For the newest tmux 2.1, to scroll with your mouse sanely, this is the right answer:
set -g mouse on
"# sane scrolling:
bind -n WheelUpPane if-shell -F -t = "#{mouse_any_flag}" "send-keys -M" "if -Ft= '#{pane_in_mode}' 'send-keys -M' 'copy-mode -e; send-keys -M'"
it's not enough to just reload your .tmux.conf you need to restart your tmux, e.g. tmux kill-server && tmux
or just quit the services that are using tmux.
taken from https://github.com/tmux/tmux/issues/145