QWindow wrapped by createWindowContainer() become invisible when Qt::WA_TranslucentBackground is set.

Say we has a QWindow called 'W', and we use createWindowContainer() to wrap it into a QWidget 'A'. 'A' has a parent widget 'P'.

Everything goes perfect until we put the following code in the parent 'P':

    this->setAttribute(Qt::WA_TranslucentBackground);
    this->setWindowFlags(Qt::FramelessWindowHint);

after that, the 'A' is still OK, but the 'W' turned to invisible.

What we need is both 'A' and 'W' are visible after the parent's background is gone.

What's going on here? Does this a known bug to Qt?

EDIT:

FYI, we are trying to implements a shadow like this: https://stackoverflow.com/a/15757358/2010078,

but we have a child QWindow indeed, so if we drop the shadow like that, our QWindow is invisible.