Sane copy pasta - tmux 2.5 + neovim + terminal (tested on Alacritty/iTerm2)

.tmux.conf

# References:
# https://github.com/tmux/tmux/issues/543
# https://github.com/tmux/tmux/issues/543#issuecomment-298193820
#
# PREREQUISITE: 
# ❯ brew install reattach-to-user-namespace --wrap-pbcopy-and-pbpaste

# Add the following snippet to your .tmux.conf -
setw -g mode-keys vi
set -g default-shell $SHELL 
set -g default-command "reattach-to-user-namespace -l ${SHELL}"
bind-key -T copy-mode-vi y send-keys -X copy-pipe-and-cancel "reattach-to-user-namespace pbcopy"
bind-key -T copy-mode-vi MouseDragEnd1Pane send-keys -X copy-pipe-and-cancel "reattach-to-user-namespace pbcopy"

.vimrc

" In your .vimrc, or neovim's init.vim file:
" Add the following line -
set clipboard=unnamed