RootedLoader.h:

#ifndef ROOTEDLOADER_H
#define ROOTEDLOADER_H

#include <QQmlEngine>
#include <QQuickItem>

class RootedLoader : public QQuickItem
{
    Q_OBJECT
    Q_PROPERTY( QString root READ root WRITE set_root NOTIFY root_changed REQUIRED )
    Q_PROPERTY( QString location READ location WRITE set_location NOTIFY location_changed )
    QML_ELEMENT

protected:
    QString _root;
    QString _location;

public:

    RootedLoader( QQuickItem* parent = nullptr );

    QString location() const;
    void set_location( const QString& location );

    QString root() const;
    void set_root( const QString& root );

protected:
    virtual void componentComplete() override;

    void load();

signals:
    void root_changed( const QString& location );
    void location_changed( const QString& location );

};

#endif // ROOTEDLOADER_H```

RootedLoader.cpp:

#include "RootedLoader.h"
#include <QQmlContext>
#include <QQmlComponent>



#include "application_engine.h"

RootedLoader::RootedLoader( QQuickItem* parent ):
    QQuickItem( parent )
{
    setFlag( QQuickItem::ItemHasContents, true );
}

QString RootedLoader::location() const
{
    return _location;
}

void RootedLoader::set_location( const QString& location )
{
    _location = location;

    if( isComponentComplete() )
        load();
}

QString RootedLoader::root() const
{
    return _root;
}

void RootedLoader::set_root( const QString& root )
{
    _root = root;
}

void RootedLoader::componentComplete()
{
    load();
}

void RootedLoader::load()
{
    auto item_width = width();
    auto item_height = height();

    qDebug() << item_width;
    qDebug() << item_height;

//    auto url = QUrl( _root + _location );
    auto url_string = QString( "qrc:/law/lawtree/testpage.qml" );
    auto url = QUrl( url_string );

    QQmlComponent* location_component = new QQmlComponent( application_engine::engine(), url );
    QQuickItem* instance = qobject_cast<QQuickItem*>( location_component->create() );

    if( childItems().count() > 0 )
    {
        QQuickItem* item = this->childItems().at( 0 );

        item->setParentItem( NULL );
        instance->setParentItem( this );
        instance->setParent( application_engine::engine() );

        item->setParent( NULL );
        item->deleteLater();
    }
    else
    {
        instance->setParent( application_engine::engine() );
        instance->setParentItem( this );
    }
}

main.qml:

import QtQuick
import QtQuick.Layouts 1.15
import QtQuick.Controls 2.15

import CustomComponents 1.0
import "."

ApplicationWindow
{
    title: qsTr( "Law and Order" )
    width: 540
    height: 960
    visible: true

    StackLayout
    {
        id: view_stack
        anchors.fill: parent
        currentIndex: 1

        LawTree
        {
            id: law_tree
        }

        Rectangle
        {
            border.width: 4
            border.color: "blue"

            Column
            {
                anchors.fill: parent

                Text { text: "testing" }

//                TestQuickItem
//                {
//                    anchors.centerIn: parent
//                    width: parent.width/2
//                    height: parent.height/2
//                }

// note that TestQuickItem displays without a problem using updatePaintNode to paint a triangle 

                RootedLoader
                {
                    id: loader
                    root: "qrc:/law/lawtree"
                    location: "/testpage.qml"
                    anchors.fill: parent
                    width: 200
                    height: 400
                }
            }
        }

    }

}

testpage.qml (loaded from RootedLoader):

Item
{
    id: view

    property string law_description : "law description ****"
    property string order_description : "order description ****"
    property string coordination_description : "coordination description ****"
    property string preawareness_description : "preawareness description ****"

    anchors.fill: parent

    Rectangle
    {
        anchors.fill: parent
        border.color: "red"
        border.width: 4

        Column
        {
            anchors.fill: parent

            Row
            {
                Text { text: "law:" }

                Text {
                    id: law_description_text
                    text: view.law_description
                    color: "blue"
                }
            }

            Row
            {
                Text { text: "order:" }

                Text {
                    id: order_description_text
                    text: view.order_description
                    color: "blue"
                }
            }

            Row
            {
                Text { text: "coordination:" }

                Text {
                    id: coordination_description_text
                    text: view.coordination_description
                    color: "blue"
                }
            }

            Row
            {
                Text { text: "preawareness:" }

                Text {
                    id: preawareness_description_text
                    text: view.preawareness_description
                    color: "blue"
                }
            }
        }
    }
}