Create a QWidget in a dialog
x has a child
y has the Qt::Popup flag set
when x gets the mousePressEvent, call this code:

const QPoint ppt = mapToGlobal(QPoint(0, height()));
m_popup->move(ppt);
m_popup->show();

When you Alt-tab to another application, the popup hides itself which is normal for a popup.

The problem is, when you alt-tab, both the application that houses x and y and the application you alt-tabbed to (explorer or what have you) seem to be activated (the top bars of both applications are highlighted) which is not normal for Windows.