Rendering bug when showing both an OpenGL widget and a WebEngine widget in the same MDI area

#include <iostream>

#include <QToolBar>
#include <QApplication>
#include <QMainWindow>
#include <QMdiArea>
#include <QOpenGLWidget>
#include <QMdiSubWindow>
#include <QWebEnginePage>
#include <QWebEngineView>

class PlainGLWidget : public QOpenGLWidget
{
protected:
    virtual void paintGL();
};


void PlainGLWidget::paintGL()
{


    int i;
    float x1, y1, x2, y2;


    glClearColor(1.0, 1.0, 1.0, 0.5);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glLineWidth(30.);
    glColor4f(0.2, 0.3, 0.1, 0.3);
    for(i=0;i<=100;i++) {
      glBegin(GL_LINE_STRIP);
      x1 = -float(rand() % 10);
      y1 = -float(rand() % 10);
      x2 = float(rand() % 10);
      y2 = float(rand() % 10);
      glVertex2f(x1, y1);
      glVertex2f(x2, y2);
      glEnd();
    }


}


int main(int argc, char *argv[]) {


    QApplication app(argc, argv);


    // Create a QMainWindow with an MDI area and a toolbar


    QMainWindow *window = new QMainWindow;


    QMdiArea *mdi = new QMdiArea;
    window->setCentralWidget(mdi);

    // Show window now - in the real application I am writing, the MDI sub-windows
    // get created on-the-fly and so the MDI area gets shown first.
    window->show();
    window->raise();

    // Create OpenGL widget

    PlainGLWidget *glwidget = new PlainGLWidget;

    // Wrap the OpenGL widget in MDI sub-window and add to MDI area

    QMdiSubWindow *sub1 = new QMdiSubWindow;
    sub1->setWidget(glwidget);
    mdi->addSubWindow(sub1);
    sub1->resize(400,400);
    sub1->show();
    
    // Create WebEngine widget
    
    QWebEnginePage *page = new QWebEnginePage;
    QWebEngineView *web = new QWebEngineView;
    web->setPage(page);
    web ->setUrl(QUrl("https://www.qt.io"));

    // Wrap the WebEngine widget in MDI sub-window and add to MDI area

    QMdiSubWindow *sub2 = new QMdiSubWindow;
    sub2->setWidget(web);
    mdi->addSubWindow(sub2);
    sub2->resize(400,400);
    sub2->move(400,0);
    sub2->show();

    return app.exec();

}