I am using Qt 5.12.9 on Windows 10. The QComboBox's stylesheet is

* {
    padding: 10px 10px 10px 10px;
    border-radius: 10px; 
}

and in mainwindow.cpp in a brand new, empty Qt project (with just the combobox added and mainwindow background set to red for visibility):

    ui->comboBox->setFrame(false);
    QFrame* window = dynamic_cast<QFrame*>(ui->comboBox->view()->window()); // this is a QComboBoxPrivateContainer, which is a subclass of QFrame
    window->setFrameStyle(QFrame::NoFrame);
    window->setStyleSheet("* { background-color: green; }");
    window->setWindowFlags(Qt::FramelessWindowHint);
    window->setAttribute(Qt::WA_TranslucentBackground, true);
    window->setAttribute(Qt::WA_OpaquePaintEvent, true);

Yet the window background is still black. Further, FramelessWindowHint prevents the dropdown from opening unless I press space with it focused (and after opening, it immediately closes)

How do I make the dropdown window background completely invisible for QComboBox?