perf: reduce iterations in the fast path for the keyer composer updates (#5797)

* perf: reduce iterations in the fast path for the keyer composer updates

* Changelog

* 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>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: OEvgeny <2841858+OEvgeny@users.noreply.github.com>

Author: OEvgeny

CHANGELOG.md

@@ -341,7 +341,8 @@ Breaking changes in this release:
 - Improved adaptive cards rendering in copilot variant, in PR [#5682](https://github.com/microsoft/BotFramework-WebChat/pull/5682), by [@OEvgeny](https://github.com/OEvgeny)
 - Bumped to [`botframework-directlinejs@0.15.8`](https://www.npmjs.com/package/botframework-directlinejs/v/0.15.8) to include support for the new `streaming` property, by [@pranavjoshi001](https://github.com/pranavjoshi001), in PR [#5686](https://github.com/microsoft/BotFramework-WebChat/pull/5686)
 - Removed unused deps `simple-git`, by [@compulim](https://github.com/compulim), in PR [#5786](https://github.com/microsoft/BotFramework-WebChat/pull/5786)
-- Improved `ActivityKeyerComposer` performance for append scenarios by adding an incremental fast path that only processes newly-appended activities, in PR [#5790](https://github.com/microsoft/BotFramework-WebChat/pull/5790), by [@OEvgeny](https://github.com/OEvgeny)
+- Improved `ActivityKeyerComposer` performance for append scenarios by adding an incremental fast path that only processes newly-appended activities, in PR [#5790](https://github.com/microsoft/BotFramework-WebChat/pull/5790), in PR [#5797](https://github.com/microsoft/BotFramework-WebChat/pull/5797), by [@OEvgeny](https://github.com/OEvgeny)
+   - Added frozen window optimization to limit reference comparisons to the last 1,000 activities with deferred verification of the frozen portion, in PR [#5797](https://github.com/microsoft/BotFramework-WebChat/pull/5797), by [@OEvgeny](https://github.com/OEvgeny)
 - Bumped to [`adaptivecards@3.0.6`](https://www.npmjs.com/package/adaptivecards/v/3.0.6) in PR [#5800](https://github.com/microsoft/BotFramework-WebChat/pull/5800) by [@compulim](https://github.com/compulim)
 
 ### Deprecated

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

@@ -1,8 +1,9 @@
 import { getActivityLivestreamingMetadata, type WebChatActivity } from 'botframework-webchat-core';
-import React, { useCallback, useMemo, useRef, type ReactNode } from 'react';
+import React, { useCallback, useEffect, useMemo, useRef, type ReactNode } from 'react';
 
 import reduceIterable from '../../hooks/private/reduceIterable';
 import useActivities from '../../hooks/useActivities';
+import usePonyfill from '../Ponyfill/usePonyfill';
 import type { ActivityKeyerContextType } from './private/Context';
 import ActivityKeyerContext from './private/Context';
 import getActivityId from './private/getActivityId';
@@ -17,6 +18,12 @@ type ActivityToKeyMap = Map<WebChatActivity, string>;
 type ClientActivityIdToKeyMap = Map<string, string>;
 type KeyToActivitiesMap = Map<string, readonly WebChatActivity[]>;
 
+/** After this many ms of no activity changes, verify that the frozen portion was not modified. */
+const FROZEN_CHECK_TIMEOUT = 10_000;
+
+/** Only the last N activities are compared reference-by-reference on each render. */
+const MUTABLE_ACTIVITY_WINDOW = 1_000;
+
 /**
  * React context composer component to assign a perma-key to every activity.
  * This will support both `useGetActivityByKey` and `useGetKeyByActivity` custom hooks.
@@ -32,6 +39,7 @@ type KeyToActivitiesMap = Map<string, readonly WebChatActivity[]>;
  * Local key are only persisted in memory. On refresh, they will be a new random key.
  */
 const ActivityKeyerComposer = ({ children }: Readonly<{ children?: ReactNode | undefined }>) => {
+  const [{ cancelIdleCallback, clearTimeout, requestIdleCallback, setTimeout }] = usePonyfill();
   const existingContext = useActivityKeyerContext(false);
 
   if (existingContext) {
@@ -52,14 +60,25 @@ const ActivityKeyerComposer = ({ children }: Readonly<{ children?: ReactNode | u
   const prevActivityKeysStateRef = useRef<readonly [readonly string[]]>(
     Object.freeze([Object.freeze([])]) as readonly [readonly string[]]
   );
+  const pendingFrozenCheckRef = useRef<
+    | {
+        readonly current: readonly WebChatActivity[];
+        readonly frozenBoundary: number;
+        readonly prev: readonly WebChatActivity[];
+      }
+    | undefined
+  >();
+  const warnedPositionsRef = useRef<Set<number>>(new Set());
 
   // Incremental keying: the fast path only processes newly-appended activities (O(delta) per render)
   // instead of re-iterating all activities (O(n) per render, O(n²) total for n streaming pushes).
   const activityKeysState = useMemo<readonly [readonly string[]]>(() => {
     const prevActivities = prevActivitiesRef.current;
 
-    // Detect how many leading activities are identical (same reference) to the previous render.
-    let commonPrefixLength = 0;
+    // Only the last MUTABLE_ACTIVITY_WINDOW activities are compared each render.
+    // Activities before the frozen boundary are assumed unchanged — O(1) instead of O(n).
+    const frozenBoundary = Math.max(0, Math.min(prevActivities.length, activities.length) - MUTABLE_ACTIVITY_WINDOW);
+    let commonPrefixLength = frozenBoundary;
     const maxPrefix = Math.min(prevActivities.length, activities.length);
 
     // eslint-disable-next-line security/detect-object-injection
@@ -78,6 +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;
@@ -118,19 +143,15 @@ const ActivityKeyerComposer = ({ children }: Readonly<{ children?: ReactNode | u
 
       prevActivitiesRef.current = activities;
 
-      if (newKeys.length) {
-        const nextKeys = Object.freeze([...prevActivityKeysStateRef.current[0], ...newKeys]);
-        const result = Object.freeze([nextKeys]) as readonly [readonly string[]];
-
-        prevActivityKeysStateRef.current = result;
-
-        return result;
+      if (!newKeys.length) {
+        // New activities might be added to existing keys — no new keys, but the keyToActivitiesMap
+        // was mutated. Return a new tuple reference so context consumers re-render and see the
+        // updated activities-per-key via getActivitiesByKey.
+        return Object.freeze([prevActivityKeysStateRef.current[0]]) as readonly [readonly string[]];
       }
 
-      // New activities were added to existing keys — no new keys, but the keyToActivitiesMap
-      // was mutated. Return a new tuple reference so context consumers re-render and see the
-      // updated activities-per-key via getActivitiesByKey.
-      const result = Object.freeze([prevActivityKeysStateRef.current[0]]) as readonly [readonly string[]];
+      const nextKeys = Object.freeze([...prevActivityKeysStateRef.current[0], ...newKeys]);
+      const result = Object.freeze([nextKeys]) as readonly [readonly string[]];
 
       prevActivityKeysStateRef.current = result;
 
@@ -180,6 +201,10 @@ const ActivityKeyerComposer = ({ children }: Readonly<{ children?: ReactNode | u
     keyToActivitiesMapRef.current = nextKeyToActivitiesMap;
     prevActivitiesRef.current = activities;
 
+    // Slow path did a full recalculation — no frozen check needed, reset warnings.
+    pendingFrozenCheckRef.current = undefined;
+    warnedPositionsRef.current.clear();
+
     const nextKeys = Object.freeze([...nextActivityKeys.values()]);
     const result = Object.freeze([nextKeys]) as readonly [readonly string[]];
 
@@ -192,10 +217,53 @@ const ActivityKeyerComposer = ({ children }: Readonly<{ children?: ReactNode | u
     activityToKeyMapRef,
     clientActivityIdToKeyMapRef,
     keyToActivitiesMapRef,
+    pendingFrozenCheckRef,
     prevActivitiesRef,
-    prevActivityKeysStateRef
+    prevActivityKeysStateRef,
+    warnedPositionsRef
   ]);
 
+  // Deferred verification: after FROZEN_CHECK_TIMEOUT of quiet, validate that activities
+  // inside the frozen portion have not actually changed. Warn once per position if they did.
+  // Uses requestIdleCallback inside the timeout to avoid contending with the first post-stream repaint.
+  useEffect(() => {
+    const pending = pendingFrozenCheckRef.current;
+
+    if (!pending) {
+      return;
+    }
+
+    let idleHandle: ReturnType<NonNullable<typeof requestIdleCallback>> | undefined;
+
+    const runCheck = () => {
+      const { current: currentActivities, frozenBoundary, prev: prevFrozenActivities } = pending;
+
+      for (let i = 0; i < frozenBoundary; i++) {
+        // eslint-disable-next-line security/detect-object-injection
+        if (prevFrozenActivities[i] !== currentActivities[i] && !warnedPositionsRef.current.has(i)) {
+          warnedPositionsRef.current.add(i);
+
+          console.warn(
+            `botframework-webchat internal: change in activity at position ${i} was not applied because it is outside the mutable window of ${MUTABLE_ACTIVITY_WINDOW}.`
+          );
+        }
+      }
+    };
+
+    const timer = setTimeout(() => {
+      if (requestIdleCallback) {
+        idleHandle = requestIdleCallback(runCheck);
+      } else {
+        runCheck();
+      }
+    }, FROZEN_CHECK_TIMEOUT);
+
+    return () => {
+      clearTimeout(timer);
+      idleHandle !== undefined && cancelIdleCallback?.(idleHandle);
+    };
+  }, [activities, cancelIdleCallback, clearTimeout, requestIdleCallback, setTimeout]);
+
   const getActivitiesByKey: (key?: string | undefined) => readonly WebChatActivity[] | undefined = useCallback(
     (key?: string | undefined): readonly WebChatActivity[] | undefined => key && keyToActivitiesMapRef.current.get(key),
     [keyToActivitiesMapRef]