bug#13623: 24.3.50; Redisplay issue with transient-mark-mode

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 05 Feb 2013 08:54:05 +0400
> From: Dmitry Antipov <dmantipov@yandex.ru>
> CC: Lawrence Mitchell <wence@gmx.li>, 13623@debbugs.gnu.org
> 
> >    -  /* If showing the region, and mark has changed, we must redisplay
> >    -     the whole window.  The assignment to this_line_start_pos prevents
> >    -     the optimization directly below this if-statement.  */
> >    -  if (((!NILP (Vtransient_mark_mode)
> >    -       && !NILP (BVAR (XBUFFER (w->buffer), mark_active)))
> >    -       != !NILP (w->region_showing))
> >    -      || (!NILP (w->region_showing)
> >    -         && !EQ (w->region_showing,
> >    -                 Fmarker_position (BVAR (XBUFFER (w->buffer), mark)))))
> >    -    CHARPOS (this_line_start_pos) = 0;
> 
> Hm.  Although this is an obvious bug, are you sure that we must redisplay
> the whole window even if the region doesn't span multiple lines? IIUC
> it should be enough to redisplay the current line only.

As long as we don't restrict the 'region' face to a very small subset
of possible face customizations (e.g., just the background and the
foreground colors), and ignore the other attributes, an arbitrary face
change on one line might potentially affect many more lines in the
window.  E.g., try customizing the 'region' face to twice its normal
height.

And I'm not sure this is worth optimizing anyway: region changes are
relatively rare and almost always driven by user input, so I don't
think redisplay will become significantly faster as result of any
optimizations in this area.