diff --git a/ui/dr-ui/src/derived_sync.rs b/ui/dr-ui/src/derived_sync.rs
index 165b589..12de793 100644
--- a/ui/dr-ui/src/derived_sync.rs
+++ b/ui/dr-ui/src/derived_sync.rs
@@ -53,6 +53,13 @@ pub struct SyncReport {
     pub catalog_uploaded: bool,
     pub catalog_merged: bool,
     pub collections_gained: usize,
+    /// Membership rows the merge brought in (FR-CAT-7).
+    ///
+    /// Counted apart from `collections_gained`, which counts the collections
+    /// themselves. A sync that files 127 photographs into a collection both
+    /// 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,
 }
 
 impl SyncReport {
@@ -334,6 +341,7 @@ async fn sync_catalog(
                     Ok(merge) => {
                         report.catalog_merged = true;
                         report.collections_gained = merge.inserted + merge.updated;
+                        report.members_gained = merge.members_added;
                     }
                     Err(e) => log::warn!("merging remote catalog: {e}"),
                 },
diff --git a/ui/dr-ui/src/library_ui.rs b/ui/dr-ui/src/library_ui.rs
index 06405dc..0e5fc37 100644
--- a/ui/dr-ui/src/library_ui.rs
+++ b/ui/dr-ui/src/library_ui.rs
@@ -3211,9 +3211,32 @@ fn start_derived_sync(window: &AppWindow, ctl: &Rc<LibraryController>) {
                         }
                         // Adopted thumbnails and merged collections both change
                         // what the grid should show.
-                        if report.thumbnails_adopted > 0 || report.collections_gained > 0 {
+                        if report.thumbnails_adopted > 0
+                            || report.collections_gained > 0
+                            || report.members_gained > 0
+                        {
                             load_window(&w, &ctl_cb);
                         }
+                        // TRACES: FR-CAT-7
+                        // And the sidebar, which the grid reload does not
+                        // touch. Membership counts as a change: a sync that
+                        // files 127 photographs into a collection both devices
+                        // already had gains no *collection*, so keying this on
+                        // `collections_gained` alone left the tree showing no
+                        // number beside a collection that had just been filled.
+                        //
+                        // The scan already does this after it finishes; the
+                        // sync merges the same tables and did not.
+                        if report.collections_gained > 0 || report.members_gained > 0 {
+                            let coll = ctl_cb.coll_ctl.borrow().as_ref().and_then(|w| w.upgrade());
+                            if let Some(coll) = coll {
+                                let borrow = ctl_cb.catalog();
+                                let borrow = borrow.borrow();
+                                if let Some(cat) = borrow.as_ref() {
+                                    crate::collections_ui::refresh_tree(&w, &coll, cat);
+                                }
+                            }
+                        }
                         stop(&ctl_cb.sync_timer);
                         return;
                     }