Steps to reproduce:

debvm-create -- --hook-dir=/usr/share/mmdebstrap/hooks/useradd \
  --include=linux-image-generic,xserver-xorg-core,xserver-xorg-input-libinput,lightdm,icewm,xterm,neovim,tmux
debvm-run -g

Inside the VM start an xterm, a tmux and inside:

vi -c 'set spell' <(echo bla blaj)

Running the same vi command outside of tmux will underline the spelling
errors but not inside tmux.

You can also run debvm-create -r testing or trixie to verify that tmux
3.5a-3 works and update just tmux to see that it is enough to trigger
the behaviour change.

Cheers Jochen

Hi,

* Jochen Sprickerhof <jspricke@debian.org> [2025-12-03 22:27]:

This is due to:

https://github.com/tmux/tmux/commit/7499d925da697498b7ee69b0effaa8b5176a50bf

I am unsure if this is a better behaviour then before given that tmux
announces Smulx support. As a workaround I switched to a terminal
emulator that supports it.

Cheers Jochen