[barrkel]

Whatever TERM is set for you outside tmux needs to have a full ncurses terminfo with all the extended ncurses-specific codes. Assuming that screen and screen-256color on your system is similarly fully-featured with respect to ncurses, then you'll be in a good position. If you're lucky, TERM=screen-256color inside tmux and you're good to go.

Without that, there are a variety of hacks at different levels of the processing stack you can use to fix things. You could set TERM to screen.<your-terminal> if your system has the right settings for screen.<your-terminal>; or you could try setting it to <your-terminal> if <your-terminal> is sufficiently xterm-like.