tmux set -g mouse-mode on doesn't work

Question

So this option has been renamed in version 2.1 (18 October 2015)

From the changelog:

 Mouse-mode has been rewritten.  There's now no longer options for:
    - mouse-resize-pane
    - mouse-select-pane
    - mouse-select-window
    - mode-mouse

  Instead there is just one option:  'mouse' which turns on mouse support

So this is what I'm using now in my .tmux.conf file

set -g mouse on

As @Graham42 noted, mouse option has changed in version 2.1. Scrolling now requires for you to enter copy mode first. To enable scrolling almost identical to how it was before 2.1 add following to your .tmux.conf.

set-option -g mouse on

# make scrolling with wheels work
bind -n WheelUpPane if-shell -F -t = "#{mouse_any_flag}" "send-keys -M" "if -Ft= '#{pane_in_mode}' 'send-keys -M' 'select-pane -t=; copy-mode -e; send-keys -M'"
bind -n WheelDownPane select-pane -t= \; send-keys -M

This will enable scrolling on hover over a pane and you will be able to scroll that pane line by line.

Source: https://groups.google.com/d/msg/tmux-users/TRwPgEOVqho/Ck_oth_SDgAJ

Just a quick heads-up to anyone else who is losing their mind right now:

https://github.com/tmux/tmux/blob/310f0a960ca64fa3809545badc629c0c166c6cd2/CHANGES#L12

so that's just

 :setw -g mouse

this should work:

setw -g mode-mouse on

then resource then config file

tmux source-file ~/.tmux.conf

or kill the server

Paste here in ~/.tmux.conf

set -g mouse on

and run on terminal

tmux source-file ~/.tmux.conf

As @Graham42 said, from version 2.1 mouse options has been renamed but you can use the mouse with any version of tmux adding this to your ~/.tmux.conf:

Bash shells:

is_pre_2_1="[[ $(tmux -V | cut -d' ' -f2) < 2.1 ]] && echo true || echo false"
if-shell "$is_pre_2_1" "setw -g mode-mouse on; set -g mouse-resize-pane on;\
      set -g mouse-select-pane on; set -g mouse-select-window on" "set -g mouse on"

Sh (Bourne shell) shells:

is_pre_2_1="tmux -V | cut -d' ' -f2 | awk '{print ($0 < 2.1) ? "true" : "false"}'"
if-shell "$is_pre_2_1" "setw -g mode-mouse on; set -g mouse-resize-pane on;\
      set -g mouse-select-pane on; set -g mouse-select-window on" "set -g mouse on"

Hope this helps

You can still using the devil logic of setting options depending on your current Tmux version: see my previous answer.

But since Tmux v1.7, set-option adds "-q" to silence errors and not print out anything (see changelog). I recommend to use this feature, it's more readable and easily expandable.

Add this to your ~/.tmux.conf:

# from v2.1
set -gq mouse on
# before v2.1
set -gq mode-mouse on
set -gq mouse-resize-pane on
set -gq mouse-select-pane on
set -gq mouse-select-window on

Restar tmux or source-file your new .tmux.conf

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Side note: I'm open to remove my old answer if people prefer this one