A floating QDockWidget is not dockable after the QMainWindow state has been restores from QSettings file via QMainWindow::restoreState() in QMainWindow constructor.

Do the following steps to reproduce the error:

1. Build and run the attached modified dockwidgets example wit Qt 5.0.1

2. Undock one dockwidget and make it floating

3. Select the menu item View -> Save Window State

4. Close the application

5. Restart the application

Now it is not posssible to dock the floating dockwidget. If I restore the state via the menu item View -> Restore Window State this does not happen. So it seems to be related to the fact the restoreState() is called in the QMainWindow constructor when the window is not visible yet.

In Qt4.8.4 this code works.