How to make WA_TranslucentBackground work on Windows without using FramelessWindowHint?

[Roberrt]

Im using setAttribute(Qt::WA_TranslucentBackground); to make my MainWindow background transparent, however, this attribute only work if:

I also set: setWindowFlag(Qt::FramelessWindowHint);
or
Have at least once QOpenGLWidget in my GUI.

I tried asking ChatGPT what happens when i add QOpenGLWidget to my window which makes the attribute to work:

Qt::WA_TranslucentBackground is an attribute that sets the background of a widget to be translucent, meaning that any widgets or windows underneath it will be visible. The attribute needs to be set on the top-level window for it to work properly.

However, for a translucent background to be properly rendered, the window needs to have an alpha channel, which is a channel in the color format that stores the transparency information for each pixel. In the case of a QMainWindow, the default color format doesn't include an alpha channel.

When you add a QOpenGLWidget to your QMainWindow, it sets the required format and creates a context for OpenGL rendering. This provides an alpha channel for the window, allowing the translucent background to be rendered correctly.

So, the reason Qt::WA_TranslucentBackground only works when you add a QOpenGLWidget to your QMainWindow is that the QOpenGLWidget is providing the required alpha channel for the translucent background to be rendered correctly.

------------------------------------------------------------------------------------------------
Q) Is possible to provide this required alpha channel without including a QOpenGLwidget to it?
------------------------------------------------------------------------------------------------

Yes, it is possible to provide the required alpha channel for a translucent background without including a QOpenGLWidget. One way to do this is by setting the window surface format for your QMainWindow explicitly.

You can do this by creating a QSurfaceFormat object, setting the required format options (e.g. an alpha channel), and then setting this format as the window surface format for your QMainWindow using the QSurfaceFormat::setDefaultFormat() static function.

I exhaustedly tried all answers it gave, specially using QSurfaceFormat, but the background remains black.

I have no experience with QOpenGlWidget, i would like to ask if is it really possible to get this attribute working using QSurfaceFormat or something like;

Or if someone knows any other 'option' than setting the FramelessWindowHint flag or creating an QOpenGL widget.

Example code:

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

    /*
    QSurfaceFormat format;
    format.setAlphaBufferSize(8);
    QSurfaceFormat::setDefaultFormat(format);


    QSurfaceFormat format;
    format.setAlphaBufferSize(8);
    format.setSamples(16);
    format.setSwapBehavior(QSurfaceFormat::DoubleBuffer);
    format.setRenderableType(QSurfaceFormat::OpenGL);
    format.setProfile(QSurfaceFormat::CoreProfile);
    QSurfaceFormat::setDefaultFormat(format);
    */

    MainWindow w;

    w.show();
    return a.exec();
}

//mainwindow.h
#include <QtWidgets/QMainWindow>
#include "ui_MainWindow.h"

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindowClass; };
QT_END_NAMESPACE

class MainWindow : public QMainWindow{
    Q_OBJECT
public:
    MainWindow(QWidget* parent = nullptr);
}

//mainwindow.cpp
MainWindow::MainWindow(QWidget *parent) 
    : QMainWindow(parent), ui(new Ui::MainWindowClass())
{
    ui->setupUi(this);

    setAttribute(Qt::WA_TranslucentBackground);

    QGridLayout* layout = new QGridLayout();
    ui->centralWidget->setStyleSheet("background-color: transparent;");
    ui->centralWidget->setLayout(layout);
    QWidget* widget = new QWidget(this);
    widget->setStyleSheet("background-color: green; border-radius: 32px;");
    layout->addWidget(widget);
    
    //QOpenGLWidget* opengl = new QOpenGLWidget(this);
    //opengl->deleteLater();
}

Creating the QOpenGlWidget and calling opengl->deleteLater() the WA_TranslucentBackground works, however, it make the application use 50mb more of ram compared to not creating the QOpenGLWidget.

This is not much if i were creating just one GUI, but I'm creating multiples.