showMinimized() misbehave when called from closeEvent under Linux/X11

# -*- coding: utf-8 -*-

from PyQt5.QtWidgets import QMainWindow, QApplication

class Dummy(QApplication):
    mainWin = None
    def __init__(self, *args):
        super().__init__(*args)

        self.mainWin = MainWin()
        self.mainWin.show()

class MainWin(QMainWindow):
    def __init__(self):
        super().__init__(None)

    def closeEvent(self, qEvent):
        qEvent.ignore()
        self.showMinimized()
        # same happens to self.setWindowState(Qt.WindowMinimized) 

if __name__ == "__main__":
    app = Dummy([])
    app.exec()

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The code above is misbehaving under Linux/X11. I tested this on Archlinux(KDE), Manjaro(Xfde), Ubuntu 13.10(Gnome fallback), they all went wrong in a same way.

1. click the close button, it minimizes to the taskbar as expected
2. click the taskbar to restore the window, working
3. click the close button again, nothing happens.
4. no matter how many times you try from now on, nothing happens.

The same code works on Windows 7 without problems.

This is not with PyQt for sure. I tried with the C++ version under Linux as well. Same problem.