perf: schedule pendingFrozenCheckRef only after confirming append-only with content changes

Agent-Logs-Url: https://github.com/microsoft/BotFramework-WebChat/sessions/d07bf01e-33c1-440a-a28e-095207d61357

Co-authored-by: OEvgeny <2841858+OEvgeny@users.noreply.github.com>

Author: Copilot

packages/api/src/providers/ActivityKeyer/ActivityKeyerComposer.tsx

@@ -86,11 +86,6 @@ const ActivityKeyerComposer = ({ children }: Readonly<{ children?: ReactNode | u
       commonPrefixLength++;
     }
 
-    // Schedule deferred verification of the frozen portion if any was skipped.
-    pendingFrozenCheckRef.current = frozenBoundary
-      ? Object.freeze({ current: activities, frozenBoundary, prev: prevActivities })
-      : undefined;
-
     const isAppendOnly = commonPrefixLength === prevActivities.length;
 
     if (isAppendOnly) {
@@ -102,6 +97,12 @@ const ActivityKeyerComposer = ({ children }: Readonly<{ children?: ReactNode | u
         return prevActivityKeysStateRef.current;
       }
 
+      // Schedule deferred verification of the frozen portion only now that we know:
+      // (1) the update is append-only and (2) there are actual content changes.
+      if (frozenBoundary) {
+        pendingFrozenCheckRef.current = Object.freeze({ current: activities, frozenBoundary, prev: prevActivities });
+      }
+
       const { current: activityIdToKeyMap } = activityIdToKeyMapRef;
       const { current: activityToKeyMap } = activityToKeyMapRef;
       const { current: clientActivityIdToKeyMap } = clientActivityIdToKeyMapRef;