Put the following Emacs Lisp into a file named repro.el:
(get-buffer-create "repro")
(fundamental-mode)
(erase-buffer)
(set-variable 'show-trailing-whitespace t)
(dotimes (_ 3)
(insert "x\t\n\f\t\n"))
(message "%s" (substitute-command-keys "Now type \\[delete-trailing-whitespace]"))
Now invoke emacs with `-Q --load repro.el`. You'll see a buffer with a
few lines, each with a red smear at the end indicating trailing
whitespace (the TAB character).
Now, like the echo area is telling you, type M-x
delete-trailing-whitespace RET. You'll see the red go away on half the
lines (the ones with an "x") but remain on the lines with form feeds.
It's not entirely clear if this is a bug at all, since the doc for
delete-trailing-whitespace doesn't specifically mention this situation;
but it does say that that function considers form feeds to be _not_
whitespace, and also that the function deletes whitespace at the ends of
lines, so it seems it ought to have deleted _all_ the tabs in this case.
In GNU Emacs 25.3.1 (x86_64-apple-darwin13.4.0, NS appkit-1265.21 Version 10.9.5 (Build 13F1911))
of 2017-09-12 built on builder10-9.local
Windowing system distributor 'Apple', version 10.3.1404
Configured using:
'configure --with-ns '--enable-locallisppath=/Library/Application
Support/Emacs/${version}/site-lisp:/Library/Application
Support/Emacs/site-lisp' --with-modules'
Configured features:
NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES
Important settings:
value of $LANG: en_US.UTF-8
locale-coding-system: utf-8-unix
Major mode: Emacs-Lisp
Minor modes in effect:
global-magit-file-mode: t
magit-auto-revert-mode: t
global-git-commit-mode: t
async-bytecomp-package-mode: t
csv-field-index-mode: t
pyvenv-mode: t
projectile-mode: t
desktop-save-mode: t
savehist-mode: t
shell-dirtrack-mode: t
global-auto-revert-mode: t
recentf-mode: t
ido-everywhere: t
diff-auto-refine-mode: t
global-hl-line-mode: t
override-global-mode: t
tooltip-mode: t
global-eldoc-mode: t
eldoc-mode: t
mouse-wheel-mode: t
file-name-shadow-mode: t
global-font-lock-mode: t
font-lock-mode: t
blink-cursor-mode: t
auto-composition-mode: t
auto-encryption-mode: t
auto-compression-mode: t
line-number-mode: t
transient-mark-mode: t
Recent messages:
History item: 279 [3 times]
Mark saved where search started
Truncate long lines enabled
Mark saved where search started
Mark set [3 times]
Invoking man open in the background
open man page formatted
Mark saved where search started
Making completion list...
Scanning for dabbrevs...done
Load-path shadows:
/Users/erichan/.emacs.d/elpa/let-alist-1.0.5/let-alist hides /Applications/Emacs.app/Contents/Resources/lisp/emacs-lisp/let-alist
Features:
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pp browse-kill-ring advice eww mm-url gnus gnus-ems nnheader mail-utils
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tree-widget wid-edit ido vc-git diff-mode ibuf-ext ibuffer ibuf-macs
hl-line dired-x dired grep compile comint ansi-color ring find-func
rotate cl cl-macs cl-seq face-remap edmacro kmacro tls gnutls
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info package epg-config seq byte-opt gv bytecomp byte-compile cl-extra
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tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
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mouse jit-lock font-lock syntax facemenu font-core frame cl-generic cham
georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese charscript case-table epa-hook
jka-cmpr-hook help simple abbrev minibuffer cl-preloaded nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote kqueue cocoa ns multi-tty
make-network-process emacs)
Memory information:
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(symbols 48 65197 0)
(miscs 40 5271 10928)
(strings 32 197662 8798)
(string-bytes 1 5932111)
(vectors 16 82902)
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(floats 8 1011 325)
(intervals 56 64230 72)
(buffers 976 134))