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

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

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’.

I came across this issue with my Logitech MX Master 2S as well, and unfortunately all the above solution didn't work to me.

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

Hope it helps.