Undocking a QDockWidget embedded inside an MdiSubWindow causes the QDockWidget to be unmovable and impossible to re-embed.

import sys
from PyQt6.QtWidgets import (
    QApplication, QMainWindow, QDockWidget, QLabel, QMdiArea, QMdiSubWindow
)
from PyQt6.QtCore import Qt

class MinimalMdiDockApp(QMainWindow):
    def __init__(self):
        super().__init__()
        self.setWindowTitle("Minimal QMdiArea with QDockWidget Inside Sub-Window")
        self.setGeometry(100, 100, 800, 600)

        self.mdi_area = QMdiArea()
        self.setCentralWidget(self.mdi_area)

        # Create a QMdiSubWindow
        sub_window = QMdiSubWindow()
        sub_window.setWindowTitle("Sub-Window with Dock Widget")

        # Create the QDockWidget
        dock_widget = QDockWidget("Internal Dock", sub_window) # Parent is sub_window, not QMainWindow

        # Set the required features for the QDockWidget
        dock_widget.setFeatures(
            QDockWidget.DockWidgetFeature.DockWidgetClosable |
            QDockWidget.DockWidgetFeature.DockWidgetMovable |
            QDockWidget.DockWidgetFeature.DockWidgetFloatable
        )

        # Create content for the dock widget
        dock_content = QLabel("This is supposed to be a dock!")
        dock_content.setAlignment(Qt.AlignmentFlag.AlignCenter)
        dock_widget.setWidget(dock_content)

        # Set the QDockWidget directly as the content of the QMdiSubWindow
        # This is the key change.
        sub_window.setWidget(dock_widget) # Attempting to put QDockWidget directly

        self.mdi_area.addSubWindow(sub_window)
        sub_window.show()

if __name__ == "__main__":
    app = QApplication(sys.argv)
    window = MinimalMdiDockApp()
    window.show()
    sys.exit(app.exec())