Tabifying docks broken if manipulating dock widgets is not animated

After disabling the use of animations for dock widget and tool bar manipulations, dock widgets can no longer be tabified (moved on top of each other to enable switching between them using tabs at the side).

To reproduce:
Change example widgets/mainwindows/dockwidgets to disable animations for dock widget manipulations. In constructor of MainWindow (mainwindow.cpp:74), insert line

setAnimated(false);

Launch application and try to move one of the dock widgets on top of the other. The preview is shown correctly but on drop, the dock widgets are not tabified.