Hi, I'm having exactly the same problem, none of the following ways works for me, any suggestions? Thanks in advance.

QWinWidget* pParentWidget = new QWinWidget(hParent);

1. pParentWidget->setAttribute(Qt::WA_TranslucentBackground, true);
pParentWidget->setWindowFlags(Qt::FramelessWindowHint);

2. pParentWidget->setWindowOpacity(0);

3. QPalette p(pParentWidget->palette());
p.setColor(QPalette::Background, Qt::transparent);
pParentWidget->setPalette(p);

4. setMask isn't suitable for me, as the UI elements are dynamic.(e.g. tree view)