I'm using iTerm2 3.0.10 on macOS which connects via SSH to an Ubuntu 14.04 VM running Neovim 0.1.6-dev and Tmux 2.0.

I have the following vim config:

packadd vim-solarized8
colorscheme solarized8_dark
set t_8f=^[[38;2;%lu;%lu;%lum
set t_8b=^[[48;2;%lu;%lu;%lum
set termguicolors

(the ^[ are actual escape sequences)

This works pretty well without Tmux, like this:

But as soon as I start nvim inside Tmux, it looks FUCKED. Like this:

My TERM is xterm-256color outside of tmux, and screen-256color inside.

I've tried pretty much "everything" but no success. Any ideas why?