Wrong global position with BypassGraphicsProxyWidget

A QComboBox is contained in a widget embedded in a QGraphicsProxyWidget.
(the combo box has a QTreeView as view here, but that should not matter)

Hover the items, which have tooltips: Tool tips are displayed at the left edge of the screen.
Involved is this line in QAbstractItemDelegate::helpEvent:
QToolTip::showText(he->globalPos(), tooltip.toString(), view);

Set the BypassGraphicsProxyWidget flag on the widget containing the combo box:
The combo box popup is displayed to the left edge of the screen.
(there is also a UI freeze happening here when hiding the popup again btw: event loop is in poll() but no input events processed)
Involved is this code line in QComboBox::showPopup:
QPoint below = mapToGlobal(listRect.bottomLeft());

Instead of a combo box, use a KLineEdit with an autocompletion popup and the BypassGraphicsProxyWidget flag:
The popup is displayed to the left edge of the screen.
Involved in kdelibs code is the code line:
d->m_parent->mapToGlobal( QPoint(0, d->m_parent->height()) );

From the code examples given, it seems that mapping to global position for embedded widgets is broken.