ComboBox Popup not showing all items

I am using Qt 5.9.1, and the following ComboBox's popup doesn't display all the elements:

import QtQuick 2.9import QtQuick.Window 2.2import QtQuick.Controls 2.2Window {    visible: true    width: 640    height: 480    title: qsTr("Hello World")    ComboBox{        id:comboNum        width:parent.width * 0.30        height:parent.height * 0.15        model: ["12","23","78","23","45","70"]        currentIndex: 0        popup: Popup{            id:popup            y: comboNum.height - 1            width: comboNum.width            height: comboNum.height * 2            padding: 1            contentItem: ListView {                id: listview                implicitHeight: popup.height                clip: true                model:comboNum.delegateModel                currentIndex: comboNum.highlightedIndex                interactive: true                highlightMoveDuration: 0                boundsBehavior: ListView.StopAtBounds                ScrollBar.vertical:ScrollBar {}            }        }    }}

Why is that, and how do I fix it?