First noticed the issue when pasting from the system clipboard stopped
working in vim (only when running inside a tmux session).
The fix is in the following Stack Overflow answer:
http://stackoverflow.com/a/40154047
The necessary wrapper used by the fixed is at:
https://github.com/ChrisJohnsen/tmux-MacOSX-pasteboard
The wrapper can be installed on the Mac with:
brew install reattach-to-user-namespace
From the tmux 2.1 changelog:
* Mouse-mode has been rewritten. There's now no longer options for:
- mouse-resize-pane
- mouse-select-pane
- mouse-select-window
- mode-mouse
Instead there is just one option: 'mouse' which turns on mouse support
entirely.