>
> So, I tried the patch (applied on top of 04ade0508d) and it does not fix 
> the problem.  After `M-x global-text-scale-adjust RET', the command 
> immediately makes the font larger (without me ever pressing `+'), and 
> `-' still does nothing.
>

Thanks.  Can you try the attached patch instead, and post what you see in 
*Messages*?