--- a/nix/rust-shell-client/src/theme_ui.rs
+++ b/nix/rust-shell-client/src/theme_ui.rs
@@ -1597,0 +1598,32 @@
+    fn prepare_ahead_latest_center_gets_its_full_dwell_before_neighbors_resume() {
+        let mut view = ThemeView::default();
+        view.page = ThemePage::List;
+        load_list(&mut view, 7, Some(3));
+        let queued = view.pending_neighbor_warms.clone();
+        assert_eq!(queued.len(), 2);
+        // Changes of direction during motion never spend or grow this queue.
+        for center in [6, 1, 5, 2, 4].into_iter().cycle().take(200) {
+            assert_eq!(view.poll_prepare_ahead_at_rest(1000, Some(center), false), None);
+            assert_eq!(view.pending_neighbor_warms, queued);
+        }
+        // A neighbor must not occupy the single prepared slot while the
+        // newly settled center is still earning its 250ms dwell.
+        assert_eq!(view.poll_prepare_ahead_at_rest(100, Some(6), true), None);
+        assert_eq!(view.pending_neighbor_warms, queued);
+        // A different settled center restarts that dwell, even without a
+        // movement poll between the two observations.
+        assert_eq!(view.poll_prepare_ahead_at_rest(100, Some(0), true), None);
+        assert_eq!(view.poll_prepare_ahead_at_rest(100, Some(0), true), None);
+        let (index, request) = view.poll_prepare_ahead_at_rest(50, Some(0), true).unwrap();
+        assert_eq!(index, 0);
+        assert_eq!(view.pending_neighbor_warms, queued);
+        view.prepare_ahead_submitted(index, 42);
+        assert_eq!(view.poll_prepare_ahead_at_rest(1000, Some(0), true), None);
+        assert!(view.prepare_ahead_reply(&ThemeReply {
+            id: 42, request, result: Err("fixture completion".into()),
+        }));
+        // Once the chosen center has been handled, neighbors may resume.
+        assert_eq!(view.poll_prepare_ahead_at_rest(16, Some(0), true).unwrap().0, 2);
+    }
+
+    #[test]