The following example is taken from the docking demo. It shows how the OnUnDock event handler of the conjoinment docking site re-enables docking in the control that is undocked (if it is a dockable form). In addition, when the next-to-last docked control is undocked, the conjoinment docking site sends itself a close message so that the last docked control is undocked to its old position and size.
procedure TConjoinDockHost.FormUnDock(Sender: TObject; Client: TControl; NewTarget: TWinControl; var Allow: Boolean); begin // Only two dock clients means the host must be destroyed and // the remaining window undocked to its old position and size. // Recall that OnUnDock gets called before the undocking // actually occurs. if Client is TDockableForm then TDockableForm(Client).DockSite := True; if (DockClientCount = 2) and (NewTarget <> Self) then PostMessage(Self.Handle, WM_CLOSE, 0, 0); UpdateCaption(Client); end;