Take the server's shards and dates when the scan completes, not after the sweep

The derived sync fired only after the metadata sweep, so a fresh device
re-derived every thumbnail it scrolled past, re-detected faces and re-read
every header for hours before adopting the shards and snapshot that held
all of it. It now fires as soon as the scan completes — the first moment
the rows the merges key on exist — and the sweep starts behind it. In
steady state that pass is one listing.

The catalog merge gains a fourth half: capture metadata (captured_at,
offset, camera, lens, ISO) for images still at metadata_state < 2, matched
by oc:fileid from a remote row at 2. A date is a fact about the file's
bytes, not local state, and the snapshot already carried it. The sweep's
per-chunk query then finds nothing left, and the timeline is whole on a
fresh device without a header fetch.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/dr-ui/src/derived_sync.rs

@@ -67,6 +67,9 @@ pub struct SyncReport {
     /// devices already had gains *no* collection, and reporting only the
     /// former left the sidebar showing no count beside a full collection.
     pub members_gained: usize,
+    /// Images dated from the remote's snapshot rather than by this device's
+    /// own sweep — what makes the timeline whole on a fresh device.
+    pub dates_gained: usize,
 
     // Face data is counted apart from thumbnails for the same reason keywords
     // are counted apart from collections: "adopted 4,812 faces" is a sentence
@@ -815,6 +818,7 @@ fn merge_downloaded(
     report.catalog_merged = true;
     report.collections_gained += merge.inserted + merge.updated;
     report.members_gained += merge.members_added;
+    report.dates_gained += merge.metadata_adopted;
     Ok(())
 }
 

ui/dr-ui/src/library_ui.rs

@@ -1378,6 +1378,15 @@ fn drain_scan(
                                     }
                                 }
                                 load_window(&w, ctl);
+                                // Take the server's shards and catalog *now*,
+                                // before the sweep: the rows they key on exist
+                                // from this moment, and on a fresh device
+                                // every thumbnail, face and collection a peer
+                                // has already made is on the server. Waiting
+                                // for the sweep — hours on a large library —
+                                // meant re-deriving all of it here first. In
+                                // steady state this is one listing.
+                                start_derived_sync(&w, ctl);
                                 // Everything the grid did not touch: the rest
                                 // of the library gets a thumbnail and a date,
                                 // so the timeline describes all of it rather
@@ -4201,8 +4210,11 @@ fn spawn_scheduled_backup(ctl: &Rc<LibraryController>) {
 
 /// Push shards and the catalog to the server, and take what it has.
 ///
-/// Fired after the sweep completes, when there is a finished index worth
-/// sharing, and from the Sync button for an explicit exchange.
+/// Fired when the scan completes, so a fresh device inherits its peers'
+/// work before deriving any of its own; after the sweep completes, when
+/// there is a finished index worth sharing; and from the Sync button for an
+/// explicit exchange. A pass still running when the next trigger fires is
+/// left to finish — the guard below.
 fn start_derived_sync(window: &AppWindow, ctl: &Rc<LibraryController>) {
     // An escape hatch for running the app against a *copied* library without
     // touching the account's real server.
@@ -4326,7 +4338,7 @@ fn start_derived_sync(window: &AppWindow, ctl: &Rc<LibraryController>) {
                     crate::derived_sync::SyncMessage::Finished(report) => {
                         log::info!(
                             "sync: {} shard(s) up, {} down ({} thumbnails), \
-                             catalog {}{}",
+                             catalog {}{}{}",
                             report.shards_uploaded,
                             report.shards_downloaded,
                             report.thumbnails_adopted,
@@ -4343,6 +4355,11 @@ fn start_derived_sync(window: &AppWindow, ctl: &Rc<LibraryController>) {
                                 format!(", {} collection(s) gained", report.collections_gained)
                             } else {
                                 String::new()
+                            },
+                            if report.dates_gained > 0 {
+                                format!(", {} date(s) gained", report.dates_gained)
+                            } else {
+                                String::new()
                             }
                         );
                         w.set_library_syncing(false);
@@ -4359,13 +4376,22 @@ fn start_derived_sync(window: &AppWindow, ctl: &Rc<LibraryController>) {
                             w.set_library_status(format!("synced · {summary}").into());
                         }
                         // Adopted thumbnails and merged collections both change
-                        // what the grid should show.
+                        // what the grid should show; adopted dates change its
+                        // order, and the timeline beside it.
                         if report.thumbnails_adopted > 0
                             || report.collections_gained > 0
                             || report.members_gained > 0
+                            || report.dates_gained > 0
                         {
                             load_window(&w, &ctl_cb);
                         }
+                        if report.dates_gained > 0 {
+                            let borrow = ctl_cb.catalog();
+                            let borrow = borrow.borrow();
+                            if let Some(cat) = borrow.as_ref() {
+                                refresh_timeline(&w, cat, &ctl_cb);
+                            }
+                        }
                         // TRACES: FR-CAT-7
                         // And the sidebar, which the grid reload does not
                         // touch. Membership counts as a change: a sync that