⚡ Bolt: [performance improvement] optimize CodeBlock render via useMemo

.jules/bolt.md

@@ -5,3 +5,7 @@
 ## 2026-10-15 - [Performance] Prevent unnecessary UI array recalculations
 **Learning:** In the chat component (`useThreadList`), derived state that requires filtering and sorting large arrays (like generating nested agent lists) was running synchronously on every render. Given that chat polling triggers frequent re-renders, this caused unnecessary CPU strain on lists that only rarely change.
 **Action:** Use `useMemo` for derived states like `rows` and `current` in custom hooks when they involve `.filter()`, `.map()`, or `.sort()` and depend on data that updates less frequently than the component renders.
+
+## 2026-10-18 - [Performance] Optimize CodeBlock Token Concatenation
+**Learning:** In the `CodeBlock.tsx` component, we used nested `.map(...).join('')` calls to derive the `shown` token on every render. Just like with the `messagesSignature`, allocating new intermediate arrays and strings dynamically repeatedly creates garbage collection overhead during chat interactions.
+**Action:** Utilized standard React memoization (`useMemo`) to wrap the derivation logic. This prevents unnecessary string and array operations on every render without sacrificing code readability. When deriving values on a hot UI path, consider whether React `useMemo` is sufficient before replacing concise functional methods with verbose loops.

apps/web/src/components/chat/CodeBlock.tsx

@@ -1,4 +1,4 @@
-import { useEffect, useRef, useState } from 'preact/hooks';
+import { useEffect, useRef, useState, useMemo } from 'preact/hooks';
 import { Check, Copy } from 'lucide-preact';
 import { copyText } from '../../lib/clipboard';
 import { highlightCode, normalizeLang, type CodeToken } from '../../lib/highlight';
@@ -26,7 +26,11 @@ export function CodeBlock({ code, lang }: { code: string; lang?: string }) {
       window.clearTimeout(t);
     };
   }, [code, lang]);
-  const shown = tokens && tokens.map((l) => l.map((t) => t.content).join('')).join('\n') === code ? tokens : null;
+
+  // Memoize tokens comparison string to avoid recalculating the string flat on every render
+  const shown = useMemo(() => {
+    return tokens && tokens.map((l) => l.map((t) => t.content).join('')).join('\n') === code ? tokens : null;
+  }, [tokens, code]);
 
   async function onCopy() {
     const ok = await copyText(code);