A window which hasn't set the DockArea.Document like the properties window of the DockSample application will dock on documents, if they are floating. This combined floating window cannot be docked anymore, if the document has DockArea.Document only and the property window hasn't set DockArea.Document. This probably isn't the intended behaviour.
A possible solution would be to prohibit a dock to a floating window if the only DockArea flag in common is Float.