Tabsheet drag & drop problem

Hello forum,
Seen a problem with tab drag and drop, and hoping the experts can shed some light on it:
I have a multi tabsheet layout (a bunch of split panes, each with its own tabsheet).
If a given tabsheet has two or more tabs (>1), drag operations are good.
If a tabsheet only has one tab, the drag operation is never initiated, and so it can’t be dragged (in my case, to another tabsheet in a different splitpane position).
Is this a bug in tabsheet behaviour, or is there a trick to get D&D across multiple tabsheets working?

Many thanks!
Peter

BTW, I’m using Vaadin 7.1