Validates the client area within the given rectangle by removing the rectangle from the update region of the window.
void ValidateRect( LPCRECT lpRect );
Points to a CRect object or RECT structure that contains client coordinates of the rectangle to be removed from the update region. If lpRect is NULL, the entire window is validated.
The BeginPaint member function automatically validates the entire client area. Neither the ValidateRect nor the ValidateRgn member function should be called if a portion of the update region needs to be validated before WM_PAINT is next generated.
Windows continues to generate WM_PAINT messages until the current update region is validated.