# Using PyQt5 5.11.2 (Qt 5.11.1) (latest at time of writing)
# To see the bug:
# - Enable "Stay on Top" in the third MDI window's window menu
# - Drag the first MDI window around on top of the second

from PyQt5.QtCore import Qt
from PyQt5.QtWidgets import QApplication, QMainWindow, QMdiArea, QMdiSubWindow, QLabel

def main(args):
    global app
    app = QApplication(args)
    win = QMainWindow()
    mdi = QMdiArea()
    widgets = [QLabel('Drag me'), QLabel('over me'), QLabel('while I stay on top')]
    for w in widgets:
        sw = QMdiSubWindow()
        sw.setWidget(w)
        sw.resize(200, 200)
        mdi.addSubWindow(sw)
    win.setCentralWidget(mdi)
    win.show()
    widgets[0].setAttribute(Qt.WA_NativeWindow)
    return app.exec_()

if __name__ == '__main__':
    import sys
    main(sys.argv)