Bug#1123509: xterm: various issues with the U+FE0F VARIATION SELECTOR-16 (VS16) character after emoji

[Vincent Lefevre <vincent@vinc17.net>]

Package: xterm
Version: 405-1
Severity: important

With xterm 405, the use of the U+FE0F VARIATION SELECTOR-16 (VS16)
character after an emoji can completely corrupt the display with
Mutt. GNU Screen also gets broken with the command below (issues
with the last line of the terminal). I suspect that this is due
to an inconsistency between the xterm behavior and wcwidth(),
which may affect various applications that rely on wcwidth().
I have not checked wcswidth().

But there are issues even with simple output. In a 80-column terminal:

  perl -C -e 'print "\x{2642}\x{FE0F}"x60, "\n"'

I get "♂♂" in the last two columns, which is inconsistent with what
is output before. And in case of scrolling, the spaces are missing
in the second line.

And selection/deselection of such output gives random behavior.
In particular:
  * Spurious ♂ characters appear where there were spaces.
  * Some ♂ characters still appear in reverse video after everything
    has been unselected.

-- System Information:
Debian Release: forky/sid
  APT prefers unstable-debug
  APT policy: (500, 'unstable-debug'), (500, 'stable-updates'), (500, 'stable-security'), (500, 'stable-debug'), (500, 'proposed-updates-debug'), (500, 'unstable'), (500, 'testing'), (500, 'stable'), (1, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 6.7.12-amd64 (SMP w/16 CPU threads; PREEMPT)
Kernel taint flags: TAINT_WARN
Locale: LANG=C.UTF-8, LC_CTYPE=C.UTF-8 (charmap=UTF-8), LANGUAGE not set
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages xterm depends on:
ii  libc6           2.42-6
ii  libfontconfig1  2.15.0-2.4
ii  libfreetype6    2.13.3+dfsg-1
ii  libice6         2:1.1.1-1
ii  libtinfo6       6.5+20251123-1
ii  libutempter0    1.2.1-4
ii  libx11-6        2:1.8.12-1
ii  libxaw7         2:1.0.16-1
ii  libxext6        2:1.3.4-1+b3
ii  libxft2         2.3.6-1+b4
ii  libxinerama1    2:1.1.4-3+b4
ii  libxmu6         2:1.1.3-3+b4
ii  libxpm4         1:3.5.17-1+b3
ii  libxt6t64       1:1.2.1-1.3
ii  xbitmaps        1.1.1-2.2

Versions of packages xterm recommends:
ii  luit [luit]  2.0.20250912-1
ii  x11-utils    7.7+7

Versions of packages xterm suggests:
pn  xfonts-cyrillic  <none>

-- no debconf information

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / Pascaline project (LIP, ENS-Lyon)