bug#23783: Emacs 25: Changing font-lock-maximum-decoration doesn't work.

[Alan Mackenzie]

Hello, Emacs.

In GNU Emacs 25.0.94.2 (x86_64-unknown-linux-gnu, GTK+ Version 3.18.7)
 of 2016-06-07 built on acm
Repository revision: 9d5ccebeba0506f7280662630f0ee85a52c8a327
Configured using:
 'configure --with-tiff=no --with-gif=no --with-gpm'

Do this:
1. emacs -Q
2. M-x customize-variable <CR> font-lock-maximum-decoration <CR>
   o - set the level to 1 (minimum decoration) and apply the new
     setting.
3. C-x C-f .../src/xdisp.c
4. Note that font locking is indeed at level 1.  Note, indeed, how fast
   scrolling works.  ;-)
5. M-x customize-variable <CR> font-lock-maximum-decoration <CR>
   o - set the level to t (maximum decoration) and apply.
6. (In the xdisp.c window) M-x font-lock-mode, twice.
7. Note that the font locking remains at level 1.

This last is surely a bug.  Enabling font locking in a buffer should use
the current valid setting of font-lock-maximum-decoration.

Discussion:
-----------

There are no instructions in the Emacs manual which tell you how to
change decoration levels.  It merely states on page "Font Lock Mode"
that "You can customize the variable `font-lock-maximum-decoration' to
alter the amount of fontification applied by Font Lock mode, for major
modes that support this feature.".  This doesn't then say what is to be
done after doing such customization, for it to take effect.

There is a non-interactive function `font-lock-refresh-defaults' which
works, but this function isn't called from anywhere in Emacs.

-- 
Alan Mackenzie (Nuremberg, Germany).