perf(chat): memoize dedup map, O(1) tool-result comparator, TanStack Virtual, perf page & tests

Addresses review findings from PR CopilotKit#3611:
- toolResultMap: pre-compute id→content Map via useMemo so MemoizedAssistantMessage
  comparator does O(k) lookups instead of O(n) linear scans
- resolveSlotComponent<T>() helper eliminates repeated slot-resolution boilerplate
- NODE_ENV gate on __perfMsgCount / __perfScrollEl (no production window pollution)
- hasScrolledToBottomRef → dep-based [shouldVirtualize, firstMessageId] in useLayoutEffect
- Merged duplicate virtual/flat render return paths; removed dead null-guards in comparator
- AssistantMessage type import + cast in fingerprint (CopilotChat.tsx)
- perf page: private → protected, lastScrollTop=NaN, STABLE_POLLS_REQUIRED constant,
  agent as any with version-mismatch comment (ag-ui 0.0.46 vs 0.0.48)
- snapshot test: act+tick flush instead of sentinel message (Observable subscription
  ends after RUN_FINISHED so new events are dropped)
- .bench.tsx extension comment: not picked up by vitest CI glob

Pre-existing test failures in use-human-in-the-loop.e2e.test.tsx (2 tests) and
@copilotkit/sqlite-runner + @copilotkit/runtime are on main; unrelated to this PR.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: marthakelly

examples/v2/react/demo/src/app/perf/page.tsx

@@ -31,8 +31,8 @@ const TEXTS = [
 // with a delay between chunks to simulate live streaming.
 // ---------------------------------------------------------------------------
 class HistoryAgent extends AbstractAgent {
-  private messageCount = 0;
-  private streamDelayMs = 0;
+  protected messageCount = 0;
+  protected streamDelayMs = 0;
 
   configure(count: number, streamDelayMs = 0) {
     this.messageCount = count;
@@ -259,10 +259,15 @@ export default function PerfPage() {
       //                        reaches `count`) AND for the scroll position to
       //                        stabilise (3 consecutive identical scrollTop readings),
       //                        which means StickToBottom's spring animation has settled.
+      // How many consecutive 50 ms polls with identical scrollTop = animation settled.
+      const STABLE_POLLS_REQUIRED = 3;
+
       let phase: "emit" | "render" = "emit";
       let emitMs = 0;
       let stableCount = 0;
-      let lastScrollTop = -1;
+      // NaN ensures we don't start counting stability until we observe at least
+      // one real scrollTop value (avoids false-positive when scrollEl is null).
+      let lastScrollTop = NaN;
 
       const poll = setInterval(() => {
         if (phase === "emit") {
@@ -289,7 +294,7 @@ export default function PerfPage() {
           lastScrollTop = currentScrollTop;
         }
 
-        if (stableCount >= 3) {
+        if (stableCount >= STABLE_POLLS_REQUIRED) {
           clearInterval(poll);
           const start =
             (window as any).__perfConnectStart ?? startTimeRef.current;
@@ -391,6 +396,7 @@ export default function PerfPage() {
       <div style={{ flex: 1, overflow: "hidden", position: "relative" }}>
         <CopilotKitProvider
           key={key}
+          // eslint-disable-next-line @typescript-eslint/no-explicit-any
           agents__unsafe_dev_only={{ default: agent as any }}
         >
           <RenderCounter

packages/react-core/src/v2/components/chat/CopilotChat.tsx

@@ -21,6 +21,7 @@ import {
 } from "../../providers/CopilotKitProvider";
 import { InlineFeatureWarning } from "../../components/license-warning-banner";
 import { AbstractAgent, HttpAgent } from "@ag-ui/client";
+import { type AssistantMessage } from "@ag-ui/core";
 import { renderSlot, SlotValue } from "../../lib/slots";
 import {
   transcribeAudio,
@@ -368,22 +369,27 @@ export function CopilotChat({
     : transcribeMode;
 
   // Memoize messages array - only create new reference when content actually changes
-  // (agent.messages is mutated in place, so we need a new reference for React to detect changes)
-
+  // (agent.messages is mutated in place, so we need a new reference for React to detect changes).
+  //
+  // Lightweight fingerprint instead of JSON.stringify(agent.messages):
+  //   - length + last-id catch new messages (O(1))
+  //   - last content covers text streaming (serializes only in-flight message)
+  //   - last tool-call args cover TOOL_CALL_CHUNK streaming
+  //
+  // Known limitation: same-length in-place mutations to non-last messages
+  // (e.g. two tool results arriving in the same tick) are not detected here.
+  // The MemoizedAssistantMessage comparator handles those cases via toolResultMap.
   const messages = useMemo(
     () => [...agent.messages],
-    // Lightweight fingerprint instead of JSON.stringify(agent.messages):
-    //   - length + last-id catch new messages (O(1))
-    //   - last content covers text streaming (serializes only in-flight message)
-    //   - last tool-call args cover TOOL_CALL_CHUNK streaming (arguments change
-    //     on the same message without touching content or length)
     // eslint-disable-next-line react-hooks/exhaustive-deps
     [
       agent.messages.length,
       agent.messages.at(-1)?.id ?? "",
       JSON.stringify(agent.messages.at(-1)?.content),
       JSON.stringify(
-        agent.messages.at(-1)?.toolCalls?.at(-1)?.function?.arguments,
+        (agent.messages.at(-1) as AssistantMessage | undefined)?.toolCalls?.at(
+          -1,
+        )?.function?.arguments,
       ),
     ],
   );