fix(website): synchronize telemetry replay playback with recorded time (Phase 4)

The replay loop advanced the cursor by one sample every 50 ms / speed, assuming a uniform
20 Hz stream. Recorded telemetry is not always uniform (segment breaks and GPS-loss gaps up
to 5 s), so the rider marker drifted out of sync with the elapsed-time readout — which is
derived from real timestamps — and "1x" did not match real time across gaps.

Make playback timestamp-driven:
- Add advancePlaybackIndex(points, fromIndex, budgetMs): a pure helper that consumes a
  budget of virtual recorded time, advancing by the real delta between samples so the
  marker tracks the elapsed-time clock. Disconnected transitions (per
  areTelemetryPointsConnected) are compressed to a single 50 ms step so playback never
  stalls across multi-second dead time. No coordinates are altered.
- Drive the rAF loop with frameDelta * playbackSpeed plus carry-over, resuming from an
  index ref so manual scrubs are honored.

Verified sound without changes: SHA-256-checked R2/inline telemetry load, CSV/GPX parse and
export integrity, loading/error/empty overlays, and tile-provider fallback.

Tests: advancePlaybackIndex covers uniform cadence, carry-over, gap compression, end-clamp,
and empty input. Existing replay/export/corner tests remain green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

website/components/web-telemetry-replay-modal.tsx

@@ -12,7 +12,7 @@ import {
   analyzeCorners,
 } from '../lib/telemetry-export';
 import { firebaseAuth } from '../lib/firebase-client';
-import { areTelemetryPointsConnected, normalizeTelemetryPoints } from '../lib/telemetry-track';
+import { areTelemetryPointsConnected, normalizeTelemetryPoints, advancePlaybackIndex } from '../lib/telemetry-track';
 import { getAwsSatelliteTileUrl, AWS_MAPS_ATTRIBUTION } from '../lib/aws-maps';
 
 interface WebTelemetryReplayModalProps {
@@ -119,10 +119,18 @@ export function WebTelemetryReplayModal({ ride, onClose }: WebTelemetryReplayMod
 
   const animationFrameRef = useRef<number | null>(null);
   const lastTimeRef = useRef<number | null>(null);
+  const indexRef = useRef(0);
+  const playbackCarryRef = useRef(0);
 
   const totalPoints = points.length;
   const currentPoint = points[currentIndex] || points[0];
 
+  // Keep a ref mirror of the cursor so the rAF loop resumes from the latest position
+  // (including manual scrubs) without re-subscribing every index change.
+  useEffect(() => {
+    indexRef.current = currentIndex;
+  }, [currentIndex]);
+
   // Dynamically load Leaflet library and CSS with multi-CDN fallback
   useEffect(() => {
     if (typeof window === 'undefined') return;
@@ -611,30 +619,34 @@ export function WebTelemetryReplayModal({ ride, onClose }: WebTelemetryReplayMod
     }
   };
 
-  // Handle Playback Loop
+  // Handle Playback Loop — timestamp-driven so the rider marker stays synchronized with the
+  // elapsed-time readout. Advances by real recorded time scaled by playbackSpeed; disconnected
+  // gaps are compressed by advancePlaybackIndex so playback never stalls across dead time.
   useEffect(() => {
     if (!isPlaying) {
       if (animationFrameRef.current) cancelAnimationFrame(animationFrameRef.current);
       lastTimeRef.current = null;
+      playbackCarryRef.current = 0;
       return;
     }
 
     const animate = (timestamp: number) => {
-      if (!lastTimeRef.current) lastTimeRef.current = timestamp;
-      const delta = timestamp - lastTimeRef.current;
+      if (lastTimeRef.current == null) lastTimeRef.current = timestamp;
+      const frameDelta = timestamp - lastTimeRef.current;
+      lastTimeRef.current = timestamp;
 
-      // 20 Hz step interval is 50 ms. With playback multiplier:
-      const stepInterval = 50 / playbackSpeed;
+      // Virtual recorded-time budget to consume this frame, at the selected speed.
+      const budget = frameDelta * playbackSpeed + playbackCarryRef.current;
+      const { index, remainingMs } = advancePlaybackIndex(points, indexRef.current, budget);
+      playbackCarryRef.current = index >= totalPoints - 1 ? 0 : remainingMs;
 
-      if (delta >= stepInterval) {
-        lastTimeRef.current = timestamp;
-        setCurrentIndex((prev) => {
-          if (prev >= totalPoints - 1) {
-            setIsPlaying(false);
-            return prev;
-          }
-          return prev + 1;
-        });
+      if (index !== indexRef.current) {
+        indexRef.current = index;
+        setCurrentIndex(index);
+      }
+      if (index >= totalPoints - 1) {
+        setIsPlaying(false);
+        return;
       }
 
       animationFrameRef.current = requestAnimationFrame(animate);
@@ -645,7 +657,7 @@ export function WebTelemetryReplayModal({ ride, onClose }: WebTelemetryReplayMod
     return () => {
       if (animationFrameRef.current) cancelAnimationFrame(animationFrameRef.current);
     };
-  }, [isPlaying, playbackSpeed, totalPoints]);
+  }, [isPlaying, playbackSpeed, totalPoints, points]);
 
   // Keyboard navigation (Space to toggle play)
   useEffect(() => {

website/lib/telemetry-track.ts

@@ -44,6 +44,41 @@ export function areTelemetryPointsConnected(a: TelemetryPoint, b: TelemetryPoint
   return a.segmentId === b.segmentId && delta >= 0 && delta <= 5000;
 }
 
+/**
+ * Advances a replay cursor by a budget of virtual milliseconds, honoring the real time
+ * between recorded samples so the rider marker stays synchronized with the elapsed-time
+ * readout (which is derived from timestamps). Playback is therefore wall-clock accurate at
+ * 1x and correctly scaled at other speeds, even when samples are non-uniform.
+ *
+ * Disconnected transitions (segment breaks or GPS loss, per areTelemetryPointsConnected)
+ * are compressed to a single nominal 20 Hz step (50 ms) so playback never stalls across
+ * dead time instead of waiting out a multi-second gap. No coordinates are altered.
+ *
+ * Returns the new index and the leftover budget to carry into the next frame.
+ */
+export function advancePlaybackIndex(
+  points: TelemetryPoint[],
+  fromIndex: number,
+  budgetMs: number
+): { index: number; remainingMs: number } {
+  if (points.length === 0) return { index: 0, remainingMs: 0 };
+  let i = Math.max(0, Math.min(fromIndex, points.length - 1));
+  let budget = Math.max(0, budgetMs);
+  while (i < points.length - 1) {
+    const connected = areTelemetryPointsConnected(points[i], points[i + 1]);
+    const dt = connected
+      ? Math.max(0, Date.parse(points[i + 1].timestamp) - Date.parse(points[i].timestamp))
+      : 50;
+    if (budget >= dt) {
+      budget -= dt;
+      i++;
+    } else {
+      break;
+    }
+  }
+  return { index: i, remainingMs: i < points.length - 1 ? budget : 0 };
+}
+
 /** Leaflet's geographic API is [latitude, longitude], never normalized screen pixels. */
 export function geographicSegments(points: TelemetryPoint[]): [number, number][][] {
   const segments: [number,number][][] = [];

website/tests/telemetry-playback-sync.test.ts

@@ -0,0 +1,69 @@
+import { describe, it, expect } from 'vitest';
+import { advancePlaybackIndex } from '../lib/telemetry-track';
+import type { TelemetryPoint } from '../lib/telemetry-export';
+
+const BASE = Date.parse('2026-09-28T14:30:00.000Z');
+
+// Minimal TelemetryPoint builder — advancePlaybackIndex only reads timestamp + segmentId.
+function mk(offsetMs: number, segmentId = 0): TelemetryPoint {
+  return {
+    timestamp: new Date(BASE + offsetMs).toISOString(),
+    latitude: 40,
+    longitude: -74,
+    altitude: 0,
+    speed: 0,
+    heading: 0,
+    gpsAccuracy: 0,
+    leanAngle: 0,
+    pitchAngle: 0,
+    accelX: 0,
+    accelY: 0,
+    accelZ: 0,
+    longitudinalG: 0,
+    lateralG: 0,
+    segmentId,
+  };
+}
+
+describe('advancePlaybackIndex (replay playback synchronization)', () => {
+  const uniform = Array.from({ length: 10 }, (_, i) => mk(i * 50)); // 20 Hz, 50 ms apart
+
+  it('advances in step with real recorded time (1x)', () => {
+    // 200 ms of budget over 50 ms samples = 4 steps.
+    const r = advancePlaybackIndex(uniform, 0, 200);
+    expect(r.index).toBe(4);
+    expect(r.remainingMs).toBe(0);
+  });
+
+  it('carries leftover budget when it does not cover a full step', () => {
+    // 120 ms consumes two 50 ms steps (100 ms), leaving 20 ms.
+    const r = advancePlaybackIndex(uniform, 0, 120);
+    expect(r.index).toBe(2);
+    expect(r.remainingMs).toBe(20);
+  });
+
+  it('does not advance when the budget is below one step', () => {
+    const r = advancePlaybackIndex(uniform, 3, 10);
+    expect(r.index).toBe(3);
+    expect(r.remainingMs).toBe(10);
+  });
+
+  it('compresses a multi-second gap to a single 50 ms step so playback never stalls', () => {
+    // Samples 0..2 uniform; sample 3 is 10 s later (disconnected). A 60 ms budget from
+    // index 2 must still cross the gap (treated as a 50 ms step), not wait 10 s.
+    const gapped = [mk(0), mk(50), mk(100), mk(10_100)];
+    const r = advancePlaybackIndex(gapped, 2, 60);
+    expect(r.index).toBe(3);
+    expect(r.remainingMs).toBe(0); // reached the end
+  });
+
+  it('clamps at the final index with no remaining budget', () => {
+    const r = advancePlaybackIndex(uniform, 8, 100_000);
+    expect(r.index).toBe(9);
+    expect(r.remainingMs).toBe(0);
+  });
+
+  it('is a no-op on empty input', () => {
+    expect(advancePlaybackIndex([], 0, 1000)).toEqual({ index: 0, remainingMs: 0 });
+  });
+});