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

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Sat, Jun 18, 2016 at 12:56:10PM +0300, Eli Zaretskii wrote:
> > 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.

I haven't read them yet, but I will.  It is not user friendly to have to
set an option before visiting a file.  In this case, a natural use case
would be to want to reduce the decoration level after finding things
being too slow at the default level.

> > 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.

Thanks!

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

OK.  I'll think about trying to fix the code in the near future.

> Thanks.

-- 
Alan Mackenzie (Nuremberg, Germany).