test.cpp

#include <QApplication>
#include <QMainWindow>
#include <windows.h>
#include <dwmapi.h>

int main(int argc, char *argv[]) {
    QApplication a(argc, argv);
    QMainWindow w;
    w.setMinimumSize(400, 300);
    
    w.setAttribute(Qt::WA_TranslucentBackground);
    // Explicitly enforce native window frame
    w.setWindowFlags(Qt::Window | Qt::WindowTitleHint | Qt::WindowSystemMenuHint | Qt::WindowMinimizeButtonHint | Qt::WindowMaximizeButtonHint | Qt::WindowCloseButtonHint);

    HWND hwnd = reinterpret_cast<HWND>(w.winId());
    int backdropType = 3; // 3 = Acrylic, 2 = Mica
    DwmSetWindowAttribute(hwnd, 38, &backdropType, sizeof(backdropType));

    w.setStyleSheet("QMainWindow { background: rgba(0,0,0,50); }");
    w.show();
    return a.exec();
}