def scroll_one_line_up(event):
    """
    scroll_offset -= 1
    """
    w = event.app.layout.current_window
    b = event.app.current_buffer
    if w:
        if w.render_info:
            info = w.render_info
            if w.vertical_scroll > 0:
                first_line_height = info.get_height_for_line(info.
                    first_visible_line())
                cursor_up = info.cursor_position.y - (info.window_height - 
                    1 - first_line_height - info.configured_scroll_offsets.
                    bottom)
                for _ in range(max(0, cursor_up)):
                    b.cursor_position += b.document.get_cursor_up_position()
                w.vertical_scroll -= 1
