diff --git a/client/src/sync/tripSyncManager.ts b/client/src/sync/tripSyncManager.ts
index 5a9c6d716..6a953ed8d 100644
--- a/client/src/sync/tripSyncManager.ts
+++ b/client/src/sync/tripSyncManager.ts
@@ -56,6 +56,17 @@ function todayStr(): string {
   return new Date().toISOString().slice(0, 10)
 }
 
+/**
+ * Run work once the browser has nothing better to do, with a ceiling so it
+ * still happens on browsers without requestIdleCallback (Safari) or in a tab
+ * that never goes idle.
+ */
+function whenIdle(fn: () => void | Promise<void>): void {
+  const run = () => { void Promise.resolve(fn()).catch(console.error) }
+  if (typeof requestIdleCallback === 'function') requestIdleCallback(run, { timeout: 10_000 })
+  else setTimeout(run, 2_000)
+}
+
 function shouldCache(trip: Trip): boolean {
   if (!trip.end_date) return true            // no end date → cache forever
   return trip.end_date >= todayStr()          // ongoing or future
@@ -195,11 +206,20 @@ export const tripSyncManager = {
       for (const trip of toSync) {
         const files = await offlineDb.tripFiles.where('trip_id').equals(trip.id).toArray()
         cacheFilesForTrip(files).catch(console.error)
+      }
 
-        if (cacheTiles) {
-          const places = await offlineDb.places.where('trip_id').equals(trip.id).toArray()
-          prefetchTilesForTrip(trip.id, places, tileUrl).catch(console.error)
-        }
+      // Map tiles last, and only once the browser goes idle. syncAll runs right
+      // after login, where the app is still mounting the first screen — starting
+      // a bulk tile download into that leaves the UI waiting behind our own
+      // background traffic.
+      if (cacheTiles) {
+        whenIdle(async () => {
+          for (const trip of toSync) {
+            if (!isAuthed() || !navigator.onLine) return
+            const places = await offlineDb.places.where('trip_id').equals(trip.id).toArray()
+            await prefetchTilesForTrip(trip.id, places, tileUrl).catch(console.error)
+          }
+        })
       }
     } finally {
       _syncing = false
@@ -209,9 +229,9 @@ export const tripSyncManager = {
   /**
    * "Prepare for offline" (#1135 ask 1): a fully-awaited sync the user runs while
    * still online so everything they need is guaranteed on-device before they go
-   * offline. Unlike syncAll, this AWAITS file-blob and map-tile downloads and
-   * reports progress, so the UI can show a real completion state instead of
-   * resolving the moment the requests are merely dispatched.
+   * offline. Unlike syncAll it awaits the file-blob and map-tile downloads up
+   * front instead of deferring them to idle time, reports progress, and forces
+   * the tile prefetch to run even for a bbox we believe is already cached.
    *
    * Returns the number of trips prepared.
    */