#include <QWidget>
#include <QKeyEvent>
#include <QApplication>
class Widget : public QWidget
{
protected:
  void keyPressEvent(QKeyEvent* e)
  {
    if(e->key() == Qt::Key_Space)
    {
      setWindowFlags(windowFlags() ^ Qt::FramelessWindowHint);
      show();
    }
  }
};

#include <QGLWidget>
class GLWidget : public QGLWidget
{
public:
  using QGLWidget::QGLWidget;

protected:
  void	paintGL() override
  {
    glClearColor(1, 0, 0, 1);
    glClear(GL_COLOR_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
  }
};

#include <QVBoxLayout>
int main(int argc, char *argv[])
{
  QApplication a(argc, argv);
  Widget w;
  w.setMinimumSize({100, 100});

  auto l = new QVBoxLayout;
  w.setLayout(l);
  l->addWidget(new GLWidget(&w));

  w.show();

  return a.exec();
}