I've been using QQmlApplicationEngine to load and display the window. If I switch it to QQuickView I also don't get a transparent window.

Here is the example code I'm running:

main.cpp:

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQuickView>

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

    // creating application using engine
    // when uncommenting this also make sure to replace
    // Rectangle by Window in Main.qml
/*    QQmlApplicationEngine engine;
    const QUrl url(u"qrc:/transparenttest/Main.qml"_qs);
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
        &app, [url](QObject *obj, const QUrl &objUrl) {
            if (!obj && url == objUrl)
                QCoreApplication::exit(-1);
        }, Qt::QueuedConnection);

    engine.load(url);*/

    // creating the application using view
    QQuickView *view = new QQuickView;
    view->setSource(QUrl("qrc:/transparenttest/Main.qml"));

    QSurfaceFormat surfaceFormat;
    surfaceFormat.setAlphaBufferSize(8);
    view->setFormat(surfaceFormat);
    view->setColor(QColor(qRgba(0,0,0,128)));
    view->show();

    return app.exec();
}

(the commented out code is the default code provided by qt creator and (afaict) the recommended way (?) of creating qml applications.)

Main.qml:

import QtQuick
import QtQuick.Controls
import QtQuick.Window

// use Window when running it with QQmlApplicationEngine
//Window {
Rectangle {
    width: 640
    height: 480
    visible: true
    color: "#88000000"
}

It also has some strange behavior when resizing the window, in particular when making it larger. The newly added areas are indeed semi-transparent. It seems like there is a central item or rectangle in the background that is all white at startup and stays around but doesn't resize...

When using QQuickView, I can make it work with the following code:

        // See https://stackoverflow.com/questions/14009549/how-to-make-a-transparent-window-with-qt-quick-2-0
        QSurfaceFormat surfaceFormat;
        surfaceFormat.setAlphaBufferSize(8);
        pQuickView->setFormat(surfaceFormat);
        pQuickView->setColor(QColor(Qt::transparent));

(a semi-transparent color should work then, as well)