fix(webui): bind landing first message to created chat

webui/src/components/thread/ThreadShell.tsx

@@ -339,6 +339,7 @@ export function ThreadShell({
   const filePreviewWidthRef = useRef(FILE_PREVIEW_DEFAULT_WIDTH);
   const filePreviewCloseTimerRef = useRef<number | null>(null);
   const pendingFirstRef = useRef<PendingFirstMessage | null>(null);
+  const [pendingFirstTargetChatId, setPendingFirstTargetChatId] = useState<string | null>(null);
   const viewportRef = useRef<ThreadViewportHandle | null>(null);
   const messageCacheRef = useRef<Map<string, UIMessage[]>>(new Map());
   /** Last chatId we associated with the in-memory thread (for cache-on-switch). */
@@ -555,15 +556,22 @@ export function ThreadShell({
     messageCacheRef.current.set(chatId, projectWebuiThreadMessages(messages));
   }, [chatId, loading, messages]);
 
+  // The landing composer queues the first message while `new_chat` is in flight.
+  // Only the chat created for that send may consume it; selecting another chat
+  // while creation is pending must not leak the message there.
   useEffect(() => {
-    if (!chatId) return;
+    if (!chatId || pendingFirstTargetChatId !== chatId) return;
     const pending = pendingFirstRef.current;
-    if (!pending) return;
+    if (!pending) {
+      setPendingFirstTargetChatId(null);
+      return;
+    }
     pendingFirstRef.current = null;
+    setPendingFirstTargetChatId(null);
     setScrollToLatestUserPromptSignal((value) => value + 1);
     send(pending.content, pending.images, pending.options);
     setBooting(false);
-  }, [chatId, send]);
+  }, [chatId, pendingFirstTargetChatId, send]);
 
   useEffect(() => {
     let cancelled = false;
@@ -585,11 +593,15 @@ export function ThreadShell({
       if (booting) return;
       setBooting(true);
       pendingFirstRef.current = { content, images, options: withWorkspaceScope(options) };
+      setPendingFirstTargetChatId(null);
       const newId = await onCreateChat?.(workspaceScope);
       if (!newId) {
         pendingFirstRef.current = null;
+        setPendingFirstTargetChatId(null);
         setBooting(false);
+        return;
       }
+      setPendingFirstTargetChatId(newId);
     },
     [booting, onCreateChat, withWorkspaceScope, workspaceScope],
   );