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Thank you for helping use to reproduce the rarely crashing issue in combo box control. To fix it, please update the source code for the following method:

LRESULT CExtComboBoxPopupListBox::WindowProc( UINT message, WPARAM wParam, LPARAM lParam )
{
    if( m_pCB != NULL && m_hWndCB != NULL && ::IsWindow( m_hWndCB ) )
    {
        LRESULT lResult = 0L;
        if( m_pCB->_OnCbLbPreWndProc( lResult, message, wParam, lParam ) )
            return lResult;
    }
    switch( message )
    {
    case WM_NCCALCSIZE:
        if( m_bSimpleModeComboBox)
        {
            NCCALCSIZE_PARAMS * myNCSIZE = (NCCALCSIZE_PARAMS*)lParam;
            CRect rcSaved = myNCSIZE->rgrc[0];
            LRESULT lResult = CWnd::WindowProc( message, wParam, lParam );
            //myNCSIZE->rgrc[0].left = rcSaved.left;
            myNCSIZE->rgrc[0].top = rcSaved.top;
            myNCSIZE->rgrc[0].bottom = rcSaved.bottom;
            myNCSIZE->rgrc[0].right ++;
            return lResult;
        }
    break;
    case (WM_USER+555):
        MoveWindow( &m_rcWndRestore );
    return 0L;
    case WM_PRINT:
    case WM_PRINTCLIENT:
        if( m_bFirstActivationInvocation )
        {
            m_bFirstActivationInvocation = false;
            if( ! m_bSimpleModeComboBox )
            {
                GetWindowRect( &m_rcWndRestore );
                MoveWindow( -32000, -32000, 0, 0 );
                PostMessage( (WM_USER+555) );
            }
        }
        if( ! m_bSimpleModeComboBox )
            return 0L;
    break;
    case WM_ERASEBKGND:
    if(     (GetStyle() & LBS_OWNERDRAWFIXED) != 0 
        ||    (GetStyle() & LBS_OWNERDRAWVARIABLE) != 0 
        )
        return TRUE;
    break;
    case WM_PAINT:
    if(     (GetStyle() & LBS_OWNERDRAWFIXED) != 0 
        ||    (GetStyle() & LBS_OWNERDRAWVARIABLE) != 0 
        )
    {
        CPaintDC dcPaint( this );
        CRect rcClient;
        GetClientRect( &rcClient );
        CExtMemoryDC dc( &dcPaint, &rcClient );
        if( m_pCB != NULL && m_hWndCB != NULL && ::IsWindow( m_hWndCB ) )
        {
            CExtPaintManager * pPM = m_pCB->PmBridge_GetPM();
            ASSERT_VALID( pPM );
            dc.FillSolidRect( &rcClient, pPM->GetColor( COLOR_WINDOW, this ) );
            HFONT hFont = m_pCB->OnQueryFont();
            if( hFont == NULL )
                hFont = (HFONT)pPM->m_FontNormal.GetSafeHandle();
            ASSERT( hFont != NULL );
            HFONT hOldFont = (HFONT)::SelectObject( dc.m_hDC, hFont );
            INT nCount = m_pCB->GetCount();
            INT nTopIndex = ((CListBox*)this)->GetTopIndex();
            for( INT nItem = nTopIndex; nItem < nCount; nItem++ )
            {
                DRAWITEMSTRUCT dis;
                ::memset( &dis, 0, sizeof( DRAWITEMSTRUCT ) );
                dis.CtlType = ODT_LISTBOX;
                dis.CtlID = GetDlgCtrlID();
                dis.itemID = nItem;
                dis.hDC = dc.GetSafeHdc();
                ((CListBox*)this)->GetItemRect( nItem, &dis.rcItem );
                dis.itemAction = ODA_DRAWENTIRE;
                dis.hwndItem = GetSafeHwnd();
                dis.itemData = m_pCB->GetItemData( nItem );
                if( rcClient.bottom < dis.rcItem.top )
                    break;
                if( ((CListBox*)this)->GetSel( nItem ) > 0 )
                    dis.itemState |= ODS_SELECTED;
                if( ((CListBox*)this)->GetCurSel() == nItem )
                    dis.itemState |= ODS_FOCUS;
                m_pCB->SendMessage( WM_DRAWITEM, (WPARAM)GetDlgCtrlID(), (LPARAM)&dis );
            }
            ::SelectObject( dc.m_hDC, hOldFont );
        }
        return TRUE;
    }
    break;
    } // switch( message )
HWND hWndCB = m_hWndCB, hWndOwn = m_hWnd;
CWnd * pThis = this;
LRESULT lResult = CWnd::WindowProc( message, wParam, lParam );
    if( ! ( hWndOwn != NULL && ::IsWindow( hWndOwn ) && CWnd::FromHandlePermanent(hWndOwn) == pThis ) )
        return lResult;
    if( hWndCB != NULL && ::IsWindow( hWndCB ) && m_pCB != NULL && LPVOID(CWnd::FromHandlePermanent(hWndCB)) == LPVOID(m_pCB) )
        m_pCB->_OnCbLbPostWndProc( lResult, message, wParam, lParam );
LONG nFrame = RDW_FRAME;
    switch( message )
    {
    case WM_VSCROLL:
    case WM_HSCROLL:
        nFrame = 0L;
    case WM_SHOWWINDOW:
    case WM_WINDOWPOSCHANGED:
        RedrawWindow(
            NULL,
            NULL,
            RDW_INVALIDATE|RDW_UPDATENOW|RDW_ERASE|RDW_ERASENOW|nFrame
            );
    break;
    } // switch( message )
    return lResult;
}