bug#25666: Screen rendering bug

[Eli Zaretskii <eliz@gnu.org>]

> From: npostavs@users.sourceforge.net
> Cc: 25666@debbugs.gnu.org,  fredde1994@gmail.com
> Date: Sun, 12 Feb 2017 13:59:41 -0500
> 
> >> > I see this with nlinum-mode as well.  My terminal is PuTTY (which
> >> > emulates xterm).
> >> 
> >> Ah, this depends on how high the terminal is.  With an 80x32 terminal I
> >> see it with nlinum-mode as well.  I think it's just a question of
> >> whether the first scroll reaches high enough line numbers to trigger a
> >> margin width adjustment.
> >
> > Not sure what you mean by that.  In my experiments, the margin starts
> > at 3 columns, and stays at 3 columns.  There's no adjustment.
> 
> With nlinum-mode it starts at 2 columns for me.  With a terminal 80x49
> or higher, it widens before the bug happens.

Most of my experiments were with 80x38 terminal, where the margin
never widens during the first 2 scrolls.  And I see the problem with
nlinum-mode with that configuration.

I don't think margin widening has anything to do with the problem.
It's enough to have a post-command-hook that changes overlays, I
think.  But that's a guess, although it's based on the fact that the
problematic window doesn't need to be redrawn at all and doesn't need
any margin width adjustment.  The selected window, which is where the
margin width might change, is updated correctly.