bug#23403: 25.1.50; cua-mode does not interact with cursor-face and themes well

[Jeremy T. Braun]

I've had issues where themes couldn't change cursor colors.  I finally
figured out the culprit, cua-mode.  Details on what I noticed and how I
found it follow.



I believe the "correct" way for themes to interact with the cursor color
is to modify the :background attribute of the face named cursor.  See:
https://www.gnu.org/software/emacs/manual/html_node/emacs/Cursor-Display.html

cua-mode defines a variable cua-normal-cursor-color.  By default this is
set to one of initial-cursor-color, cursor-color from initial-frame-alis
or default-frame-alist, the frame-parameter 'cursor-color, or "red" if
those are all not true.

When cua-enable-cursor-indications is enabled, cua--update-indications
is run as a part of cua--post-command-handler, which is added to
post-command-hook.

When run, cua-enable-cursor-indications typically calls set-cursor-color
to one of the cua-*-cursor-color variables.

The result?  Themes can change the cursor-color (via the face or via
set-cursor-color, but after they are loaded/run, the post-command-hook
eventually runs cua--update-indications, which overwrites the theme's
new "normal" cursor color with whatever was previously stored in
cua-normal-cursor-color (for "normal" buffers, anyway).

I first noticed this because I run emacs in --daemon mode a lot.  When
emacs is in --daemon mode, I think that .emacs/init.el is run in the
frame/context of the terminal or background emacs process. When my
init.el is run, (frame-parameter nil 'cursor-color) returns "white",
which cua-normal-cursor-color is set to.  When I switch between the
background-mode = dark/light color themes linked below, the cursor
always stays white, and it's illegible in the "light" theme.
https://github.com/sellout/emacs-color-theme-solarized

When I launch emacs without --daemon, my init.el loads the theme first,
which sets frame-parameter.cursor-color to #839496, which initializes
cua-normal-cursor-color, which is legible in both dark and light modes.

I'm not sure how "best" to fix this.  Options I see:
* Change documentation on custom themes, requiring that in addition to
  modifying the cursor face, they should also modify cua-*-cursor-color.
* Change cua-mode to be aware that themes may modify the cursor face,
  and either update cua-normal-cursor-face to reflect the cursor face's
  :background attribute, or maybe have cua--update-indications use
  something like:
    (or cua-normal-cursor-face (grab the :background attribute of 
cursor-face))
  And then the user can set/customize cua-normal-cursor-face to nil to
  allow the theme to control it.
* Document this, and require the user to hook theme loading and
  after-make-frame-functions to update cua-normal-cursor-color to be
  (frame-parameter frame 'cursor-color), which would have been set by
  the theme at that point.
* Other?

Thanks,
Jeremy

In GNU Emacs 25.1.50.2 (x86_64-pc-linux-gnu, GTK+ Version 3.10.8)
 of 2016-04-25 built on lgw01-57
Windowing system distributor 'The X.Org Foundation', version 11.0.11501000
System Description:    Ubuntu 14.04.4 LTS

Configured using:
 'configure --build=x86_64-linux-gnu --prefix=/usr
 '--includedir=${prefix}/include' '--mandir=${prefix}/share/man'
 '--infodir=${prefix}/share/info' --sysconfdir=/etc --localstatedir=/var
 '--libdir=${prefix}/lib/x86_64-linux-gnu'
 '--libexecdir=${prefix}/lib/x86_64-linux-gnu' --disable-maintainer-mode
 --disable-dependency-tracking --prefix=/usr --sharedstatedir=/var/lib
 --program-suffix=-snapshot --with-x=yes --with-x-toolkit=gtk3
 'CFLAGS=-g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat
 -Werror=format-security' CPPFLAGS=-D_FORTIFY_SOURCE=2
 'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro''

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-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:
For information about GNU Emacs and the GNU system, type C-h C-a.

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message puny seq byte-opt gv bytecomp
byte-compile cl-extra help-mode cconv cl-loaddefs pcase cl-lib dired
dired-loaddefs format-spec rfc822 mml easymenu mml-sec password-cache
epa derived epg epg-config gnus-util rmail rmail-loaddefs mm-decode
mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader
sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
time-date mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel term/x-win x-win term/common-win x-dnd tool-bar
dnd fontset image regexp-opt fringe tabulated-list newcomment elisp-mode
lisp-mode prog-mode register page menu-bar rfn-eshadow timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors 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
obarray 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
dbusbind inotify dynamic-setting system-font-setting font-render-setting
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 97251 6761)
 (symbols 48 20808 0)
 (miscs 40 433 132)
 (strings 32 17940 4659)
 (string-bytes 1 555138)
 (vectors 16 13743)
 (vector-slots 8 447013 4512)
 (floats 8 181 19)
 (intervals 56 231 0)
 (buffers 976 11)
 (heap 1024 33150 1055))