Re-docking of floating window does not work

[CCB67]

In the DockMvvmSample:
If I undock a panel and release it without docking it anywhere else, it is displayed in a floating window, which is good.
But then I can't integrate it back into the main window, i.e. dock it somewhere again.
The control for selecting the docking position appears and the view of the selected position is also highlighted, but the docking does not work.