Tmux capture the history message

To scroll back tmux history, run
Control + B, then PageUp or PageD 

Alternatively, we can also capture the history to a file by executing tmux capture-pane command in another pane:

tmux capture-pane -S -2000 -J -p -t %123 >tmux.log

Where %123 is the unique pane id. To find out the pane id, run the following command inside a target pane:
echo $TMUX_PANE

See man tmux for other parameters.