Docks tabbed together outside of main window don't behave correctly.

Steps to reproduce:

Create QMainWindow and set QMainWindow::GroupedDragging option.
Create two docks: A and B.
Drag A out of main window.
Drag B out of main window and tab together with A.
Drag B out.

Issues at this point:
1. Both docks windows have B window title.
2. Dock A can't be tabbed with B anymore. B can still be tabbed with A.
3. Double clicking on A's title bar moves it to position 0,0 on screen with frame outside of the screen area (it should dock A to main window).
4. Neither of the docks is now accessible from the main window context menus (normally there's a show/hide action for them there).
5. Dragging A in and out of main window does not emit topLevelChanged signal anymore.

Example code:

#include <QApplication>
#include <QMainWindow>
#include <QDockWidget>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QMainWindow w;
    w.addToolBar("Dummy"); //right click the toolbar to see dock actions
    w.setDockOptions(QMainWindow::GroupedDragging);
    w.addDockWidget(Qt::LeftDockWidgetArea, new QDockWidget("A"));
    w.addDockWidget(Qt::RightDockWidgetArea, new QDockWidget("B"));
    w.show();

    return a.exec();
}