Transparent QOpenGLWidget in Dialog

[Sivan]

When I add a QOpenGLWidget in a frameless dialog, and open the dialog on a qwidget, I can see the dialog is transparent as shown in the screenshot. What I am expecting is for the red to have alpha but it shouldn't see the button in the main widget.

#include "Widget.h"

#include <QHBoxLayout>
#include <QOpenGLWidget>
#include <QOpenGLFunctions>
#include <QPushButton>

class MyOpenGLWidget : public QOpenGLWidget, protected QOpenGLFunctions
{
public:
    explicit MyOpenGLWidget(QWidget* p_Parent = nullptr)
        : QOpenGLWidget(p_Parent)
    {
        QSurfaceFormat newFormat = format();
        newFormat.setSwapInterval(0);
        setFormat(newFormat);
    }

protected:
    virtual void initializeGL() override
    {
        initializeOpenGLFunctions();
    }

    virtual void paintGL() override
    {
        glClearColor(0, 0, 0, 1);
        glClear(GL_COLOR_BUFFER_BIT);

        glEnable(GL_BLEND);
        glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
        glColor4f(1, 0, 0, 0.5);
        glRectf(-0.5, -1, 0.5, 1);
    }
};

class MyDialog : public QDialog
{
public:
    MyDialog(QWidget* p_Parent = nullptr)
        : QDialog(p_Parent, Qt::FramelessWindowHint | Qt::Dialog)
    {
        MyOpenGLWidget* myOpenGLWidget = new MyOpenGLWidget(this);

        setAttribute(Qt::WA_TranslucentBackground, false);
        QHBoxLayout* layout = new QHBoxLayout;
        layout->addWidget(myOpenGLWidget);
        setLayout(layout);

        setMinimumSize(400, 400);
    }

    virtual ~MyDialog()
    {
    }
};


Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    QPushButton* btn = new QPushButton("Press Me!");

    QHBoxLayout* layout = new QHBoxLayout;
    layout->addWidget(btn);
    setLayout(layout);

    setMinimumSize(500, 500);

    connect(btn, &QPushButton::clicked, [this]
    {
        MyDialog dialog;
        dialog.exec();
    });
}

Widget::~Widget()
{
}

It seems to me that the usage of QSurfaceFormat::setSwapInterval(0) together with Qt::FramelessWindowHint is causing this issue. Changing either one of them eliminate the problem. Could anyone please advice on what I am doing wrong here? Thanks in advance.

I am using Qt-5.15.2 and on Mac OS 10.15.7

[Sivan]

For now, I manage to fix this by changing

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

to

glBlendEquationSeparate(GL_FUNC_ADD, GL_FUNC_ADD);
glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_CONSTANT_ALPHA, GL_ONE);

However, I would still like to understand the root cause or better alternative to fix this issue.