All the windows with this style are excluded from their parent's visible region. This style protects windows but increases the amount of time necessary to calculate the parent's visible region. This style normally is not necessary, because if the parent and child windows overlap and both are invalidated, the parent window is drawn before the child window. If the child window is invalidated independently from its parent window, only the child window is redrawn. If the update region of the parent window does not intersect the child window, drawing the parent window does not disturb the child window.