fix(frontend): isolate new chat thread messages (#3508)

* fix(frontend): isolate new chat thread messages

* fix(frontend): keep live messages visible in new chat

* fix(frontend): reset thread-local message refs

frontend/src/components/workspace/chats/use-thread-chat.ts

@@ -1,29 +1,44 @@
 "use client";
 
 import { useParams, usePathname, useSearchParams } from "next/navigation";
-import { useEffect, useState } from "react";
+import { useCallback, useEffect, useRef, useState } from "react";
 
 import { uuid } from "@/core/utils/uuid";
 
 export function useThreadChat() {
   const { thread_id: threadIdFromPath } = useParams<{ thread_id: string }>();
   const pathname = usePathname();
+  const actualPathname =
+    typeof window === "undefined" ? pathname : window.location.pathname;
+  const isNewPath = actualPathname.endsWith("/new");
+  const newThreadIdRef = useRef<string | null>(
+    threadIdFromPath === "new" ? uuid() : null,
+  );
+
+  if (isNewPath && !newThreadIdRef.current) {
+    newThreadIdRef.current = uuid();
+  }
 
   const searchParams = useSearchParams();
-  const [threadId, setThreadId] = useState(() => {
-    return threadIdFromPath === "new" ? uuid() : threadIdFromPath;
+  const [threadId, setThreadIdState] = useState(() => {
+    return threadIdFromPath === "new"
+      ? (newThreadIdRef.current ?? uuid())
+      : threadIdFromPath;
   });
 
-  const [isNewThread, setIsNewThread] = useState(
+  const [isNewThreadState, setIsNewThreadState] = useState(
     () => threadIdFromPath === "new",
   );
 
   useEffect(() => {
-    if (pathname.endsWith("/new")) {
-      setIsNewThread(true);
-      setThreadId(uuid());
+    if (isNewPath) {
+      const nextThreadId = newThreadIdRef.current ?? uuid();
+      newThreadIdRef.current = nextThreadId;
+      setIsNewThreadState(true);
+      setThreadIdState(nextThreadId);
       return;
     }
+    newThreadIdRef.current = null;
     // Guard: after history.replaceState updates the URL from /chats/new to
     // /chats/{UUID}, Next.js useParams may still return the stale "new" value
     // because replaceState does not trigger router updates.  Avoid propagating
@@ -32,9 +47,28 @@ export function useThreadChat() {
     if (threadIdFromPath === "new") {
       return;
     }
-    setIsNewThread(false);
-    setThreadId(threadIdFromPath);
-  }, [pathname, threadIdFromPath]);
+    setIsNewThreadState(false);
+    setThreadIdState(threadIdFromPath);
+  }, [isNewPath, threadIdFromPath]);
+
+  const setThreadId = useCallback((nextThreadId: string) => {
+    newThreadIdRef.current = null;
+    setThreadIdState(nextThreadId);
+  }, []);
+
+  const setIsNewThread = useCallback((nextIsNewThread: boolean) => {
+    if (!nextIsNewThread) {
+      newThreadIdRef.current = null;
+    }
+    setIsNewThreadState(nextIsNewThread);
+  }, []);
+
   const isMock = searchParams.get("mock") === "true";
-  return { threadId, setThreadId, isNewThread, setIsNewThread, isMock };
+  return {
+    threadId: isNewPath ? (newThreadIdRef.current ?? threadId) : threadId,
+    setThreadId,
+    isNewThread: isNewPath ? true : isNewThreadState,
+    setIsNewThread,
+    isMock,
+  };
 }