using t_Cs to set undercurl works in vim but not neovim

[Jiyangson]

Problem

The cterm=undercurl command is not working in nvim. I searched for some solutions and had some progress.

1. when setting cterm=undercurl in neovim, it will fall back to underline; but in vim, nothing is shown.
2. set let &t_Cs = "\e[4:3m" in neovim, it still fall back to underline; but in vim, undercurl occurs.
   what should I do to make undercurl occur in neovim. Thanks.

Steps to reproduce

nvim --clean
# type `asdf`
:se spell 
:let &t_Cs = "\e[4:3m"
:hi SpellBad     gui=undercurl guisp=red term=undercurl cterm=undercurl
# underline is shown under `asdf`

vim --clean
# type `asdf`
:se spell 
:let &t_Cs = "\e[4:3m"
:hi SpellBad     gui=undercurl guisp=red term=undercurl cterm=undercurl
# **undercurl** is shown under `asdf`

Expected behavior

nvim shows undercurl

Neovim version (nvim -v)

0.10.0-dev

Vim (not Nvim) behaves the same?

no, 8.1.3741

Operating system/version

20.04.1-Ubuntu

Terminal name/version

bash, version 5.0.17

$TERM environment variable

xterm

Installation

appimage

[zeertzjq]

t_Cs is not supported

[Jiyangson]

Is there a way to work around that do the t_Cs thing in neovim? Or what method should I try? change a terminal or something?

[zeertzjq]

What terminal are you using? Bash is not a terminal and XTerm doesn't supoort undercurl.

[Jiyangson]

I use kitty and it works fine (it shows undercurl as usual). But when I open nvim in a docker container, it has this problem.

[zeertzjq]

Hmm, the value of $TERM should be xterm-kitty inside kitty, not xterm.

[JCallicoat]

I have the same issue with st 0.9 (with undercurl patch) and tmux next-3.5 built from git head like a week ago. I have TERM=screen-256color inside of tmux (which I need to use for key compatibility with old terminfo databases shipped on servers I administer via ssh). Undercurl works fine outside of tmux (TERM=st-256color) in neovim, but shows only an underline inside tmux.

I have the following overrides in tmux.conf just to ensure the feature is enabled.

set -as terminal-overrides ',*:Smulx=\E[4::%p1%dm'  # undercurl support
set -as terminal-overrides ',*:Setulc=\E[58::2::%p1%{65536}%/%d::%p1%{256}%/%{255}%&%d::%p1%{255}%&%d%;m'  # underscore colours - needs tmux-3.0

And outside of neovim undercurl is displayed correctly inside of tmux with, e.g., printf '\e[4:3mUndercurl\e[0m\n'. But inside neovim running inside tmux I only get an underline.

I assume there is some kind of feature detection based on the value of TERM? Is there any way to override that to force the feature to be enabled?

[Jiyangson]

For anyone who has a similar problem, I asked chatgpt and got a solution. Here is the answer from gpt

When you use docker exec to enter a Docker container, the $TERM environment variable is set to xterm by default because it is a widely recognized and compatible setting for most programs. This environment variable informs programs running on the terminal about the type and capabilities of the terminal so that they can adjust their behavior accordingly, such as how to interpret colors and handle cursor movements.
xterm-kitty is specifically for the Kitty terminal emulator, and Kitty uses it to enable additional features that may not be available in other terminal environments. When you start a Docker container terminal from Kitty, you need to explicitly set the terminal type to retain some of the underlying features of Kitty.
To set the $TERM variable in the container to xterm-kitty, you can pass the environment variable when connecting to the container:

docker exec -it -e TERM=xterm-kitty [container-id] /bin/bash

Note that to fully utilize the features provided by xterm-kitty, your container needs to have the appropriate terminal information files (terminfo). If they are not present, some features specific to xterm-kitty may not work as expected. If the base image of the container does not include the Kitty terminfo file, you may need to copy this file from the host machine to the corresponding location in the container, which is usually /usr/share/terminfo/x/xterm-kitty.

For me, I just copy xterm-kitty in my kitty install path and it works.

docker cp ~/.local/kitty.app/share/terminfo/x/xterm-kitty container_id:/usr/share/terminfo/x/