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Dear Emil,

There are two ways to avoid flickering slider controls in resizable windows.

1) Adding the WS_CLIPCHILDREN style to your dialog allows all sliders in the dialog to have their own background. In other words this style clips all other overlapped child windows out of the region of the dialog to be updated. This method is not very good because it causes some other controls (e.g. a static control) to flicker.

2) Repainting the slider control would be much better:

class CCustomPaintedSliderCtrl : public CSliderCtrl
 {
 protected:
  virtual LRESULT WindowProc(
   UINT message,
   WPARAM wParam,
   LPARAM lParam
   )
  {
   ASSERT_VALID( this );
   if( message == WM_ERASEBKGND )
    return TRUE;
   if( message == WM_NCCALCSIZE )
    return 0L;
   if( message == WM_PAINT )
   {
    CPaintDC dcPaint( this );
    CExtPaintManager::stat_ExcludeChildAreas(
     dcPaint.GetSafeHdc(),
     GetSafeHwnd()
     );
    CRect rcClient;
    GetClientRect( &rcClient );
    CExtMemoryDC dc(
     &dcPaint,
     &rcClient
     );
    if( g_PaintManager>GetCb2DbTransparentMode(this) )
     g_PaintManager>PaintDockerBkgnd( dc, this );
    else
     dc.FillSolidRect(
      &rcClient,
      g_PaintManager->GetColor(
       CExtPaintManager::CLR_3DFACE_OUT
       )
      );
    DWORD dwWndStyle = GetStyle();
    bool bHorz = ( (dwWndStyle&TBS_VERT) != 0 ) ? false : true;
    COLORREF clrFace = g_PaintManager->GetColor( COLOR_3DFACE );
    COLORREF clrHilight = g_PaintManager->GetColor( COLOR_3DHILIGHT );
    COLORREF clrShadow = g_PaintManager->GetColor( COLOR_3DSHADOW );
    CRect rcChannel;
    //CSliderCtrl::GetChannelRect( &rcChannel );
    rcChannel = rcClient;
    CRect rcThumb;
    CSliderCtrl::GetThumbRect( &rcThumb );
    if( bHorz )
    {
     rcChannel.top = rcThumb.top;
     rcChannel.bottom = rcThumb.bottom;
     rcChannel.DeflateRect(
      rcThumb.Width(),
      ( rcChannel.Height() - rcThumb.Width() ) / 2
      );
    } // if( bHorz )
    else
    {
     rcChannel.left = rcThumb.left;
     rcChannel.right = rcThumb.right;
     rcChannel.DeflateRect(
      ( rcChannel.Width() - rcThumb.Height() ) / 2,
      rcThumb.Height()
      );
    } // else from if( bHorz )
    dc.FillSolidRect( &rcChannel, clrFace );
    dc.Draw3dRect( &rcChannel, clrShadow, clrHilight );
    dc.FillSolidRect( &rcThumb, clrFace );
    dc.Draw3dRect( &rcThumb, clrHilight, clrShadow );
    return TRUE;
   } // if( message == WM_PAINT )
   LRESULT lResult = CSliderCtrl::WindowProc( message, wParam, lParam );
   return lResult;
  }
 }; // class CCustomPaintedSliderCtrl

You can add the CCustomPaintedSliderCtrl class to your project and use it inside your dialog (instead of CSliderCtrl). Of course, if a new look of the slider control does not fit your requirements, we can modify it.