bug#10960: linum-mode does not work properly when changing font size

[Eli Zaretskii <eliz@gnu.org>]

> From: Glenn Morris <rgm@gnu.org>
> Date: Tue, 06 Mar 2012 12:07:37 -0500
> Cc: 10960@debbugs.gnu.org
> 
> Le Wang wrote:
> 
> > Here is the stackoverflow question:
> >
> > http://stackoverflow.com/questions/9304192/emacs-linum-mode-and-size-of-font-unreadable-line-numbers
> 
> Here is the existing Emacs report
> 
> http://debbugs.gnu.org/cgi/bugreport.cgi?bug=1255

I don't understand Stefan's response in that bug: what effect exactly
are text-scale-* functions supposed to produce?  Should the text in
the margins be enlarged to the same size as the text in the "text
area" of the window, or should the text in the margins retain its
original size?

And whichever of these two is TRT, why is this considered to be a
problem in linum-mode, and not in the infrastructure (a.k.a. the
display engine)?