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

#include <QApplication>
#include <QMainWindow>
#include <QMdiArea>
#include <QMdiSubWindow>
#include <QDockWidget>
#include <QLabel>
#include <QVBoxLayout>

class MinimalMdiDockApp : public QMainWindow {
public:
    MinimalMdiDockApp(QWidget *parent = nullptr) : QMainWindow(parent) {
        setWindowTitle("Minimal QMdiArea with QDockWidget Inside Sub-Window");
        resize(800, 600);

        // Create QMdiArea and set it as central widget
        QMdiArea *mdiArea = new QMdiArea(this);
        setCentralWidget(mdiArea);

        // Create QMdiSubWindow
        QMdiSubWindow *subWindow = new QMdiSubWindow;
        subWindow->setWindowTitle("Sub-Window with Dock Widget");

        // Create QDockWidget, parented to subWindow
        QDockWidget *dockWidget = new QDockWidget("Internal Dock", subWindow);
        dockWidget->setFeatures(
            QDockWidget::DockWidgetClosable |
            QDockWidget::DockWidgetMovable |
            QDockWidget::DockWidgetFloatable
        );

        // Create content for the dock
        QLabel *dockContent = new QLabel("This is supposed to be a dock!");
        dockContent->setAlignment(Qt::AlignCenter);
        dockWidget->setWidget(dockContent);

        // Set QDockWidget directly as the widget of the subwindow
        subWindow->setWidget(dockWidget);

        // Add subwindow to mdi area
        mdiArea->addSubWindow(subWindow);
        subWindow->show();
    }
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    MinimalMdiDockApp window;
    window.show();
    return app.exec();
}

QT += widgets

CONFIG += c++17 console
CONFIG -= app_bundle  # Only needed on macOS to avoid .app packaging

TARGET = MinimalMdiDockApp
TEMPLATE = app

SOURCES += main.cpp