draw_set_alpha(0.85);
draw_rectangle_color(40, 380, 760, 500,
    c_black, c_black, c_black, c_black, false);
draw_set_alpha(1);

chars = min(chars + 2, string_length(lines[page]));
draw_text_ext(60, 400,
    string_copy(lines[page], 1, chars), 24, 640);