StackLayout does not set the correct size for a dynamically created element

import QtQuick 2.15
import QtQuick.Window 2.15
import QtQuick.Layouts 1.15
import QtQuick.Controls 2.12
Window {
 width: 800
 height: 600
visible: true
Component {
 id: stackItemComponent
Rectangle {
 color: Qt.rgba(Math.random(),Math.random(),Math.random(),1)
 Layout.fillWidth: true
 Layout.fillHeight: true
 Text {
 text: "Hello world"
 anchors.centerIn: parent
 }
 }
 }
MouseArea {
 anchors.fill: parent
onClicked: {
 var item = stackItemComponent.createObject(stackLayout)
 stackLayout.children.push(item)
 }
 }
StackLayout {
 id: stackLayout
 anchors.fill: parent
 }
}