Macvim can not load custom Powerline fonts when started from tmux session
I stumbled over the solution with some searching. The issue issue can be fixed with the reattach-to-user-namespace tool.

$ brew install reattach-to-user-namespace


Add the following line to the end of your ~/.tmux.conf file

set-option -g default-command "reattach-to-user-namespace -l zsh"