QWindow using QBackingStore behaves differently depending on the platform

Yujeonja

I'm testing QWindow with QBackingStore to draw a window. I started from this example: https://doc.qt.io/qt-5/qtgui-rasterwindow-example.html

And I added some code in the constructor for alpha channel.

    QSurfaceFormat format;
    format.setAlphaBufferSize(8);
    this->setFormat(format);

I need semi-transparent content so I modified the code as below.

void Window::setColor(const QColor &color)
{
    this->m_color = color;
}

void Window::renderNow()
{
    if (!isExposed()) {
        return;
    }

    QRect rect(0, 0, width(), height());
    m_backingStore->beginPaint(rect);

    QPaintDevice *device = m_backingStore->paintDevice();
    QPainter painter(device);

    if (this->parent() == nullptr) {
        painter.fillRect(0, 0, width(), height(), QColor(255, 0, 0, 255));
    } else {
        painter.fillRect(0, 0, width(), height(), this->m_color);
    }
    render(&painter);
    painter.end();

    this->m_backingStore->endPaint();
    this->m_backingStore->flush(rect);
}

And I want to draw some rectangles in toplevel window so I created sub-class of QWindow as a child of the window.

    QApplication a(argc, argv);
    Window win;
    Window win2(&win);
    win2.setWidth(20);
    win2.setHeight(20);
    win2.setX(0);
    win2.setY(0);
    win2.setColor(QColor(100, 100, 255, 100));

    win.show();
    win2.show();

    return a.exec();

But the result was different across the platforms.

Linux - X11
Child window is transparent through the parent window. It means, while parent window(red) is opaque, the area of child window is semi-transparent so I can see the content of other window below my window.
Linux - Wayland
Semi-transparent child window is blended with below parent windows color. This is the result exactly what I want.
Windows
Child window is not transparent as there is black background under the window. Transparency is not working.

Why are the results different for different platforms? Is there something I missed? Are these undefined behaviors? If then, what should I do for the result that I want as second one.

I used Qt 5.15.2 for all platforms.

Kent-Dorfman

@Yujeonja said in QWindow using QBackingStore behaves differently depending on the platform:

Why are the results different for different platforms?

because each GUI handles it differently and has differing capabilities.

Is there something I missed?

No.