FYI - after you do this on Mac OS you can't copy to the system clipboard from the buffer using mouse unless you push `fn` before you click on the pane.
I think there are workarounds to get from the tmux copy buffer onto the system clipboard but I usually just `Ctrl-b z` to zoom the pane to full-screen, hold down `fn` button to select text, then `Ctrl-C` like normal.
IIRC, this was fixed in the last few releases- it had to do with OS X's weird user-session/permissions management. At the very least, I was able to comment out the following workaround:
1. If I click my mouse to drag and select what I want to copy, the selection is copied to the clipboard when I release. Selection is highlighted with a yellow background.
2. If I want to double-click to copy, I just hold down the `option` key, then double click and it is copied to the clipboard. Selection is highlighted with a white background.
In both instances, I don't even need to use `ctrl+c`.