Hello Jack - if you're using Babel not sure what the answer is. But my
GetInitialExtent does the following (sets start and end cols to be the same)
which seems to look okay when displayed.

STDMETHODIMP CTypeBrowserCompletionSet::GetInitialExtent(long *piLine, long
*piStartCol, long *piEndCol) {
   try
   {
       ASSERT(piLine && piStartCol && piEndCol);
       if (!piLine || !piStartCol || !piEndCol)
           return E_POINTER;

       if (m_pTextView->GetCaretPos(piLine, piStartCol) != S_OK)
           return S_FALSE;

       *piEndCol = *piStartCol;

       m_bVisible = true;

       return S_OK;
   }
   catch(...)
   {
       ATLASSERT(false);
   }

   return S_FALSE;
}