QGraphicsOpacityEffect does not work for widget, which is owned to window with mask

Unick

Hello.

In my case, QGraphicsOpacityEffect does not work for widget, if the owner of it has a mask. Here is what i do:

QMainWindow mainWindow; // Main Window

QWidget widget(&mainWindow, Qt::Window | Qt::FramelessWindowHint); // I creates QWidget with onwer
widget.setAttribute(Qt::WA_NoSystemBackground, true);
widget.setAttribute(Qt::WA_TranslucentBackground, true);

QWidget background(&widget);
background.setStyleSheet("QWidget {background: rgba(255, 50, 80, 1.0)}"); // setup background.

QTimer::singleShot(2000, [&]()
{
	auto effect = new QGraphicsOpacityEffect(&widget);
	widget.setGraphicsEffect(effect);
	auto animation = new QPropertyAnimation(effect, "opacity");
	animation->setDuration(3000);
	animation->setEndValue(0.01);
	animation->start();  // Start fade out in 2 sec.
});

mainWindow.show();
widget.show();

it works correctly. In 2 seconds widget will start to disappear.

If i add this code after QMainWindow mainWindow:

QMainWindow mainWindow
QRegion reg(0, 0, mainWindow.width(), mainWindow.height());
mainWindow.setMask(reg);

QGraphicsOpacityEffect stops work. The widget will not disappear.

Is there a method to fix my problem?