fix(P0): 修复发送消息后主区域跳回欢迎屏

langgraph/src/main.tsx

@@ -98,7 +98,7 @@ function App() {
   }, []);
 
   // Clear error and stale stream state when thread switches.
-  // Skip when doSubmit already called switchThread manually (to avoid clearing mid-stream).
+  // Skip when doSubmit set the flag (new thread created during submit — don't clear mid-stream).
   useEffect(() => {
     if (skipNextSwitchRef.current) {
       skipNextSwitchRef.current = false;
@@ -217,12 +217,11 @@ function App() {
       setConcurrentStatus("idle");
       return;
     }
-    // Only need to switchThread manually when a new thread was just created.
-    // For existing threads, currentThreadId didn't change so useEffect won't fire.
-    // Setting the skip flag prevents the useEffect from re-firing and clearing messages.
+    // For a newly created thread, currentThreadId just changed so the useEffect
+    // would call thread.switchThread() which clears thread.messages mid-stream.
+    // Skip it — useStream already uses the updated threadId prop from currentThreadId.
     if (!currentThreadId) {
       skipNextSwitchRef.current = true;
-      thread.switchThread(activeThreadId);
     }
 
     try { localStorage.removeItem(`draft_${activeThreadId}`); } catch { /* ignore */ }