判断某个窗口是否是活动窗口（当前正在使用、操作的窗口）

    // 各种浏览器兼容
var hidden, state, visibilityChange;
if (typeof document.hidden !== "undefined") {
  hidden = "hidden";
  visibilityChange = "visibilitychange";
  state = "visibilityState";
} else if (typeof document.mozHidden !== "undefined") {
  hidden = "mozHidden";
  visibilityChange = "mozvisibilitychange";
  state = "mozVisibilityState";
} else if (typeof document.msHidden !== "undefined") {
  hidden = "msHidden";
  visibilityChange = "msvisibilitychange";
  state = "msVisibilityState";
} else if (typeof document.webkitHidden !== "undefined") {
  hidden = "webkitHidden";
  visibilityChange = "webkitvisibilitychange";
  state = "webkitVisibilityState";
}

// 添加监听器，在title里显示状态变化
document.addEventListener(visibilityChange, function() {
  document.title = document[state];
}, false);

// 初始化
document.title = document[state];