QDialog* dialog = new QDialog(this);
dialog->setWindowFlags(Qt::FramelessWindowHint | Qt::Window);
dialog->setAttribute(Qt::WA_TranslucentBackground);

QWidget* test = new QWidget(this);
test->setFixedSize(400,300);

QGraphicsDropShadowEffect* effect = new QGraphicsDropShadowEffect;
const int blur = 38 * 3;
effect->setBlurRadius(blur);
effect->setXOffset(0);
effect->setYOffset(24);
effect->setColor(QColor(0, 0, 0, 75));
test->setGraphicsEffect(effect);
test->setContentsMargins(blur, blur, blur, blur);

QVBoxLayout* layout = new QVBoxLayout;
layout->addWidget(test);

dialog->setLayout(layout);

dialog->show();