Leaves One

# Create QMdiArea
w.mdi = QMdiArea(w)
w.mdi.setGeometry(w.geometry())

# Create subwindow
sub = QMdiSubWindow()
sub.setWidget(QTextEdit("Alan Richard"))
sub.setWindowTitle("SubWindow")

w.mdi.addSubWindow(sub)  # Add to MDI area

# !!! Try to hide close button. It works for main window, but not for MDI subwindow.
sub.setWindowFlags(sub.windowFlags() | Qt.WindowType.CustomizeWindowHint)
sub.setWindowFlags(sub.windowFlags() & ~Qt.WindowType.WindowCloseButtonHint)

# ...
# Create subwindow
sub = QTextEdit("Alan Richard")
w.mdi.addSubWindow(sub, Qt.WindowType.WindowMinMaxButtonsHint)  # <--
sub.setWindowTitle("SubWindow")

w.mdi.addSubWindow(sub)  # Add to MDI area

# Removed `setWindowFlag` lines