Cannot copy/paste anymore in tmux 2.1
This is happening because you enabled your program to capture and use your mouse inputs so that your terminal app has to forward all mouse events.

However, most terminals have 'override' capability. In most cases holding something like Shift or Ctrl key brings control back to terminal and enables to select text, right-click and make other actions.

I found out that for Terminal app the override key is Fn; for iTerm2 it's Option.