fix(FN-912): move scrollback buffer reset to WebSocket onopen handler

Move initialBufferRef reset from the beginning of connect() to inside
ws.onopen to ensure the buffer is fresh only when the new connection is
established. This prevents a race condition where the buffer could be
cleared before the WebSocket is ready, causing scrollback data to be
lost when terminal tabs are switched.

packages/dashboard/app/hooks/useTerminal.ts

@@ -192,8 +192,6 @@ export function useTerminal(sessionId: string | null): UseTerminalReturn {
     }
 
     isManualCloseRef.current = false;
-    // Reset buffer for new connection — previous session's data is stale
-    initialBufferRef.current = createEmptyBuffer();
     setConnectionStatus("connecting");
 
     // Build WebSocket URL
@@ -204,6 +202,10 @@ export function useTerminal(sessionId: string | null): UseTerminalReturn {
     wsRef.current = ws;
 
     ws.onopen = () => {
+      // Reset buffer ONLY when connection is established — ensures any
+      // late-arriving messages from a previous session are discarded and
+      // the new session's scrollback/data is captured in a fresh buffer.
+      initialBufferRef.current = createEmptyBuffer();
       setConnectionStatus("connected");
       reconnectAttemptsRef.current = 0;