在Chrome浏览器中添加以下代码，当检测到断开连接时，停止投屏并弹出相关提示：

var session = null;
var stopCasting = function() {
  if (session != null) {
    session.stop();
    session = null;
    alert('Chromecast已断开连接');
  }
};

var initializeCastApi = function() {
  var sessionRequest = new chrome.cast.SessionRequest(chrome.cast.media.DEFAULT_MEDIA_RECEIVER_APP_ID);
  var apiConfig = new chrome.cast.ApiConfig(sessionRequest, sessionListener, receiverListener);

  chrome.cast.initialize(apiConfig, onInitSuccess, onError);
};

var sessionListener = function(e) {
  session = e;
};

var receiverListener = function(e) {
  if( e === chrome.cast.ReceiverAvailability.AVAILABLE) {
    // Chromecast可用，可以投屏
  }
};

var onError = function(e) {
  console.log('Error', e);
};

var onInitSuccess = function() {
  // 初始化成功
};

document.addEventListener('chromecastAvailable', initializeCastApi);

window.addEventListener('beforeunload', stopCasting);
window.addEventListener('unload', stopCasting);