ComboBox wrong touch behavior

import QtQuick 2.9
import QtQuick.Controls 2.2
import QtQuick.Layouts 1.3
import QtQuick.Controls.Material 2.2

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    ComboBox {
        id: myCombo
        anchors.top: parent.top
        anchors.left: parent.left
        anchors.topMargin: 48
        anchors.leftMargin: 48
        currentIndex: -1
        model: ["Alpha","Beta"]
        displayText: currentIndex < 0? "Select" : currentText
        // displayText: currentText
        onCurrentIndexChanged: {
            if(currentIndex >= 0) {
                console.log("something selected")
            }
        }
    }

    Button {
        anchors.centerIn: parent
        text: "the Button"
        onClicked: {
            console.log("Button clicked")
        }
    }

}