fix(maps): make offline tiles cover real trips (cap coherence + zoom-clamp) (#1177)

Closes BLOCKER B5 — the offline map was blank for most real trips:

- The Workbox 'map-tiles' cache held only 1000 entries while the prefetcher
  budgeted ~3413, so prefetched tiles were evicted on arrival. Both caps are
  now a coherent 12288 (~180 MB), kept in sync with cross-referencing comments.
- prefetchTilesForTrip skipped a trip entirely when its all-zooms estimate
  exceeded the cap, so region/road-trip bboxes got no tiles. Removed the
  all-or-nothing guard; prefetchTiles already fills zooms low→high and stops at
  the budget, so large trips now cache the zooms that fit instead of nothing.

client/src/sync/tilePrefetcher.ts

@@ -17,11 +17,18 @@ import { offlineDb, upsertSyncMeta } from '../db/offlineDb'
 
 // ── Constants ─────────────────────────────────────────────────────────────────
 
-/** Estimated average tile size in KB (road/transit tiles ~15 KB). */
+/** Estimated average tile size in KB (raster basemap tiles ~15 KB). */
 const AVG_TILE_KB = 15
 
-/** Hard cap: ~50 MB worth of tiles. */
-export const MAX_TILES = Math.floor((50 * 1024) / AVG_TILE_KB) // ≈ 3413
+/**
+ * Hard cap on prefetched tiles (~180 MB).
+ *
+ * MUST stay in sync with the Workbox 'map-tiles' `maxEntries` in
+ * client/vite.config.js (kept equal). If this budget exceeds the SW cache size,
+ * the LRU evicts freshly-prefetched tiles on arrival and the offline map goes
+ * blank — which is exactly the bug this value was raised (from ~3413) to fix.
+ */
+export const MAX_TILES = Math.floor((180 * 1024) / AVG_TILE_KB) // = 12288
 
 const DEFAULT_TILE_URL =
   'https://{s}.basemaps.cartocdn.com/light_all/{z}/{x}/{y}{r}.png'
@@ -177,15 +184,13 @@ export async function prefetchTilesForTrip(
   const bbox = computeBbox(places)
   if (!bbox) return
 
-  // Size guard: if total tile count across all zooms exceeds cap, skip
-  const estimated = countTiles(bbox, 10, 16)
-  if (estimated > MAX_TILES) {
-    console.warn(
-      `[tilePrefetch] trip ${tripId}: estimated ${estimated} tiles exceeds cap (${MAX_TILES}), skipping`,
-    )
-    return
-  }
-
+  // Zoom-clamp rather than skip: prefetchTiles fills zooms low→high and stops
+  // once MAX_TILES is reached, so large (region / road-trip) bboxes still get
+  // their lower zooms cached instead of being skipped entirely.
+  //
+  // NOTE: opaque (no-cors) tile responses are padded by Chromium to ~7 MB each
+  // for quota accounting, so the real on-disk budget is far below 180 MB. That
+  // (audit H8) and navigator.storage.persist() (M6) are tracked separately.
   const fetched = await prefetchTiles(bbox, template)
 
   // Update syncMeta with bbox and tile count

client/tests/unit/sync/tilePrefetcher.test.ts

@@ -207,17 +207,42 @@ describe('prefetchTilesForTrip', () => {
     expect(meta!.tilesBbox).toHaveLength(4);
   });
 
-  it('skips prefetch when estimated tiles exceed MAX_TILES', async () => {
+  it('zoom-clamps instead of skipping when the bbox exceeds MAX_TILES', async () => {
     await upsertSyncMeta({ tripId: 1, lastSyncedAt: Date.now(), status: 'idle', tilesBbox: null, filesCachedCount: 0 });
 
-    // Places far apart → huge bbox → estimate > MAX_TILES
+    // ~4° road-trip span: low zooms fit the budget, high zooms (z14+) blow past
+    // it. The old guard skipped the whole trip; now we keep what fits.
     const places = [
-      buildPlace({ trip_id: 1, lat: -60, lng: -170 }),
-      buildPlace({ trip_id: 1, lat: 60, lng: 170 }),
+      buildPlace({ trip_id: 1, lat: 45.0, lng: 0.0 }),
+      buildPlace({ trip_id: 1, lat: 49.0, lng: 4.0 }),
     ];
     await prefetchTilesForTrip(1, places, 'https://{s}.example.com/{z}/{x}/{y}.png');
 
-    // No fetches should have been made
-    expect(vi.mocked(fetch)).not.toHaveBeenCalled();
+    // Previously this skipped entirely; now it prefetches a clamped subset.
+    const calls = vi.mocked(fetch).mock.calls.length;
+    expect(calls).toBeGreaterThan(0);
+    expect(calls).toBeLessThanOrEqual(MAX_TILES);
+  });
+
+  it('prefetches a region-sized (0.5°) trip that the old all-or-nothing guard would have skipped', async () => {
+    await upsertSyncMeta({ tripId: 1, lastSyncedAt: Date.now(), status: 'idle', tilesBbox: null, filesCachedCount: 0 });
+
+    const places = [
+      buildPlace({ trip_id: 1, lat: 48.6, lng: 2.1 }),
+      buildPlace({ trip_id: 1, lat: 49.1, lng: 2.6 }),
+    ];
+    await prefetchTilesForTrip(1, places, 'https://{s}.example.com/{z}/{x}/{y}.png');
+
+    const calls = vi.mocked(fetch).mock.calls.length;
+    expect(calls).toBeGreaterThan(0);
+    expect(calls).toBeLessThanOrEqual(MAX_TILES);
+  });
+});
+
+// ── cap coherence ───────────────────────────────────────────────────────────────
+
+describe('MAX_TILES budget', () => {
+  it('matches the Workbox map-tiles maxEntries in vite.config.js (drift guard)', () => {
+    expect(MAX_TILES).toBe(12288);
   });
 });

client/vite.config.js

@@ -15,21 +15,25 @@ export default defineConfig({
         runtimeCaching: [
           {
             // Carto map tiles (default provider)
+            // maxEntries MUST stay >= MAX_TILES in src/sync/tilePrefetcher.ts
+            // (both are 12288) so prefetched tiles aren't evicted on arrival.
             urlPattern: /^https:\/\/[a-d]\.basemaps\.cartocdn\.com\/.*/i,
             handler: 'CacheFirst',
             options: {
               cacheName: 'map-tiles',
-              expiration: { maxEntries: 1000, maxAgeSeconds: 30 * 24 * 60 * 60 },
+              expiration: { maxEntries: 12288, maxAgeSeconds: 30 * 24 * 60 * 60 },
               cacheableResponse: { statuses: [0, 200] },
             },
           },
           {
             // OpenStreetMap tiles (fallback / alternative)
+            // Shares the 'map-tiles' cache; keep maxEntries equal to the Carto
+            // rule above and MAX_TILES in src/sync/tilePrefetcher.ts (12288).
             urlPattern: /^https:\/\/[a-c]\.tile\.openstreetmap\.org\/.*/i,
             handler: 'CacheFirst',
             options: {
               cacheName: 'map-tiles',
-              expiration: { maxEntries: 1000, maxAgeSeconds: 30 * 24 * 60 * 60 },
+              expiration: { maxEntries: 12288, maxAgeSeconds: 30 * 24 * 60 * 60 },
               cacheableResponse: { statuses: [0, 200] },
             },
           },