Re: `set t_Co=16` in vimrc file has no effect

On 2020-09-26, Nikolay Belikov wrote:
> Hello all,
>
> I've stumbled upon strange behavior in Vim 8.2 regarding `t_Co`
> variable. This might be a bug, but more probably is just a case of
> PEBCAK, so maybe someone could point me towards what I'm doing wrong.
>
> Background: I want to use my terminal emulator (which is plain old
> `xterm`) color scheme across all TUI programs, including Vim. I've
> added `set t_Co=16` to my `.vim/vimrc` file and expected Vim to limit
> itself to 16 base colors (at least this is what all sources on Internet
> say to do, which I managed to find). However, it appears that Vim does
> not honor my will and resets `t_Co` to 256, unless I manually type `:set
> t_Co=16` in command-line mode.
>
> I reduced my `vimrc` to the bare minimum needed to reproduce the
> problem, and now it contains a single line:
>
> set t_Co=16
>
> At this point I'm unsure what to do and ask for your assistance. I
> attach a log captured by running `vim -V100vim.log` as well as version
> info (`vim --version >vim.version`), and I'm willing to provide any
> other info that may be of help.

Vim may change t_Co when it receives a terminal's termresponse.
I noticed the same strange behavior some years ago with Vim setting
t_Co to 256 in an xterm. See

:help v:termresponse
:help t_RV
:helpgrep termresponse

You can test this by adding this to your vimrc,

set t_RV=

and seeing if t_Co is still mysteriously reset to 256.

You may be able to reset t_Co to 16 with this in your vimrc:

autocmd TermResponse * set t_Co=16

I say "may" because I've found that not everything works as I would
expect it to in the environment of a TermResponse autocommand.

See

:help TermResponse

Regards,
Gary

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