Why does this TextOut() sometimes hang up?

I tried to make the equivalent of a simple ProgressBar by
displaying a count in the window of the outermost one of
several nested loops. This process is very CPU intensive so
I have to force screen updates. The count works correctly
unless I move the window, and then the window is no longer
updated. What is causing this, and how can I fix it?

// The TextOut() display code inside a very tight loop
    ClientArea.TextOut(0, 0, String, strlen(String));
    MainWin->InvalidateRect(NULL);
    MainWin->UpdateWindow();

// Window member data
  CDC ClientArea; // virtual window device context
  CBitmap m_bmp; // virtual window bitmap

// Window Constructor
MainWindow::MainWindow()
{
  ScreenWidth = GetSystemMetrics(SM_CXSCREEN);
  ScreenHeight = GetSystemMetrics(SM_CYSCREEN);
  Create(NULL, "SeeScreen the Screen Recognizer",
WS_OVERLAPPEDWINDOW | WS_HSCROLL | WS_VSCROLL, rectDefault,
NULL, "TextMenu");
  CClientDC DC(this);

  // Create bitmap for virtual window
  ClientArea.CreateCompatibleDC(&DC);
  m_bmp.CreateCompatibleBitmap(&DC, ScreenWidth,
ScreenHeight);
  ClientArea.SelectObject(&m_bmp);

  // use standard background
  MainBrush.CreateStockObject(WHITE_BRUSH);
  ClientArea.SelectObject(&MainBrush);

  // paint background of virtual window
  ClientArea.PatBlt(0, 0, ScreenWidth, ScreenHeight,
PATCOPY);
}

// Update screen using contents of virtual window.
afx_msg void MainWindow::OnPaint()
{
  CPaintDC DC(this);
  DC.BitBlt(0, 0, ScreenWidth, ScreenHeight, &ClientArea, 0,
0, SRCCOPY);
}