createWindowContainer() creates a widget which wraps a QWindow. It lacks stacking functionality because of preventing all widgets to be native. I think that it should works if user wants turn all widgets into native:

QWidget *wc = QWidget::createWindowContainer(window, parentWidget);
wc->setAttribute(Qt::WA_NativeWindow);

The window container should not paint the widget background, because it is covered up by a window, so it causes only performance issues.

See Bad.png and Good.png.