Terminal Preferences

The enlightened tmux perspective is that you should always be using the keyboard to navigate between panes because it's better and there's no reason to use your mouse anymore, and that selecting text just sucks but you can always C-b+! to break a pane into a new window and then select it as normal.

lc's response is kinder than mine.  I was about to respond "that's because focus-follows-mouse is pure evil and anti-productive (which is a direct result of the fact that mice are useful for graphics and browser-like-navigation, and the keyboard should control everything beyond that)".  

[ edit: I apologize for the tone of the previous paragraph.  I intended it as jokey-flamewar-style, but I later re-read it and found that I just sound like a jerk.  Sorry. ]

I do agree that for the special case of text selection that requires scrolling (because it's larger than the pane), tmux gets in the way.  This is a rare enough thing for me that it's well worth it to do everything in a tmux session.  YMMV, obviously.

Note: I do recommend to anyone who spends more than a few hours per day doing this kind of work, in addition to getting good monitors, investing in a very good keyboard.  I've grown to love https://ultimatehackingkeyboard.com/ - the split keeps my wrists straight, which minimizes my RSI issues, the right-thumb trackpoint and lack of 10-key means I don't have to move far when forced to use the mouse, and the programmable layers mean I stay near the home row most of the time, and use VI movement for navigation (arrow, home/end, pgup/pgdn all use VI bindings on the mod layer).  I still use a timer to stand and stretch every hour, but this makes it much more comfortable and efficient for long periods of use.

When I tried using tmux to get a similar effect the main problem I had was that if I tried to select text with the mouse it didn't understand my panes. And I couldn't figure out how to get it to do focus-follows-mouse between the panes

This works in Ubuntu 20.04:

# selection
## Source: https://www.rockyourcode.com/copy-and-paste-in-tmux/
set-option -g mouse on ## key line
set-option -s set-clipboard off
bind-key -T copy-mode-vi MouseDragEnd1Pane send-keys -X copy-pipe-and-cancel 'xclip -se c -i'

## Bonus: Pasting, vim bindings
bind P paste-buffer
bind-key -T copy-mode-vi v send-keys -X begin-selection
unbind -T copy-mode-vi Enter
bind-key -T copy-mode-vi Enter send-keys -X copy-pipe-and-cancel 'xclip -se c -i'
bind-key -T copy-mode-vi 'y' send-keys -X copy-pipe-and-cancel 'xclip -se c -i'

From the place I got it: 

The above commands use xclip, a Linux command line tool for X11. You can replace xclip -se c -i with a platform-specific command like pbcopy (MacOS) or wl-copy (Wayland).

More generally, here is my .tmux.conf, in which copying with the mouse works, as does copying and pasting. 

Ah that makes sense. In my usage of terminal text editors I had left my text editor handle loading the different files and splitting the window, but I can see why you would want to just load them with separate terminals.