https://wiki.archlinux.org/index.php/Tmux#Mouse_scrolling
Mouse scrolling
Note: This interferes with selection buffer copying and pasting. To copy/paste to/from the selection buffer hold the shift
key.
If you want to scroll with your mouse wheel, ensure mode-mouse is on in .tmux.conf
set -g mouse on
You can set scroll History with:
set -g history-limit 30000
For mouse wheel scrolling as from tmux 2.1 try adding one or both of these to ~/.tmux.conf
bind-key -T root WheelUpPane if-shell -F -t = "#{alternate_on}" "send-keys -M" "select-pane -t =; copy-mode -e; send-keys -M"
bind-key -T root WheelDownPane if-shell -F -t = "#{alternate_on}" "send-keys -M" "select-pane -t =; send-keys -M"
Copy from tmux to clipboard
Hold the shift
key to select the text, then copy & paste.