Custom ComboBox not scrolling

[Saviz]

I used the QtQuick documentation to customize a ComboBox and successfully modified its colors. While the ComboBox generally works as expected, I encountered an issue when it contains a large number of items (around 35–50). The dropdown doesn't allow scrolling, and as a result, not all items are visible at once. I expected the ComboBox to handle this with scrolling but it doesn't. Could you help me identify the cause? Here's my code:

pragma ComponentBehavior: Bound

import QtQuick
import QtQuick.Controls.Basic
import QtQuick.Layouts

// Custom CPP Registered Types
import AppTheme 1.0

ComboBox {
    id: control

    property real dropDownTopMarign: 0

    opacity: enabled ? 1.0 : 0.5

    delegate: ItemDelegate {
        id: itemDelegate

        width: control.width
        height: 30

        required property var model
        required property int index

        contentItem: Text {
            leftPadding: 7

            text: itemDelegate.model[control.textRole]

            color: {
                if (itemDelegate.highlighted) {
                    Qt.color(AppTheme.colors["UFO_ComboBox_Item_Text_Highlighted"])
                }

                else {
                    Qt.color(AppTheme.colors["UFO_ComboBox_Item_Text_Normal"])
                }
            }

            font: control.font
            elide: Text.ElideRight
            verticalAlignment: Text.AlignVCenter
        }

        background: Rectangle {
            width: control.width

            radius: 0
            color: itemDelegate.highlighted ? Qt.color(AppTheme.colors["UFO_ComboBox_Item_Background_Highlighted"]) : Qt.color(AppTheme.colors["UFO_ComboBox_Item_Background_Normal"])
        }

        highlighted: control.highlightedIndex === index
    }

    contentItem: Text {
        leftPadding: 20
        rightPadding: control.indicator.width + control.spacing

        text: control.displayText
        font: control.font
        color: control.pressed ? Qt.color(AppTheme.colors["UFO_ComboBox_Text_Pressed"]) : Qt.color(AppTheme.colors["UFO_ComboBox_Text_Normal"])
        verticalAlignment: Text.AlignVCenter
        elide: Text.ElideRight
    }

    background: Rectangle {
        implicitWidth: 120
        implicitHeight: 40

        radius: 0
        color: Qt.color(AppTheme.colors["UFO_ComboBox_Background"])
        border.color: control.pressed ? Qt.color(AppTheme.colors["UFO_ComboBox_Border_Pressed"]) : Qt.color(AppTheme.colors["UFO_ComboBox_Border_Normal"])
        border.width: control.visualFocus ? 2 : 1
    }

    indicator: Canvas {
        id: canvas

        x: control.width - width - control.rightPadding
        y: control.topPadding + (control.availableHeight - height) / 2

        width: 12
        height: 8

        contextType: "2d"

        Connections {
            target: control

            function onPressedChanged() {
                canvas.requestPaint()
            }
        }

        onPaint: {
            context.reset()
            context.moveTo(0, 0)
            context.lineTo(width, 0)
            context.lineTo(width / 2, height)
            context.closePath()
            context.fillStyle = control.pressed ? Qt.color(AppTheme.colors["UFO_ComboBox_Arrow_Background_Pressed"]) : Qt.color(AppTheme.colors["UFO_ComboBox_Arrow_Background_Normal"])
            context.fill()
        }
    }

    popup: Popup {
        y: control.height -1

        width: control.width
        implicitHeight: contentItem.implicitHeight

        padding: 0

        contentItem: ListView {
            implicitHeight: contentHeight

            clip: true
            model: control.popup.visible ? control.delegateModel : null
            currentIndex: control.highlightedIndex

            ScrollIndicator.vertical: ScrollIndicator {}
        }

        background: Rectangle {
            radius: 0
            color: Qt.color(AppTheme.colors["UFO_ComboBox_Popup_Background"])
            border.color: Qt.color(AppTheme.colors["UFO_ComboBox_DropBox_Border"])
        }
    }
}