QDialogs, QMessageBoxes not centering properly in light weigth Window Managers

When using some "light-weight" windows managers such as FVMW2 and IceWM, QDialogs and QMessageboxes created by the running app intended to be centered sometimes are not.

Example 1:
When the MainWindow is setup to be fullscreen with QDesktopWidget:

QDesktopWidget* desktop = QApplication::desktop();
const QRect& ssize = desktop->screenGeometry(-1);
m_MainWindow->setGeometry (0, 0, ssize.width(), ssize.height() );
m_MainWindow->showFullScreen();

Pop up dialogs which are children of a main window are created, but they do not center properly, but only in the FVWM2 window manager. In Gnome, KDE, and without a windows manager in X11 dialogs and message boxes are displayed centered correctly, but not in FVWM2.

Example 2:
The same goes for using IceWM. An example tested in the IceWM environment using two Messageboxes will result in two message boxes both placed in non-centered pos where as in X11 without a windows manager, or KDE/Gnome both messages are centered.

ex:
QMessageBox::question(this,"test", "Long test Message 1");
QMessageBox::question(0,"test","Long test Message 2");