问题在于，Scrollbar的默认notificationPredicate定义为：

bool defaultScrollNotificationPredicate(ScrollNotification notification) {
  return notification.depth == 0;
}

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因此，由于第二个Scrollbar的深度是1（因为它是嵌套的），并且默认 predicate 只检查0，所以您看不到嵌套Scrollbar的拇指。
解决方案：当深度大于0时也会得到通知。
见备注：

@override
Widget build(BuildContext context) {
  /// The default 'notificationPredicate
  return Scrollbar(
    controller: _horizontalController,
    thumbVisibility: true,
    /// Nest the vertical 'Scrollbar' right below the horizontal one.
    child: Scrollbar(
      controller: _verticalController,
      thumbVisibility: true,
      /// Notification.depth == 1 here,
      /// but the default predicate checks whether Notification.depth == 0.
      notificationPredicate: (notification) => notification.depth >= 0,
      child: SingleChildScrollView(
        scrollDirection: Axis.horizontal,
        controller: _horizontalController,
        child: SingleChildScrollView(
          scrollDirection: Axis.vertical,
          controller: _verticalController,
          child: Container(
            width: 800.0,
            height: 1200.0,
            color: Colors.red,
          ),
        ),
      ),
    ),
  );
}

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