ComboBox Popup not showing all items

Question

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

import QtQuick 2.9
import QtQuick.Window 2.2
import QtQuick.Controls 2.2

Window {
    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?

Answer 1

Looking at the customisation docs, I can see that it sets a model conditionally, based on whether or not the popup is visible. Doing the same for your snippet makes it work for me:

model: popup.visible ? comboNum.delegateModel : null

However, the Default style implementation of ComboBox doesn't do that, so I'm not sure why it's necessary in your case.