starting tmux or doing tmux attach generates input buffer garbage only under urxvt

[idallen]

Issue description

When starting tmux or doing a tmux attach under urxvt, a line of garbage appears in the input buffer:

$ TERM=xterm SHELL=/bin/sh tmux -vv -Ltest -f/dev/null new
$ ^[]10;rgb:0000/0000/0000^[^[]11;rgb:ff00/ff00/dd00^[

The garbage does not appear when starting tmux or doing the attach from inside of gnome-terminal or xterm.
The same urxvt garbage appears when I set TERM to any of xterm, rxvt-unicode-256color, or xterm.
I can get rid of the input buffer garbage using either backspaces or line-erase (CTRL-U).

Required information

This is the February 7 tmux version next-3.4 patched with the recent zero-width patch.
(The zero-width patch did not cause this; it was happening before the patch.)
This "next-3.4" clone is newer than the previous "next-3.4" I cloned last June.
The one from last June works fine. The newer clone (from February 7) does not.

$ uname -sp && tmux -V && echo $TERM
Linux x86_64
tmux next-3.4
tmux-256color
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 20.04.5 LTS
Release:        20.04
Codename:       focal

tmux-out-1089532.log
tmux-server-1089532.log
tmux-client-1089530.log

[nicm]

This is a bug in urxvt.

[nicm]

You can make this change to urxvt:

https://cvsweb.openbsd.org/cgi-bin/cvsweb/~checkout~/ports/x11/rxvt-unicode/patches/patch-src_command_C?rev=1.6&content-type=text/plain

[idallen]

This is a bug in urxvt.

Indeed it is. How did you know where to find the fix?
Are you watching the bug lists of all the world's terminal emulators? :-)

Building urxvt from source required not only the source here https://github.com/exg/rxvt-unicode
but also libraries http://dist.schmorp.de/libev/ and http://dist.schmorp.de/libptytty/
Then the patch can be applied and a new version compiled and installed. It works!

Closed.

[nicm]

Someone else already reported it to me :-).

[github-actions]

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