Issue description

On Fedora, 3.3a-1 was the last version of tmux where allow-passthrough on was working with vim's t_RB setting to request the terminal bg color.

Set up a bare environment:

podman run -it --rm fedora:37
dnf install vim tmux
echo 'let &t_RB="\ePtmux;\e\e]11;?\007\e\\"' > ~/.vimrc
echo 'set -g allow-passthrough on' > ~/.tmux.conf
tmux

And then run the following inside of vim:

echo v:termrbgresp

Returns the correct RBG sequence, but moving to fedora:38 (the first broken version is building from commit b202a2f) causes the sequence to stop working, and v:termrbgresp is empty. I have confirmed this also happens building from tmux master.

Required information

Please provide the following information:

• tmux version (tmux 3.3a).
• Platform (Linux x86_64).
• $TERM inside and outside of tmux (tmux-256color).
• Logs from tmux (tmux kill-server; tmux -vv new).