bug#12277: 24.1.50; Graphical issues with linum-mode and fringe

[Eli Zaretskii <eliz@gnu.org>]

> From: Jan Djärv <jan.h.d@swipnet.se>
> Date: Sat, 8 Sep 2012 14:22:39 +0200
> Cc: Eli Zaretskii <eliz@gnu.org>,
>  12277@debbugs.gnu.org
> 
> But I found a simpler way to reproduce the bug:
> 
> ./emacs -Q
> M-x scroll-bar-mode (to turn off scrollbars).
> C-x 3
> M-x linum-mode
> 
> Then double-click on a line to mark a word on that line.  The redisplay engine will then redraw the whole line, but the black border between windows will be gone from that line and to the end of the window

Thanks, I can reproduce this here.  Will take a look when I have time.

> why Emacs redisplays from the marked word to the end of the window I
> don't know, it does not seem efficient

Probably because the 'region' face can specify a different font,
which, if its size is significantly different from the default, could
require redrawing of more than one screen line.  Perhaps it's possible
to invent optimizations that redraw less in at least some of the
cases, but no one has come up with such optimizations, and I'm not
sure it's worth our while.  Each optimization adds to the complexity
of the display engine, so IMO we should only introduce new ones if the
normal redisplay is unbearably slow.