bug#19721: 25.0.50; Mode-line not redrawn with expose events

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 01 Feb 2015 17:30:06 +0100
> From: martin rudalics <rudalics@gmx.at>
> CC: mituharu@math.s.chiba-u.ac.jp, 19721@debbugs.gnu.org
> 
>  > and (b) there's a tricky condition near the beginning of
>  > redisplay_window that would bypass redisplaying a window, under some
>  > conditions, even if its update_mode_line flag was set (my last patch
>  > attempts at fixing that).
> 
> So if he applies your code and mine we'd probably find out more.  I was
> exactly once able to trigger his initial scenario here on Windows with
> both upper modelines completely disappearing but was not able to repeat
> that experience after that.

I couldn't reproduce the problem with redrawing on expose events
(which is not surprising, since on Windows expose events are almost
never used, except when the frame is first displayed after it's
created).  But I have no trouble at all seeing that the mode-line
glyph row is disabled, by using dump-glyph-matrix.

>  > So I'm not sure this is the right solution.  The situation described
>  > in this report is quite unique, in that the face used for the active
>  > mode line causes the window glyph matrices to be resized each time a
>  > window becomes non-selected one.
> 
> Could you optimize that away (reserving one line more than needed)?

That was my first attempt, and it didn't do a thorough enough job, as
you can see from previous discussions.

>  > It is because of this resizing that
>  > the mode-line row of the current matrix becomes disabled.
> 
> So you mean that we have to update the mode lines of both - the selected
> and the deselected window?

Yes, but not because they become selected/deselected, because their
glyph matrices are resized, and the mode-line row gets disabled in the
process.