This function seems to be doing something weird when emacs is running in tmux. It makes `C-y` evaluate as 'mark set'. :(

wtf.el

;; normalize copy and paste for osx
(defun copy-from-osx ()
  (shell-command-to-string "pbpaste"))
(defun paste-to-osx (text &optional push)
  (let ((process-connection-type nil))
    (let ((proc (start-process "pbcopy" "*Messages*" "pbcopy")))
      (process-send-string proc text)
      (process-send-eof proc))))
(if (eq system-type 'darwin)
    (progn
      (setq interprogram-cut-function 'paste-to-osx)
      (setq interprogram-paste-function 'copy-from-osx)))