Tmux can't focus with mouse and mouse scrolling behaviour changed

I recently upgraded to Ubuntu 16.04 (XUbuntu) and I noticed that the behaviour of tmux has been altered.

I was previous on 14.04 where I was able to focus individual panes by clicking on them with the mouse. In 16.04 this no longer seems to work.

Furthermore, since the upgrade to 16.04, scrolling with the mouse wheel now seems to scroll through the previous terminal commands rather than scrolling up and down of the window pane contents.

This is my .tmux.conf (I prefer Ctrl+A over Ctrl+B):

# remap prefix to Control + a
set -g prefix C-a
# bind 'C-a C-a' to type 'C-a'
bind C-a send-prefix
unbind C-b

set -g mode-mouse on
set -g mouse-resize-pane on
set -g mouse-select-pane on
set -g mouse-select-window on

How can I bring back mouse click focus and pane content mouse wheel scrolling?