bug#19551: 25.0.50; No lighter shown for `text-scale-mode' when it should be `+0'

[Drew Adams]

> > I don't know how or why this happens, but when buffer text is zoomed out
> > and back in, so that the resulting text scale is neutral (100%, zero
> > increase/decrease), the mode-line lighter should say `+0', as specified
> > in the code (`face-remap.el').  For some reason, the lighter just
> > disappears, as if the value of `text-scale-mode-lighter' were nil.
> 
> I don't think this is a bug.  If I understand the code correctly, the
> lighter disappears because the minor mode is turned off when the text
> scale is 0; see this line in `text-scale-increase':
> 
> (text-scale-mode (if (zerop text-scale-mode-amount) -1 1))

Ah, right you are.  I didn't notice that.  I should have guessed this.

> I also don't see anywhere in face-remap.el that specifies the lighter is
> "+0" when the text scale is 0 (the default value of
> `text-scale-mode-lighter' is "+0", but changes as soon as
> text-scale-mode is enabled).

It's done in `text-scale-mode':
(setq text-scale-mode-lighter
        (format (if (>= text-scale-mode-amount 0) "+%d" "%d")
                text-scale-mode-amount))

That sets `text-scale-mode-lighter' to "+0" when `text-scale-mode-amount' is 0.

I'm OK with the bug being closed.