Copy, with line wrapping!
If you've been trying to copy/paste text from a multi-pane tmux session with the mouse, you've probably been pretty pissed at the blissful ignorance a terminal application has of the rodent in your hand.
The alternative, which is quote-unqoute native copy/pasting using copy-mode takes a bit to get used to. So this is one solution for copying and pasting lines from a session with correct line wrapping behaviour, albeit keyboard only.
Disclaimer
Since copy-mode has similar concepts of marks, regions, and temp buffers to Emacs .. you'll probably find it straight forward if you're familar with Emacsen. For people using vi-mode in tmux, the same still applies but obviously the default key bindings will differ alot from what I show below.