Widgets with transparent backgrounds over a QGraphicsView with a QGLWidget viewport

Question

I recently tried setting the viewport of my QGraphicsView to a QGLWidget to see how it performs. What I noticed is that widgets which previously had transparent backgrounds (styled buttons, in-game menus that have no background) now have black backgrounds. Is there a simple way to keep the transparency and still use a QGLWidget viewport?

#include <QtCore>
#include <QtWidgets>
#include <QGLWidget>

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

    QMainWindow mainWindow;

    QGraphicsView *view = new QGraphicsView(&mainWindow);

    view->setViewport(new QGLWidget(QGLFormat(QGL::SampleBuffers)));
    view->setViewportUpdateMode(QGraphicsView::FullViewportUpdate);

    QPushButton *button = new QPushButton(&mainWindow);
    button->setStyleSheet("QPushButton {"
      "border: 2px solid #8f8f91;"
      "border-radius: 6px;"
      "background-color: qlineargradient(x1: 0, y1: 0, x2: 0, y2: 1,stop: 0 #f6f7fa, stop: 1 #dadbde);"
      "min-width: 80px;"
      "}");

    QWidget *widget = new QWidget(&mainWindow);
    widget->move(0, 100);

    mainWindow.setCentralWidget(view);
    mainWindow.resize(200, 200);
    mainWindow.show();

    return app.exec();
}

Answer 1

According to the resolution of the bug report in my comment:

this is an unavoidable consequence of how QGLWidget works

It was suggested elsewhere that QGraphicsProxyWidget might solve the problem:

#include <QtCore>
#include <QtWidgets>
#include <QGLWidget>

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

    QMainWindow mainWindow;

    QGraphicsView *view = new QGraphicsView(&mainWindow);
    QGraphicsScene *scene = new QGraphicsScene;
    view->setScene(scene);

    view->setViewport(new QGLWidget(QGLFormat(QGL::SampleBuffers)));
    view->setViewportUpdateMode(QGraphicsView::FullViewportUpdate);

    QPushButton *button = new QPushButton;
    button->setStyleSheet("QPushButton {"
      "border: 2px solid #8f8f91;"
      "border-radius: 6px;"
      "background-color: qlineargradient(x1: 0, y1: 0, x2: 0, y2: 1,stop: 0 #f6f7fa, stop: 1 #dadbde);"
      "min-width: 80px;"
      "}");

    scene->addWidget(button);

    mainWindow.setCentralWidget(view);
    mainWindow.resize(200, 200);
    mainWindow.show();

    return app.exec();
}