Scroll the contents of this window, both pixels and children, by the given amount. The window itself does not move. Portions of the window that the scroll operation brings in from offscreen areas are invalidated. The invalidated region may be bigger than what would strictly be necessary. (For X11, a minimum area will be invalidated if the window has no subwindows, or if the edges of the window's parent do not extend beyond the edges of the window. In other cases, a multi-step process is used to scroll the window which may produce temporary visual artifacts and unnecessary invalidations.)