iterm + tmux isn't allowing me to scroll up with the mouse

Solution 1:

The recent release of tmux 2.1 changed some of the configuration options. This bit of code (taken from here) restores the previous functionality:

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 

Add/place these in the ~/.tmux.conf file

Apply with tmux source-file ~/.tmux.conf

Solution 2:

In iTerm2 all you need to do is to go to Preferences > Profile > Terminal and check ‘Save lines to scrollback when an app status bar is present’.

Solution 3:

One cause of this is the "Secure Keyboard Entry" feature of iTerm.

To solve this problem, you can go with iTerms -> Secure Keyboard Entry, and de-select that item.

I came across this issue with my Logitech MX Master 2S as well, and unfortunately other solutions mentioned here didn't work to me.