Is tmux supports cterm=undercurl used in VIM?

Question

In my vim config I've added undercurl value for badly spelled words

hi SpellBad guifg=NONE cterm=undercurl

Outside the tmux it works fine, but inside of it I continue to see like it is set to underline instead of undercurl.

To clarify that even in vim I couldn't see curly line until I added to my .vimrc this:

let &t_Cs = "\e[4:3m"
let &t_Ce = "\e[4:0m"

I did some long research, it's normal for tmux to behave like this? It seems to me that it doesn't support it at all.

Software conf: Ubuntu 18.04.3 LTS, Tmux 3.0a, Vim 8.0.1453

Answer 1

If your tmux is new enough to support this then you need to add the Smul capability, see https://man.openbsd.org/tmux#TERMINFO_EXTENSIONS:

set -as terminal-overrides ',*:Smulx=\E[4::%p1%dm'

If you want coloured underlines you wil also need to add Smulc.