Qt 5.9 QML如何使用Flickable TextArea在材质样式中向上/向下显示ScrollBar

我想在 Flickable 中添加一些文本，也可以使用键向上和向下滚动 . 它可以工作，但滚动条在已经褪色后用键滚动时不会显示 . 鼠标滚轮滚动工作正常 .

-style universal 没有显示
-style material 没有显示
-style default 确实显示

这是我的代码：

import QtQuick 2.7
import QtQuick.Controls 2.2

ApplicationWindow
{
    visible: true
    width: 640
    height: 480

    Flickable
    {
        anchors.fill: parent
        flickableDirection: Flickable.VerticalFlick
        focus: true

        TextArea.flickable: TextArea 
        {
            text:

        "Lorem ipsum dolor sit amet, consectetur adipisicing elit, " +
        "sed do eiusmod tempor incididunt ut labore et dolore magna " +
        "aliqua. Ut enim ad minim veniam, quis nostrud exercitation " +
        "ullamco laboris nisi ut aliquip ex ea commodo cosnsequat. " +

        "Lorem ipsum dolor sit amet, consectetur adipisicing elit, " +
        "sed do eiusmod tempor incididunt ut labore et dolore magna " +
        "aliqua. Ut enim ad minim veniam, quis nostrud exercitation " +
        "ullamco laboris nisi ut aliquip ex ea commodo cosnsequat. " +

        "Lorem ipsum dolor sit amet, consectetur adipisicing elit, " +
        "sed do eiusmod tempor incididunt ut labore et dolore magna " +
        "aliqua. Ut enim ad minim veniam, quis nostrud exercitation " +
        "ullamco laboris nisi ut aliquip ex ea commodo cosnsequat. " +

        "Lorem ipsum dolor sit amet, consectetur adipisicing elit, " +
        "sed do eiusmod tempor incididunt ut labore et dolore magna " +
        "aliqua. Ut enim ad minim veniam, quis nostrud exercitation " +
        "ullamco laboris nisi ut aliquip ex ea commodo cosnsequat. " ;


            font.pixelSize: 25
            wrapMode: TextArea.Wrap
            readOnly: true
        }

        ScrollBar.vertical: ScrollBar { id: scrollBar }

        // up/down keys described here:
        // https://doc-snapshots.qt.io/qt5-5.8/qml-qtquick-controls2-scrollbar.html
        Keys.onUpPressed: scrollBar.decrease()
        Keys.onDownPressed: scrollBar.increase()

    }
}

谢谢你的帮助 .