GridLayout fails to position its children items

import QtQuick
import QtQuick.Controls
import QtQuick.Layouts

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("GridTest")

    property int gridModel: 0

    Timer {
        running: true
        repeat: false
        triggeredOnStart: false
        interval: 3000
        onTriggered: gridModel += 5
    }

    Flickable {
        anchors.fill: parent
        contentHeight: columnLayout.height

        ColumnLayout {
            id: columnLayout
            anchors.top: parent.top
            anchors.left: parent.left
            anchors.right: parent.right

            ProgressBar {
                Layout.fillWidth: true
                Layout.leftMargin: width / 4
                Layout.rightMargin: width / 4
            }

            GridLayout {
                Layout.fillWidth: true
                columns: 2
                visible: gridModel != 0

                Repeater {
                    model: gridModel

                    delegate: Loader {
                        sourceComponent: testElement
                        Layout.fillWidth: true
                    }
                }
            }
        }
    }

    Component {
        id: testElement
        Rectangle {
            implicitWidth: 300
            implicitHeight: width * 0.75
            color: "red"
            border.width: 2
        }
    }
}

ProgressBar {
    Layout.alignment: Qt.AlignHCenter
    Layout.preferredWidth: parent.width / 2
}