This problem can be seen in the ToolKitPro examples, CommonControls when you add the following to the MainFrm OnCreate function:

 GetCommandBars()->GetCommandBarsOptions()->bShow ExpandButtonAlways = FALSE;

Try to move the Animation, slider and the one with the words "Opened Documents" toolbars to the far right of the window and you can see it will stop exactly one dropdown button width from the right side.

You can override CXTPToolBar:

class CToolBoxBar : public CXTPToolBar
{
public:
 DECLARE_XTP_COMMANDBAR(CToolBoxBar)

 CSize CalcDockingLayout(int nLength, DWORD dwMode, int nWidth);

}

IMPLEMENT_XTP_COMMANDBAR(CToolBoxBar, CXTPToolBar)

CSize CToolBoxBar::CalcDockingLayout(int nLength, DWORD dwMode, int nWidth)
{
 GetCommandBars()->GetCommandBarsOptions()->bShow ExpandButtonAlways = FALSE;
 CSize sz = CXTPToolBar::CalcDockingLayout(nLength, dwMode, nWidth);
 GetCommandBars()->GetCommandBarsOptions()->bShow ExpandButtonAlways = TRUE;

 return sz;
}

usage:

 CXTPToolBar* pToolBox = (CXTPToolBar*)
        pCommandBars->Add(_T("ToolBox"), xtpBarLeft, RUNTIME_CLASS(CToolBoxBar));

but you must make CXTPToolBar::CalcDockingLayout virtual (in next version it will be)