The show-trailing-whitespace option was set with custom-set-variables,
probably because it doesn't work with plain setq (becausee it's a
buffer-local variable). However setq-default does the job, and
prevents the emacs UI from interfering with it.
(put 'downcase-region 'disabled nil)
;; I want to see trailing whitespace.
(put 'downcase-region 'disabled nil)
;; I want to see trailing whitespace.
-(custom-set-variables '(show-trailing-whitespace t))
+(setq-default show-trailing-whitespace t)