ssh Breaks vim Color Scheme When Using tmux

Question

In tmux, my terminal's vim color scheme changes after connecting then immediately disconnecting from a remote server via ssh. The exact command sequence is:

tmux
vim <my_file>  # vim colors correct
ssh <my server> 
exit  # Immediately disconnect
vim <my_file> # vim colors WRONG

Note: No difference was observed between tmux and tmux -2. I also saw no difference in tmux info before and after the ssh connection.

I am unable to determine why connecting/disconnecting via ssh affects how vim runs on my local machine. This only happens with tmux. It does not happen with my normal terminal.

Color Comparison:

Image Comparison of the Terminal

Setup Info:

• MacOS Mojave 10.14.6
• Terminal: MacOS 2.9.5
• Shell: oh-my-zsh
• tmux: 2.9a (installed via brew)
• vim: 8.1
• Vim Package Manager: Vundle
• Color Scheme: Gruvbox

MWE .zshrc

"     Vundle Start
set rtp+=~/.vim/bundle/Vundle.vim
call vundle#begin()
Plugin 'VundleVim/Vundle.vim'
Plugin 'morhetz/gruvbox'
call vundle#end() 
filetype plugin indent on
"     Vundle END

syntax on

colorscheme gruvbox
set background=dark

(Adding set t_Co=256 to .vimrc seemed to have no effect)

.tmux.conf

set-option -g default-shell /usr/local/bin/zsh
set -g default-terminal "screen-256color"

Answer 1

This is probably being caused by an ANSI Escape Sequence in your prompt of the remote server, that is perhaps putting the terminal in a certain mode and not resetting it back to normal when done.

It's hard to tell which sequence exactly might be causing this... Perhaps ESC [ 2m for "low intensity mode"?

The reason why this only happens inside tmux is that tmux implements large part of its own escape sequences, so sometimes it will behave differently from the terminal itself.

In order to further troubleshoot this issue, first try to capture the escape sequences you are getting when you connect to the remote server. You can use the script(1) command to do that:

$ script
$ ssh <my_server>
my_server$ exit
$ exit
Saved output in file `typescript`
$

You need an extra exit to stop the script session. The characters will be saved in file typescript.

You can then inspect the contents of the file with:

$ cat -v typescript

Which will show the escape sequences in a printable way, so that they don't activate and you can see them. The ESC key is showed as ^[. So if you see a sequence ^[[2m, that's the "low intensity" one I mentioned above.

Even if that's not the actual one that's triggering this problem, you could edit your question to update it with the exact escape sequences you're seeing, that would help us narrow out which one is triggering this particular behavior.

If it's really being caused by escape sequences from the remote server, then check your prompt on that machine to see whether that's what's sending those particular sequences. (In bash, look at $PS1 and perhaps $PROMPT_COMMAND.) You might want to fix them to either stop using that particular sequence, or to reset settings or undo the effects of the particular sequence once done with it.

---

UPDATE: After using the procedure above to diagnose the issue, we noticed that the cause of the problem was the server-side .zshrc file gruvbox_256palette_osx.sh, which modifies the color palette of the terminal when executed.

The server side has $TMUX unset so the gruvbox script was not using the proper escape sequences, getting the color palette out of sync and producing the difference in colors that was observed here.