#include <QApplication>
#include <QDebug>

#include <QGraphicsView>
#include <QGraphicsScene>
#include <QOpenGLWidget>
#include <QResizeEvent>
#include <QGraphicsRectItem>

class Item : public QGraphicsRectItem
{
public:
  Item()
    : QGraphicsRectItem(0,0,100,100)
  {
    setAcceptHoverEvents(true);
    setPen(Qt::NoPen);
    setBrush(Qt::green);
  }

protected:
  void hoverEnterEvent(QGraphicsSceneHoverEvent* e)
  {
    setBrush(Qt::red);
  }
  void hoverLeaveEvent(QGraphicsSceneHoverEvent* e)
  {
    setBrush(Qt::green);
  }

};

class View : public QGraphicsView
{
public:
  View(QWidget *parent = {}):
    QGraphicsView(parent)
  {
    setFrameStyle(QFrame::NoFrame);
    setBackgroundBrush(Qt::black);

    setViewport(new QOpenGLWidget); //< this causes problems

    setScene(new QGraphicsScene);
    scene()->addItem(new Item);
  }

  void resizeEvent(QResizeEvent* e)
  {
    scene()->setSceneRect({{0,0}, e->size()});
  }

  void mouseDoubleClickEvent(QMouseEvent* e)
  {
    if(windowFlags() & Qt::FramelessWindowHint)
    {
      setWindowFlags(windowFlags() & ~Qt::FramelessWindowHint);
    }
    else
    {
      setWindowFlags(windowFlags() | Qt::FramelessWindowHint);
    }
    show();
  }

};

#include <QOpenGLWindow>
#include <QPainter>

using OpenGLClass = QOpenGLWidget; //< QOpenGLWindow or QOpenGLWidget

class Widget : public OpenGLClass
{
  using Base = OpenGLClass;

public:
  Widget()
  {
    // QOpenGLWidget case

    setMouseTracking(true);
  }

protected:
  void mouseMoveEvent(QMouseEvent * e)
  {
    _itemBrush = (_itemRect.contains(e->pos()) ? Qt::red : Qt::green);
    update();
  }

  bool event(QEvent *e)
  {
    switch(e->type())
    {
    case QEvent::Leave:
      _itemBrush = Qt::green;
      update();
      break;
    default:
      break;
    }

    return Base::event(e);
  }


  void mouseDoubleClickEvent(QMouseEvent* e)
  {
    // QOpenGLWidget case

    if(windowFlags() & Qt::FramelessWindowHint)
    {
      setWindowFlags(windowFlags() & ~Qt::FramelessWindowHint);
    }
    else
    {
      setWindowFlags(windowFlags() | Qt::FramelessWindowHint);
    }

    // QOpenGLWindow case

//    if(flags() & Qt::FramelessWindowHint)
//    {
//      setFlags(flags() & ~Qt::FramelessWindowHint);
//    }
//    else
//    {
//      setFlags(flags() | Qt::FramelessWindowHint);
//    }

    show();
  }

  void paintGL()
  {
    QPainter p(this);

    p.setPen(Qt::NoPen);

    p.setBrush(_itemBrush);
    p.drawRect(_itemRect);
  }

private:
  QRectF _itemRect {0.,0.,100.,100.};
  QBrush _itemBrush {Qt::green};

};

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

//  View w; //< scene view case
  Widget w;

  w.resize(200,200);
  w.show();

  return app.exec();
}