Transparency issue around winId() and QWebEngineView

#include <QtCore>
#include <QtWidgets>
#include <QtWebEngineWidgets>
#include <QGraphicsOpacityEffect> 

class TransparentWindow : public QWidget
{
public:
	TransparentWindow(QWidget *parent);
protected:
	virtual void paintEvent(QPaintEvent *event);
};

TransparentWindow::TransparentWindow(QWidget *parent)
	: QWidget(parent, Qt::Widget | Qt::FramelessWindowHint | Qt::WindowStaysOnTopHint | Qt::X11BypassWindowManagerHint)
{
	resize(parent->width(), parent->height() / 2);
	
	QGraphicsOpacityEffect *effect = new QGraphicsOpacityEffect;
	effect->setOpacity(0.5);
	setGraphicsEffect(effect);
}

void TransparentWindow::paintEvent(QPaintEvent *event)
{
	QPainter painter(this);
	
	painter.setPen(Qt::NoPen);
	painter.setBrush(QColor(200, 20, 20, 255));
	painter.drawRoundedRect(painter.viewport(), 50, 50, Qt::AbsoluteSize);

	painter.setPen(QColor(Qt::white));
	painter.drawText(rect(), Qt::AlignCenter | Qt::TextWordWrap, "Transparency bug around child winId() and QWebEngineView");
}


int main(int argc, char * argv[])
{
	QApplication app(argc, argv);
	
	QMainWindow mw;
	mw.resize(1000, 1000);
	
	QSplitter * splitter = new QSplitter(Qt::Horizontal);
	
	QWidget *child = new QWidget;
	child->winId();	// ! Removing this line fixes the issue !

	splitter->addWidget(child);
	splitter->addWidget(new QWebEngineView);	// ! Replacing QWebEngineView with some another widget also fixes the issue !
	//splitter->addWidget(new QWidget);
	mw.setCentralWidget(splitter);
	mw.show();
	
	// show the transparent popup
	TransparentWindow *transparentWindow = new TransparentWindow(&mw);
	transparentWindow->show();
	
	return app.exec();
}