feat(ui): implement message queuing during streaming responses (#6049)

Co-authored-by: Jacob Richman <jacob314@gmail.com>

packages/cli/src/ui/hooks/useMessageQueue.ts

@@ -0,0 +1,69 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { useCallback, useEffect, useState } from 'react';
+import { StreamingState } from '../types.js';
+
+export interface UseMessageQueueOptions {
+  streamingState: StreamingState;
+  submitQuery: (query: string) => void;
+}
+
+export interface UseMessageQueueReturn {
+  messageQueue: string[];
+  addMessage: (message: string) => void;
+  clearQueue: () => void;
+  getQueuedMessagesText: () => string;
+}
+
+/**
+ * Hook for managing message queuing during streaming responses.
+ * Allows users to queue messages while the AI is responding and automatically
+ * sends them when streaming completes.
+ */
+export function useMessageQueue({
+  streamingState,
+  submitQuery,
+}: UseMessageQueueOptions): UseMessageQueueReturn {
+  const [messageQueue, setMessageQueue] = useState<string[]>([]);
+
+  // Add a message to the queue
+  const addMessage = useCallback((message: string) => {
+    const trimmedMessage = message.trim();
+    if (trimmedMessage.length > 0) {
+      setMessageQueue((prev) => [...prev, trimmedMessage]);
+    }
+  }, []);
+
+  // Clear the entire queue
+  const clearQueue = useCallback(() => {
+    setMessageQueue([]);
+  }, []);
+
+  // Get all queued messages as a single text string
+  const getQueuedMessagesText = useCallback(() => {
+    if (messageQueue.length === 0) return '';
+    return messageQueue.join('\n\n');
+  }, [messageQueue]);
+
+  // Process queued messages when streaming becomes idle
+  useEffect(() => {
+    if (streamingState === StreamingState.Idle && messageQueue.length > 0) {
+      // Combine all messages with double newlines for clarity
+      const combinedMessage = messageQueue.join('\n\n');
+      // Clear the queue and submit
+      setMessageQueue([]);
+      submitQuery(combinedMessage);
+    }
+  }, [streamingState, messageQueue, submitQuery]);
+
+  return {
+    messageQueue,
+    addMessage,
+    clearQueue,
+    getQueuedMessagesText,
+  };
+}