How to add a maximize button in dialog title bar, which is a child of another widget ?

[liugang]

code: (it is ok in MS windows, but no Maximize button in linux)

import sys
from PyQt5.QtGui import *
from PyQt5.QtWidgets import *
from PyQt5.QtCore import *


class DialogDemo(QMainWindow):
    def __init__(self, parent=None):
        super(DialogDemo, self).__init__(parent)
        self.setWindowTitle('Main Window')
        self.resize(350, 300)

        self.btn = QPushButton(self)
        self.btn.setText('Popup Dialog')
        self.btn.move(50, 50)
        self.btn.clicked.connect(self.showdialog)

    def showdialog(self):
        dialog = QDialog(parent=self)
        btn = QPushButton('ok', dialog)
        btn.move(50, 50)
        dialog.setWindowTitle("Dialog")

        flags = dialog.windowFlags()
        flags |= Qt.CustomizeWindowHint
        flags |= Qt.WindowTitleHint
        flags |= Qt.WindowSystemMenuHint
        flags |= Qt.WindowCloseButtonHint
        flags |= Qt.WindowStaysOnTopHint
        flags |= Qt.WindowMaximizeButtonHint
        dialog.setWindowFlags(flags)

        dialog.showMaximized()


if __name__ == '__main__':
    app = QApplication(sys.argv)
    demo = DialogDemo()
    demo.show()
    sys.exit(app.exec_())