Judge, Condemn and Redeem

/* The following three hooks are used when we're doing a thorough
   redisplay of the frame.  We don't explicitly know which scroll bars
   are going to be deleted, because keeping track of when windows go
   away is a real pain - "Can you say set-window-configuration, boys
   and girls?"  Instead, we just assert at the beginning of redisplay
   that *all* scroll bars are to be removed, and then save a scroll bar
   from the fiery pit when we actually redisplay its window.  */

/* Arrange for all scroll bars on FRAME to be removed at the next call
   to `*judge_scroll_bars_hook'.  A scroll bar may be spared if
   `*redeem_scroll_bar_hook' is applied to its window before the judgment.  */

static void
w32_condemn_scroll_bars (struct frame *frame)
{
  if (!NILP (FRAME_SCROLL_BARS (frame)))
    {
      if (!NILP (FRAME_CONDEMNED_SCROLL_BARS (frame)))
        {
          /* Prepend scrollbars to already condemned ones.  */
          Lisp_Object last = FRAME_SCROLL_BARS (frame);

          while (!NILP (XSCROLL_BAR (last)->next))
            last = XSCROLL_BAR (last)->next;

          XSCROLL_BAR (last)->next = FRAME_CONDEMNED_SCROLL_BARS (frame);
          XSCROLL_BAR (FRAME_CONDEMNED_SCROLL_BARS (frame))->prev = last;
        }

      fset_condemned_scroll_bars (frame, FRAME_SCROLL_BARS (frame));
      fset_scroll_bars (frame, Qnil);
    }
}


/* Un-mark WINDOW's scroll bar for deletion in this judgment cycle.
   Note that WINDOW isn't necessarily condemned at all.  */

static void
w32_redeem_scroll_bar (struct window *w)
{
  struct scroll_bar *bar;
  Lisp_Object barobj;
  struct frame *f;

  /* We can't redeem this window's scroll bar if it doesn't have one.  */
  if (NILP (w->vertical_scroll_bar) && NILP (w->horizontal_scroll_bar))
    emacs_abort ();

  if (!NILP (w->vertical_scroll_bar) && WINDOW_HAS_VERTICAL_SCROLL_BAR (w))
    {
      bar = XSCROLL_BAR (w->vertical_scroll_bar);
      /* Unlink it from the condemned list.  */
      f = XFRAME (WINDOW_FRAME (w));
      if (NILP (bar->prev))
        {
          /* If the prev pointer is nil, it must be the first in one of
             the lists.  */
          if (EQ (FRAME_SCROLL_BARS (f), w->vertical_scroll_bar))
            /* It's not condemned.  Everything's fine.  */
            goto horizontal;
          else if (EQ (FRAME_CONDEMNED_SCROLL_BARS (f),
                       w->vertical_scroll_bar))
            fset_condemned_scroll_bars (f, bar->next);
          else
            /* If its prev pointer is nil, it must be at the front of
               one or the other!  */
            emacs_abort ();
        }
      else
        XSCROLL_BAR (bar->prev)->next = bar->next;

      if (! NILP (bar->next))
        XSCROLL_BAR (bar->next)->prev = bar->prev;

      bar->next = FRAME_SCROLL_BARS (f);
      bar->prev = Qnil;
      XSETVECTOR (barobj, bar);
      fset_scroll_bars (f, barobj);
      if (! NILP (bar->next))
        XSETVECTOR (XSCROLL_BAR (bar->next)->prev, bar);
    }

 horizontal:
  if (!NILP (w->horizontal_scroll_bar) && WINDOW_HAS_HORIZONTAL_SCROLL_BAR (w))
    {
      bar = XSCROLL_BAR (w->horizontal_scroll_bar);
      /* Unlink it from the condemned list.  */
      f = XFRAME (WINDOW_FRAME (w));
      if (NILP (bar->prev))
        {
          /* If the prev pointer is nil, it must be the first in one of
             the lists.  */
          if (EQ (FRAME_SCROLL_BARS (f), w->horizontal_scroll_bar))
            /* It's not condemned.  Everything's fine.  */
            return;
          else if (EQ (FRAME_CONDEMNED_SCROLL_BARS (f),
                       w->horizontal_scroll_bar))
            fset_condemned_scroll_bars (f, bar->next);
          else
            /* If its prev pointer is nil, it must be at the front of
               one or the other!  */
            emacs_abort ();
        }
      else
        XSCROLL_BAR (bar->prev)->next = bar->next;

      if (! NILP (bar->next))
        XSCROLL_BAR (bar->next)->prev = bar->prev;

      bar->next = FRAME_SCROLL_BARS (f);
      bar->prev = Qnil;
      XSETVECTOR (barobj, bar);
      fset_scroll_bars (f, barobj);
      if (! NILP (bar->next))
        XSETVECTOR (XSCROLL_BAR (bar->next)->prev, bar);
    }
}

/* Remove all scroll bars on FRAME that haven't been saved since the
   last call to `*condemn_scroll_bars_hook'.  */

static void
w32_judge_scroll_bars (struct frame *f)
{
  Lisp_Object bar, next;

  bar = FRAME_CONDEMNED_SCROLL_BARS (f);

  /* Clear out the condemned list now so we won't try to process any
     more events on the hapless scroll bars.  */
  fset_condemned_scroll_bars (f, Qnil);

  for (; ! NILP (bar); bar = next)
    {
      struct scroll_bar *b = XSCROLL_BAR (bar);

      x_scroll_bar_remove (b);

      next = b->next;
      b->next = b->prev = Qnil;
    }

  /* Now there should be no references to the condemned scroll bars,
     and they should get garbage-collected.  */
}

Source.