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

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 17 Jun 2016 10:25:00 +0000
> From: Alan Mackenzie <acm@muc.de>
> 
> 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.

It's quite clear from the code that this is the intended behavior.
The comments for font-lock-set-defaults (the function and the
variable) and for font-lock-refresh-defaults are especially revealing.

> 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.

I have added to the Emacs manual explanation of how to make the
customization of font-lock-maximum-decoration effective in a file
buffer.  In a nutshell, you need to either customize before visiting
the file, or kill the buffer and revisit the file after the
customization.

I think this fixes this bug, so I'm closing it.

Thanks.