This is mostly a post for myself to remind myself how to do this. I typically program on a remote server. I ssh in via the terminal and use tmux to manage sessions. I then edit code in neovim. This setup has a bunch of advantages and a bunch of disadvantages as well. One of the disadvantages is that I found it difficult to copy and paste from the tmux session in my terminal, to a completely different program running on my local machine, such as say my web-browser, or a chat client.
Today I came across this blog post explaining how to copy and paste from within tmux. Fortunately that also works for me, and copies it to my local machine's clipboard which is great because I can now use this to copy-and-paste lines of code elsewhere.
So, how to do this, without any additional configuration. First you enter 'navigation' mode in tmux, I'm told you can do this using Alt-Space but that is bound to something in my window manager, so you can also do prefix-[, where prefix is by default bound to Ctrl-b but almost everyone sets it to something else (Ctrl-a for me).
When in navigation mode you can use the normal vim keys to move up and down etc. Then, to start selecting you press Space, to copy the selection you press Enter and that's it. Selection is done in a block manner, but that can be quite useful, it's at least more useful than copying across splits.
One definitely slightly bad thing I still have is that it copies 'terminal' wise, therefore it doesn't know anything about your vim setup. For example I have visual whitespace which I find really helpful for seeing the (relative) line numbers of indented code. But that means when I copy I get the 'visual space' characters. So for example instead of x = 10 I get x␣=␣10. Not great. However, I can instead just 'cat' or 'less' the file in the terminal directly and copy-and-paste from there, rather than from within Vim.
Anyway I doubt this post is helpful to anyone but me, but there you have it.