Material style ToolButton's checked state is only visible when hovered

import QtQuick 2.7
import QtQuick.Layouts 1.1
import QtQuick.Controls 2.1

ApplicationWindow {
    visible: true
    width: 640
    height: 480

    header: ToolBar {
        RowLayout {
            ToolButton {
                text: qsTr("B")
                hoverEnabled: true
                checkable: true
            }

            ToolButton {
                text: qsTr("I")
                hoverEnabled: true
                checkable: true
            }

            ToolButton {
                text: qsTr("U")
                hoverEnabled: true
                checkable: true
            }
        }
    }
}