FIX: MFC ActiveX controls paint incorrectly when scrolling the HTML page (233391)

SYMPTOMS

An MFC-based windowed ActiveX control on an HTML page paints incorrectly when you scroll the HTML page in the browser. The control appears distorted, showing successively larger bands at the bottom or top as it moves off the visible portion of the HTML page.

In versions of Microsoft Internet Explorer later than version 5, the control may paint correctly. However, child windows on the control still experience the banding effects while scrolling.

CAUSE

A performance enhancement was added to Internet Explorer 5 to improve the rendering of windowed ActiveX controls by manipulating the available window and clip regions for the control's window. When this code operates on MFC controls, portions of the control's window outside the clip region are invalidated. Because the control's OnDraw cannot draw outside the clip region, these areas show stripes of the background color.

RESOLUTION

To resolve this problem, upgrade clients to version 5.01 or later of Internet Explorer.

Note A change was made in Internet Explorer to fix the problem that is described in the following Microsoft Knowledge Base (KB) article:

307978 FIX: MFC controls in overlapped IFRAMEs receive unnecessary WM_PAINT messages

However, this change reintroduced the problem in Internet Explorer 6 Service Pack 1.

WORKAROUND

To work around this problem, use one of the following methods:

Repaint the control when the cnscroll event fires, as in the following sample code.

<SCRIPT>
function workaround()
{
// "a" is the name of the control.
window.document.all.item("a").style.display = "none"
window.document.all.item("a").style.display = ""
}
</SCRIPT>
<BODY onscroll="workaround();">

If the control class is derived from the COleControl class, it implements a virtual method that is named OnSetObjectRects. You can override this method, as in the following sample code.
```
BOOL CmfcaxCtrl::OnSetObjectRects(LPCRECT lpRectPos, LPCRECT lpRectClip) 
{ return TRUE; 
}
```
Use ATL instead of MFC to develop the ActiveX control.

Unfortunately, MFC controls that contain child windows will still have painting problems in Internet Explorer. You can resolve these problems by forcing a redraw on all the child windows during handling of the OnPaint message. To do this, you must add a message map entry for WM_PAINT to the COleControl-derived class. In the OnPaint handler, use code that is similar to the following.

// NUMBER_OF_CHILDREN is predefined as the number of child windows
// that are hosted on this control

// m_Children is a member variable of the CWindowedCtrl class that
// stores an array of CWnd references to the child windows on the control.

void CWindowedCtrl::OnPaint()
{
   CPaintDC dc(this); // device context for painting<BR/>

   for(int i = 0 ; i < NUMBER_OF_CHILDREN ; i++)
   {
      m_Children[i].RedrawWindow(NULL,NULL,RDW_INVALIDATE | RDW_FRAME);   
   }

   COleControl::OnPaint(&dc);
}