How to show large data set in qml ListView

Is it legal to write a list model which returns large data set for listview ?

Say a c++ model like:

int TestModel::rowCount(const QModelIndex &parent) const
{
    return 50000000;
}

QVariant TestModel::data(const QModelIndex &index, int role) const
{
    int row = index.row();
    if (role == 0x100)  return row;
    if (role == Qt::SizeHintRole) return QSize(300,30);

    return QString("hello %1").arg(row);
}

QHash<int, QByteArray> TestModel::roleNames() const
{
    return QHash<int, QByteArray>({
                                      {0x100, "line"},
                                      {0x101, "log"}
                                  });
}

Qml:

Item {
  Component {
    id: simple
    Rectangle{
      height: 10
      width: 10
      color: 'blue'
    }
  }

  ListView {
    id: viewer
    anchors.fill: parent
    model: testmodel
    delegate: simple
    ScrollBar.vertical: ScrollBar{
    }
  }
}

The memory, cpu, rendering...everything goes fine:

But, when scroll to about 3500000 lines:

Some space appear between few lines.

Is there a Qt bug, or I'm making some mistakes ?