nextjs-app/components/ComparisonView.tsx

@@ -133,15 +133,16 @@ export function ComparisonView({
     // Guard against out-of-order responses: while the basket hydrates from
     // localStorage it can transiently hold a DIFFERENT school set than the
     // URL, firing a fetch for schools the user is no longer comparing. That
-    // stale response must not replace data for the current set — replacing
-    // it blanked every section until a hard refresh. We (a) drop responses
-    // from superseded effect runs and (b) merge rather than replace, so data
-    // for the current schools always survives.
+    // stale response must not replace data for the current set — it blanked
+    // every section until a hard refresh. Cleanup marks the run cancelled
+    // when urnKey moves on, so only the current selection's response is
+    // applied (replacing the map keeps it bounded and guarantees a re-added
+    // school is refetched fresh rather than served a lingering old entry).
     let cancelled = false;
     fetchComparison(urnKey, { cache: 'no-store' })
       .then((data) => {
         if (cancelled) return;
-        setComparisonData((prev) => ({ ...(prev ?? {}), ...data.comparison }));
+        setComparisonData(data.comparison);
         setNationalAverages(data.national_averages);
         setBenchmarks(data.benchmarks);
       })