bug#16475: enhancement request: remove vertical scroll bar automatically when not needed

[Keith David Bershatsky]

Thank you, Eli and Martin:

I have incorporated the comment of Eli about being consistent by using BEGV/ZV 
vs BUF_BEGV/BUF_ZV.

I have created a second draft based (in part) on the comment of Martin:  "The 
test for the vertical scroll bar case is whether the buffer beginning and its 
end are both visible in the window, taking into account visibility, overlays 
and the like."

The visibility test for the beginning/ending points can be added in future 
drafts.

Scroll bars are not immediately removed, however, when 
`window_contents_visible_p` is true and this-command is a keyboard stroke.  I 
encountered the same problem with the first draft, and that is why I looked for 
a means to force immediate removal of the scroll bars by using the 
condemn/judge combination.  Subsequently using the mouse scroll wheel *does* 
trigger scroll bar removal.

Any ideas on how to immediately remove scroll bars from the target window when 
`window_contents_visible_p` is true?  [I have chosen the end of 
`redisplay_window` to perform the test because window start/end *should be* 
accurate at that location of the code.]

  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);

 finish_scroll_bars:

   if ((WINDOW_HAS_VERTICAL_SCROLL_BAR (w) || WINDOW_HAS_HORIZONTAL_SCROLL_BAR 
(w))
        && !window_contents_visible_p)
    {
      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);
    }

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DATE:  [10-25-2017 07:40:24] <25 Oct 2017 17:40:24 +0300>
FROM:  Eli Zaretskii <eliz@gnu.org>
> 
> * * *
> 
> It at least should be consistent about using BEGV/ZV vs
> BUF_BEGV/BUF_ZV.  Either the code assumes that W displays the current
> buffer, or it doesn't.
> 
> Also, I don't understand the need for this part:
> 
> >     else
> >       {
> >         (*FRAME_TERMINAL (f)->condemn_scroll_bars_hook) (f);
> >         (*FRAME_TERMINAL (f)->judge_scroll_bars_hook) (f);
> >       }
> 
> We already call condemn_scroll_bars_hook at the beginning of a
> redisplay cycle, and call judge_scroll_bars_hook at its end.  So why
> did you need to do it here again, for each window separately?