#include <QtWidgets>

int main(int argc, char *argv[]) {
   QApplication app(argc, argv);
   QPushButton button("Toggle List");
   QListWidget listWidget;

   button.setMinimumSize(320, 200);
   Q_ASSERT(!listWidget.testAttribute(Qt::WA_DeleteOnClose));
   listWidget.setWindowFlags(Qt::Window | Qt::FramelessWindowHint);
   listWidget.setDragDropMode(QAbstractItemView::DragDrop);
   listWidget.setDefaultDropAction(Qt::MoveAction);
   for (auto s : {"first", "second", "third"})
        listWidget.addItem(QLatin1String(s));

   QObject::connect(&button, &QPushButton::pressed, [&] {
      static bool toggle;
      if ((toggle = !toggle)) {
         listWidget.setParent(&button, listWidget.windowFlags());
         listWidget.show();
      } else {
         listWidget.setParent(nullptr);
         listWidget.close();
      }
   });

   button.show();
   return app.exec();
}