1. Have 2 docked QDockWidgets on a QMainWindow.
2. Put restoreGeometry/restoreState in QMainWindow's constructor and saveGeometry/saveState in its closeEvent.
3. Run the program, make one QDockWidget floating and hide it.
4. Close the program.
5. Run the program again.
6. From the context menu on the widget header of the visible QDockWidget make the hidden QDockWidget visible. It'll be floating, ok.
7. Try to dock the floating QDockWidget (by dragging it around or clicking on the titlebar). You'll fail.
8. But setting programmatically

ui.myDock->setFloating(false);

will succeed.