QDockWidget losing decoration on undocking when GroupedDragging is enabled

This problem was only observed on Windows. I couldn't reproduce the problem on Linux.

• In the Dock Widgets example, add the following line to MainWindow::createDockWindows:

    setDockOptions(QMainWindow::AllowNestedDocks | QMainWindow::AllowTabbedDocks | QMainWindow::GroupedDragging);
      

• Run the example and undock both dock widgets. Dock one of the floating dock widgets on the other.
• Double click on the title bar. One of the dock widgets will be stuck on the upper left corner of the screen without window decorations.

Please see the the attached animated GIF.