FCM推送通知不显示横幅，仅显示在系统托盘上并发出声音

Question

我正在测试按照here（FCM消息传递的官方Github快速入门）的指导发送一些简单的通知。它确实已将通知正确发送到我的设备，即它确实发出了通知声音并显示在我的系统托盘中，但未显示通知横幅。我完全遵循了官方文档中提供的代码。供参考，这是我的sendNotification部分的代码：

private fun sendNotification(messageBody: String) {
    val intent = Intent(this, HomeActivity::class.java)
    intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP)
    val pendingIntent = PendingIntent.getActivity(this, 0 /* Request code */, intent,
        PendingIntent.FLAG_ONE_SHOT)

    val channelId = getString(R.string.default_notification_channel_id)
    val defaultSoundUri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION)
    val notificationBuilder = NotificationCompat.Builder(this, channelId)
        .setSmallIcon(R.drawable.ic_launcher_foreground)
        .setContentTitle(getString(R.string.fcm_message))
        .setContentText(messageBody)
        .setAutoCancel(true)
        .setSound(defaultSoundUri)
        .setContentIntent(pendingIntent)

    val notificationManager = getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager

    // Since android Oreo notification channel is needed.
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        val channel = NotificationChannel(channelId,
            "Channel human readable title",
            NotificationManager.IMPORTANCE_DEFAULT)
        notificationManager.createNotificationChannel(channel)
    }

    notificationManager.notify(0 /* ID of notification */, notificationBuilder.build())
}

如何显示通知横幅？