OK, just filled in a bug report for fill-column-indicator.el. Linked issues
so far:

https://github.com/alpaker/Fill-Column-Indicator/issues/71

https://github.com/syl20bnr/spacemacs/issues/6707





2016-10-07 20:55 GMT+02:00 Eli Zaretskii <eliz@gnu.org>:

> > From: Kristóf Katus <kristof.katus@gmail.com>
> > Date: Fri, 7 Oct 2016 16:50:14 +0200
> >
> > 1. Start a graphical emacs:
> > $ emacs -Q
> >
> > 2. Open a new buffer, type some text leaving some lines blank
> >
> > 3. Enable linum-mode:
> > M-x linum-mode
> >
> > 4. Fetch, then load fill-column-indicator.el by:
> > M-x load-file
> >
> > 5. Enable fci-mode:
> > M-x fci-mode
> >
> > 6. Decrease the window width, so that the fill column indicator is not
> shown
> >
> > Observed behaviour
> >
> > On all the empty lines, the line numbers disappear -- except on that
> empty line where your cursor might rest.
> >
> > Expected behaviour
> >
> > Line numbers should be shown on all lines, including empty lines
> independent of the visibility of the fill column
> > indicator.
>
> Please bring this first to the attention of the developers of
> fill-column-indicator.el.  I'm not at all sure this is a core Emacs
> problem.
>
> Thanks.
>