#include "QMainWindow"
#include "QApplication"
#include "QPaintEvent"
#include "QPainter"
#include "QDebug"

// A widget that fills its background with a random color
class CMyWidget : public QWidget
{
public:
  explicit CMyWidget(QWidget* parent = nullptr, Qt::WindowFlags f = Qt::WindowFlags()) : QWidget(parent, f) {}

protected:
  virtual void paintEvent(QPaintEvent *event) override
  {
    qDebug () << "PAINT" << this << event->rect();
    QPainter painter(this);
    painter.fillRect(event->rect(), QBrush(QColor::fromRgb(rand())));
  }
};

class CMainWindow : public QMainWindow
{
public:
  CMyWidget *m_pPopup;

  CMainWindow ()
  {
    CMyWidget *pCentralWidget = new CMyWidget(this);
    pCentralWidget->setObjectName("CENTRAL");
    setCentralWidget(pCentralWidget);

    // When you replace Qt::Popup by Qt::Widget, everything is drawn correctly
    m_pPopup = new CMyWidget(pCentralWidget, Qt::Popup);
    m_pPopup->resize(100, 100);
    m_pPopup->move(200, 100);
  }

  // A popup message is toggled when clicked inside the main window
  void mousePressEvent(QMouseEvent * /*event*/)
  {
    update();
    if (m_pPopup->isVisible())
      m_pPopup->hide();
    else
    {
      m_pPopup->show();
      m_pPopup->update();
    }
  }
};

int main(int argc, char *argv[])
{
  QApplication a(argc, argv);
  CMainWindow w;
  w.resize(600, 400);

  w.show();

  return a.exec();
}