Why do Vim colors look different inside and outside of tmux?

Question

Environment:

• Fedora 25 (4.9.12-200.fc25.x86_64)
• GNOME Terminal 3.22.1 Using VTE version 0.46.1 +GNUTLS
• VIM - Vi IMproved 8.0 (2016 Sep 12, compiled Feb 22 2017 16:26:11)
• tmux 2.2

I recently started using tmux and have observed that the colors within Vim change depending on whether I'm running inside or outside of tmux. Below are screenshots of Vim outside (left) and inside (right) of tmux while viewing a Git diff:

My TERM variable is

• Outside tmux: xterm-256color
• Inside tmux: screen-256color

Vim reports these terminal types as expected (via :set term?):

• Outside tmux: term=xterm-256color
• Inside tmux: term=screen-256color

Vim also reports both instances are running in 256-color mode (via :set t_Co?):

• Outside tmux: t_Co=256
• Inside tmux: t_Co=256

There are many similar questions out there regarding getting Vim to run in 256-color mode inside tmux (the best answer I found is here), but I don't think that's my problem given the above information.

I can duplicate the problem outside of tmux if I run Vim with the terminal type set to screen-256color:

$ TERM=screen-256color vim

So that makes me believe there's simply some difference between the xterm-256color and screen-256color terminal capabilities that causes the difference in color. Which leads to the question posed in the title: what specifically in the terminal capabilities causes the Vim colors to be different? I see the differences between running :set termcap inside and outside of tmux, but I'm curious as to which variables actually cause the difference in behavior.

Independent of the previous question, is it possible to have the Vim colors be consistent when running inside or outside of tmux? Some things I've tried include:

• Explicitly setting the default terminal tmux uses in ~/.tmux.conf to various values (some against the advice of the tmux FAQ):

    set -g default-terminal "screen-256color"
    set -g default-terminal "xterm-256color"
    set -g default-terminal "screen.xterm-256color"
    set -g default-terminal "tmux-256color"

• Starting tmux using tmux -2.

In all cases, Vim continued to display different colors inside of tmux.

Solution 1

tmux doesn't support the terminfo capability bce (back color erase), which vim checks for, to decide whether to use its "default color" scheme.

That characteristic of tmux has been mentioned a few times -

• Reset background to transparent with tmux?
• Clear to end of line uses the wrong background color in tmux

Solution 2

I had the similar issue before. Comments in blue in Vim were hard to read. In .tmux.conf I set this:

set -g default-terminal "screen-256color"

And in .vimrc:

set background=dark

Now it looks as follows and works both in Gnome Terminal and Cygwin:

Solution 3

Thanks to @egmont's analysis of what colors Vim was outputting when TERM=screen-256color, I was inspired to look at the color scheme Vim is using in the two scenarios.

Vim reports it is using the default color scheme in both cases. I thought that odd because the default color scheme on Fedora 25 (/usr/share/vim/vim80/colors/default.vim) doesn't appear to match the colors I actually see when TERM=xterm-256color. If I explicitly set the color scheme using :colorscheme default when TERM=xterm-256color, Vim's appearance changes to that when TERM=screen-256color. To get the colors back to what they were when I first start Vim, I had to use the ron color scheme. Progress!

I found an Ask Ubuntu answer that suggests that when Vim reports it is using the default color scheme, it doesn't necessarily mean default.vim but rather some theme-specific color scheme. As the answer points out, a dark theme (which I am using) corresponds to the ron color scheme, just as I discovered above. (Even though this post is with respect to Ubuntu, I'm assuming the OP was using GNOME.)

I also found another question that seems to describe the same problem I'm having. I came across it while searching before I posted this question, but, for some reason, the color scheme didn't strike me as being relevant.

I ended up doing what @LapshinDmitry did in his answer and explicitly set colorscheme ron in my ~/.vimrc file. Now, whether I start Vim inside or outside of tmux, the colors appear the same. The only drawback is if I ever change my desktop theme from a dark flavor to a light flavor, Vim won't automatically switch to the "default" light theme color scheme, which is apparently peachpuff. I can live with that, as I'm unlikely to ever change my theme.

I'm not going to accept this answer because I consider explicitly setting the color scheme in my ~/.vimrc a workaround rather than the solution. If someone can explain why Vim loads a different "default" color scheme depending on the value of TERM, I'll be happy to accept that answer, as I'm more interested in understanding the root cause. I suspect it has something to do with how Vim interprets the terminal capabilities between the two terminfo files.

Comments

[egmont]

Could you please start script, start vim, quit vim and then quit script's shell, and attach the resulting typescript file? Then repeat the same steps with TERM=screen-256color vim instead.

[Rusty Shackleford]

@egmont Here you go: typescript-xterm-256color and typescript-screen-256color.

[egmont]

I can see 256-color escape sequences in the xterm-256color version (search for "38;5;" in the file), but cannot in the screen-256color. E.g. wherever there's a \e[38;5;81m in xterm-256color (color #81 of the 256-color palette, a middle blue shade), screen-256color contains \e[34m instead (the standard blue of the 8-color palette). So the main question is why does vim not use the 256-color palette in case of TERM=screen-256color.

[egmont]

Another interesting thing is that the xterm-256color version seems to print the screen twice, first with the legacy colors and then with the 256-color palette. Search for e.g. "F1" in the file, you'll find two matches in xterm-256color, the first with \e[34m color, the second with \e[38;5;81m. I don't know what's going on here.

[Rusty Shackleford]

I've only noticed the problem in Vim so far (like I said, I only recently started using tmux). For example, my terminal prompt and Git CLI output appear to have the same colors both inside and outside of tmux. I do not set termguicolors in my .vimrc and :set termguicolors? reports notermguicolors in both cases. I'll look at the help topic you referenced; thanks!

[egmont]

I'm also curious why vim behaves this way. After all, the bce capability should have nothing to do with the available number of colors.

[Rusty Shackleford]

@egmont I interpreted @ThomasDickey's answer as: 1) Vim checks the bce capability of the terminal, 2) if present, select color scheme "A", 3) otherwise, select color scheme "B". I believe the color depth is the same in both cases (i.e. 256 colors, as t_Co shows). It's just that color scheme "B" on my system happens to only use an 8-color palette even though 256 colors are possible (I think that explains what you observed in your typescript analysis; please correct me if I'm wrong). Remember, after I run :colorscheme ron, I see 256-color escape sequences in the same Vim session.

[egmont]

Thanks for the explanation - I still cannot see any rationale behind this though. Nevermind. vim has really weird design decisions.

[Adam Keenan]

This solved it for me. For some reason, inside tmux, vim wasn't detecting a dark background. Explicitly setting bg=dark in my vimrc fixed it.

[Abinash Dash]

Worked for me too. I thought the problem was with stterm because in xfce4-terminal everything looked comfortable(comments appearing in cyan instead of blue which is very hard to read with dark background). after this change everything works fine. But remember to add unlet! skip_defaults_vim source $VIMRUNTIME/defaults.vim to vimrc file to use syntax highlighting etc...