Setting WA_NativeWindow on QQuickWidget doesn't render anymore, all black

We wanted to embed a QQuickWidget in a docking container within our QMainWindow, without running through the render surface type conversion of the mainwindow's default raster surface to direct3d/opengl surface. According to the recent fixes done for QTBUG-119221 , this should be now doable by setting the Qt::WA_NativeWindow on the quickwidget. 
See comment from vestbo in QTBUG-119221 and related defect:

Set Qt::WA_NativeWindow on the QOpenGLWidget/QRhiWidget/QQuickWidget/QWebEngineView, or one of its parent widgets, to composit only that widget's hiearchy using RHI. The parent widget hierarchy should not be affected (WA_DontCreateNativeAncestors and AA_DontCreateNativeWidgetSiblings still apply as normal).

541664 adds support for separate RHI flushing per native window.

But when setting the Qt::WA_NativeWindow flag the quickwidget doesn't render anymore and window content is black.

This is a simplified repro sample with a quickwidget in a dialog, instead of a dockwidget, but the same occurs when you try to put the quickwidget in a dockwidget:

#include <QtWidgets/QApplication>
#include <QtWidgets/QtWidgets>
#include <qquickwidget.h>

class QuickWidgetDialog : public QDialog
{
public:
	QuickWidgetDialog(QWidget* parent = nullptr)
		: QDialog(parent, Qt::Dialog)
	{
		setWindowTitle("QuickWidget Dialog Host");
		setAttribute(Qt::WA_DeleteOnClose, true);

		auto horzLayout = new QHBoxLayout(this);
		auto quickWidget = new QQuickWidget();
		quickWidget->setAttribute(Qt::WA_NativeWindow);
		quickWidget->setSource(QUrl::fromLocalFile("Panel.qml"));
		horzLayout->addWidget(quickWidget);
	}
};

int main(int argc, char** argv)
{
	QApplication app(argc, argv);

	auto dlg = new QuickWidgetDialog();
	dlg->resize(640, 480);
	dlg->show();

	return app.exec();
}

Note, that with the recent changes in QWidget::setParent() releated to RHI & setting up the backing store, everything strongly depends on whether at parenting time the parent's windowHandle() is already created or not, see closestParentWidgetWithWindowHandle().