bug#50474: 28.0.50; whitespace-mode in non-editable Gnus buffers

[Stephen Berman]

Enabling whitespace-mode in Gnus Group, Summary or Article buffers
removes the Gnus-specific fontification in these buffers.  This happens
when whitespace-color-on calls font-lock-add-keywords, which tests (and
font-lock-mode (not (or font-lock-keywords font-lock-defaults))), and in
the Gnus buffers font-lock-mode is t and font-lock-keywords and
font-lock-defaults are both nil, so the test succeeds and consequently
the following sexps are evaluated:

           (font-lock-mode -1)
           (setq-local font-lock-defaults '(nil t))
           (font-lock-mode 1)

which removes the buffer fontification.  Adding `(font-lock-mode -1)' to
the gnus-{group,summary,article}-mode definitions prevents this (i.e.,
keeps the Gnus-specific fontification), but then the whitespace-mode
faces are not applied to the Gnus buffers.


In GNU Emacs 28.0.50 (build 16, x86_64-pc-linux-gnu, GTK+ Version 3.24.29, 
cairo version 1.17.4)
 of 2021-09-08 built on strobelfs
Repository revision: 439ca062c830a7f3288a2dac48457edf5b1bfc59
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Linux From Scratch 10.2-rc1

Configured using:
 'configure --with-native-compilation 'CFLAGS=-Og -g3'
 PKG_CONFIG_PATH=/opt/qt5/lib/pkgconfig'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LCMS2 LIBSYSTEMD LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG
RSVG SECCOMP SOUND THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM
GTK3 ZLIB