: to } = view.value;
+  const pivot = offView(at.value) ? (from + to) / 2 : at.value;
+  const share = (pivot - from) / (to - from);
+  const next = Math.max(MIN_VIEW_MS, length);
+  showView(next, pivot + (1 - share) * next);
+}
+const zoomBy = (factor) => zoomTo(viewLength() * factor);
+// A view dragged off the handle stays where it was put while the handle only
+// creeps on with now; playing or a move of the handle takes it back.
+let panned = false;
+const follow = (ms) => {
+  panned = false;
+  showView(zoom.value.length, ms + zoom.value.length / 2);
+};
+const offView = (ms) => ms < view.value.start || ms > view.value.end;
+const showWhole = () => {
+  zoom.value = null;
+  panned = false;
+};
+// The span narrowing to the zoom or less, or going, lets go of the zoom.
+watch(span, (next) => {
+  if (zoom.value && (!next || zoom.value.length >= next.end - next.start))
+    showWhole();
+});
+watch(at, (ms) => {
+  if (!zoom.value || ms === null || !offView(ms)) return;
+  if (panned && time.value.live && !time.value.playing) return;
+  follow(ms);
+});
+// Left behind as now moves the view on, the handle takes the view back.
+watch(view, () => {
+  if (zoom.value && !panned && at.value !== null && offView(at.value))
+    follow(at.value);
+});
+const pan = (ms) => {
+  panned = true;
+  showView(zoom.value.length, view.value.end + ms);
+};
+// A wheel's lines and pages, in pixels.
+const WHEEL_UNITS = [1, 16, 400];
+const onWheel = (event) => {
+  const unit = WHEEL_UNITS[event.deltaMode] ?? 1;
+  const sideways = Math.abs(event.deltaX) > Math.abs(event.deltaY);
+  // A sideways swipe, or a wheel with shift, drags; a wheel zooms.
+  if (sideways || event.shiftKey) {
+    if (!zoom.value) return;
+    event.preventDefault();
+    const width = event.currentTarget.clientWidth;
+    const pixels = (sideways ? event.deltaX : event.deltaY) * unit;
+    if (width > 0) pan((pixels / width) * viewLength());
+    return;
+  }
+  event.preventDefault();
+  zoomBy(Math.exp(event.deltaY * unit * WHEEL_ZOOM));
+};
+// A double-click starts with presses that move the time, so it puts back the
+// time from before its first; so does a pinch, which may start on the handle.
+let before = null;
+let pressedAt = -Infinity;
+const onPress = (event) => {
+  if (event.timeStamp - pressedAt > DOUBLE_PRESS_MS)
+    before = { live: time.value.live, timeMs: time.value.timeMs };
+  pressedAt = event.timeStamp;
+};
+const restore = () => {
+  if (!before) return;
+  if (before.live) store.goLive();
+  else store.scrub(before.timeMs, 0);
+};
+const showAll = () => {
+  restore();
+  showWhole();
+};
+
+// The handle drags the time. The rest of the track takes the pointer: a tap
+// goes to the time under it, a drag drags a zoomed track along (or the time,
+// with all of it shown), and two fingers pinch it.
+const DRAG_PX = 4;
+const pointers = new Map();
+let gesture = null;
+const timeAt = (clientX, track) => {
+  const { left, width } = track.getBoundingClientRect();
+  const share = width > 0 ? Math.min(1, Math.max(0, (clientX - left) / width)) : 1;
+  return view.value.start + share * viewLength();
+};
+// Whether the first finger has moved the time, for a pinch to put it back.
+let movedTime = false;
+const onPointerDown = (event) => {
+  // Only the main button: a right-click is for the menu.
+  if (event.button > 0) return;
+  const onHandle = event.target instanceof HTMLInputElement;
+  pointers.set(event.pointerId, { x: event.clientX, onHandle });
+  if (pointers.size === 1) {
+    onPress(event);
+    movedTime = onHandle;
+    // The keys zoom and step the track as soon as it is touched.
+    if (!onHandle)
+      event.currentTarget.querySelector('input')?.focus({ preventScroll: true });
+  }
+  if (pointers.size === 2) {
+    const [a, b] = [...pointers.values()];
+    if (movedTime) restore();
+    gesture = {
+      kind: 'pinch',
+      spread: Math.abs(a.x - b.x),
+      length: viewLength(),
+    };
+    return;
+  }
+  if (onHandle || pointers.size !== 1) return;
+  // The drag goes on past the track's ends; without the capture, to them.
+  try {
+    event.currentTarget.setPointerCapture?.(event.pointerId);
+  } catch {
+    // A pointer the page no longer knows of drags no further than the track.
+  }
+  gesture = { kind: 'press', x: event.clientX, end: view.value.end };
+};
+const onPointerMove = (event) => {
+  const pointer = pointers.get(event.pointerId);
+  if (!pointer) return;
+  pointer.x = event.clientX;
+  if (!gesture) return;
+  const track = event.currentTarget;
+  if (gesture.kind === 'pinch') {
+    const [a, b] = [...pointers.values()];
+    const spreadNow = Math.abs(a.x - b.x);
+    if (spreadNow > 0) zoomTo((gesture.length * gesture.spread) / spreadNow);
+    return;
+  }
+  if (gesture.kind === 'press') {
+    if (Math.abs(event.clientX - gesture.x) < DRAG_PX) return;
+    gesture.kind = zoom.value ? 'pan' : 'scrub';
+    movedTime = gesture.kind === 'scrub';
+  }
+  if (gesture.kind === 'pan') {
+    if (!track.clientWidth) return;
+    panned = true;
+    const moved = ((gesture.x - event.clientX) / track.clientWidth) * viewLength();
+    showView(zoom.value.length, gesture.end + moved);
+  } else store.scrub(timeAt(event.clientX, track), viewLength());
+};
+const onPointerUp = (event) => {
+  if (!pointers.delete(event.pointerId)) return;
+  if (gesture?.kind === 'press' && event.type === 'pointerup')
+    store.scrub(timeAt(event.clientX, event.currentTarget), viewLength());
+  // A finger lifted from a pinch ends it; the other one moves nothing.
+  gesture = null;
+};
+
+const place = (ms) => {
+  const { start, end } = view.value;
   return end > start ? ((ms - start) / (end - start)) * 100 : 100;
 };
-const ticks = computed(() =>
+const ticks = computed(() => {
-  span.value.marks.map((mark) => `${place(mark)}%`),
+  const { start, end } = view.value;
+  return span.value.marks
+    .filter((mark) => mark >= start && mark <= end)
+    .map((mark) => `${place(mark)}%`);
+});
+const shown = computed(() =>
+  Math.min(view.value.end, Math.max(view.value.start, at.value)),
 );
-const scrub = (event) => store.scrub(Number(event.target.value));
+const handleOff = computed(() => at.value !== shown.value);
+const scrub = (event) => {
+  // A pinch only zooms; the input goes back to the handle.
+  if (gesture?.kind === 'pinch') {
+    event.target.value = shown.value;
+    return;
+  }
+  store.scrub(Number(event.target.value), viewLength());
+};
 // Arrows go to the previous or next mark (a launch, a frame), or a step along
-// the track; Page keys take a tenth of it; Home and End go to its ends.
+// the track as shown; Page keys take a tenth of it; Home and End go to the
+// ends of all of it. Plus and minus zoom, and zero shows it all; with a
+// modifier they are the browser's own zoom.
+const ZOOM_KEYS = ['+', '=', '-', '0'];
 const onKey = (event) => {
+  if (ZOOM_KEYS.includes(event.key)) {
+    if (event.ctrlKey || event.metaKey || event.altKey) return;
+    event.preventDefault();
+    if (event.key === '-') zoomBy(ZOOM_STEP);
+    else if (event.key === '0') showWhole();
+    else zoomBy(1 / ZOOM_STEP);
+    return;
+  }
   const { start, end, marks } = span.value;
   const now = at.value;
-  const length = end - start;
+  const length = viewLength();
   let to = null;
   if (event.key === 'ArrowLeft' || event.key === 'ArrowDown')
     to = marks.findLast((mark) => mark < now) ?? now - length * KEY_STEP;
@@ -64,7 +322,7 @@ const onKey = (event) => {
   else if (event.key === 'End') to = end;
   if (to === null) return;
   event.preventDefault();
-  store.scrub(Math.min(end, Math.max(start, to)));
+  store.scrub(Math.min(end, Math.max(start, to)), length);
 };
 </script>
 
@@ -72,24 +330,26 @@ const onKey = (event) => {
   <div v-if="time.supported && time.span" id="world-timeline" role="group" :aria-label="t('World time')"
     ><UiButton class="world-timeline-play" :label="time.playing ? t('Pause') : t('Play')" @click="store.togglePlay()"
       ><UiIcon :name="time.playing ? 'pause' : 'play_arrow'" :size="20" /></UiButton
-    ><time class="world-timeline-time" :datetime="new Date(time.timeMs).toISOString()" aria-live="off">{{ readout }}</time
+    ><UiSelect class="gev-field gev-field--compact world-timeline-speed" :aria-label="t('Play speed')" :title="t('Play speed')" :options="speeds" :model-value="speed" @update:model-value="store.setSpeed($event)"
-    ><span class="world-timeline-edge">{{ formatDate(span.start, EDGE_DATE) }}</span
+    /><div class="world-timeline-track" :class="{ 'is-zoomed': zoomed }" :title="t('Scroll or pinch to zoom, drag to move along · double-click to see it all')" @wheel="onWheel" @dblclick="showAll" @pointerdown="onPointerDown" @pointermove="onPointerMove" @pointerup="onPointerUp" @pointercancel="onPointerUp"
-    ><div class="world-timeline-track"
+      ><time class="world-timeline-time" :class="{ 'is-waiting': waiting }" :style="{ '--at': place(shown) / 100 }" :datetime="new Date(time.timeMs).toISOString()" aria-live="off"
+        ><span v-if="waiting" class="world-timeline-spinner" aria-hidden="true"></span>{{ bubble }}</time
       ><span v-for="(left, index) in ticks" :key="index" class="world-timeline-tick" :style="{ left }" aria-hidden="true"></span
-      ><span class="world-timeline-fill" :style="{ width: `${place(at)}%` }" aria-hidden="true"></span
+      ><span class="world-timeline-fill" :style="{ width: `${place(shown)}%` }" aria-hidden="true"></span
       ><input
         class="world-timeline-scrub"
+        :class="{ 'is-off-view': handleOff }"
         type="range"
         :aria-label="t('World time')"
         :aria-valuetext="readout"
-        :min="span.start"
+        :min="view.start"
-        :max="span.end"
+        :max="view.end"
         step="any"
-        :value="at"
+        :value="shown"
         @input="scrub"
+        aria-keyshortcuts="+ - 0"
         @keydown="onKey"
     /></div
-    ><span class="world-timeline-edge">{{ formatDate(span.end, EDGE_DATE) }}</span
     ><UiButton class="world-timeline-live" :label="time.live ? t('Live') : t('Back to live')" :pressed="time.live" pressed-class="is-live" :disabled="time.live" @click="store.goLive()">{{ t('Live') }}</UiButton
   ></div>
 </template>