Feature request 16475.

redisplay_window

/* *************************************************************************** */
/* BEGIN Feature Request # 16475:  Automatically add/remove scroll bars. */

 finish_scroll_bars:
  ;

  //  ptrdiff_t bob_disregard_narrow = BUF_BEG (buffer);
  ptrdiff_t eob_disregard_narrow = BUF_Z (buffer);

  ptrdiff_t bob_respect_narrow = BUF_BEGV (buffer);
  ptrdiff_t eob_respect_narrow = BUF_ZV (buffer);

  ptrdiff_t window_start = marker_position (w->start);
  ptrdiff_t window_end = eob_disregard_narrow - w->window_end_pos;

  //  ptrdiff_t buffer_size = eob_disregard_narrow - bob_disregard_narrow;

  //  bool buffer_narrowed_p = (eob_respect_narrow - bob_respect_narrow != buffer_size);

  bool window_contents_visible_p = (bob_respect_narrow == window_start
                                    && eob_respect_narrow == window_end);

   if (!window_contents_visible_p)
     w->vertical_scroll_bar_type = Qright;

   if (!window_contents_visible_p
       && (WINDOW_HAS_VERTICAL_SCROLL_BAR (w)
           || WINDOW_HAS_HORIZONTAL_SCROLL_BAR (w)))
    {
      if (WINDOW_HAS_VERTICAL_SCROLL_BAR (w))
	/* Set the thumb's position and size.  */
	set_vertical_scroll_bar (w);

      if (WINDOW_HAS_HORIZONTAL_SCROLL_BAR (w))
	/* Set the thumb's position and size.  */
	set_horizontal_scroll_bar (w);

      /* Note that we actually used the scroll bar attached to this
	 window, so it shouldn't be deleted at the end of redisplay.  */
      if (FRAME_TERMINAL (f)->redeem_scroll_bar_hook)
        (*FRAME_TERMINAL (f)->redeem_scroll_bar_hook) (w);
    }
    else if (!MINI_WINDOW_P (w)
             && !NILP (w->vertical_scroll_bar_type))
      {
        w->vertical_scroll_bar_type = Qnil;
        clear_glyph_matrix (w->desired_matrix);
        SET_FRAME_GARBAGED (f);
      }

/* END Feature Request # 16475:  Automatically add/remove scroll bars. */
/* *************************************************************************** */