Put "mouse-select-pane on" in .tmux.conf.

If you check the tmux manpage and search for 'mouse', there's a whole bunch of mouse-enabling options; you might as well turn them all on -- except for "mouse-utf8", which is a compatibility option that can potentially screw things up.

The downside to enabling mouse support in tmux is that it disables mouse support in the outer terminal (the one tmux is running inside). If you select text in tmux, it will go to the tmux clipboard instead of your system clipboard. You can get around that by holding shift before selecting text, but some people get annoyed by the extra action required.

Try these options:

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

Learned Tmux a few weeks ago and use it every day now. You can enable Mouse Mode, on Macs you can install extra Tmux features, and iTerm2 has mouse mode built in.

If you really want this, you should use a tiling window manager and use "focus follows cursor" or "focus on click".