With tmux's mouse mode enabled (set-option -g -q mouse on), you can use the
mouse to make a drag selection of some text. You can then paste it within a
tmux session using Cmd+v.
If you are using tmux within the Kitty terminal emulator, you'll get more text pasted than you bargained for. Kitty's clipboard appends by default. So after several selections have made there way into tmux's buffer history, a paste will result in all of those buffers values being appended together and output to the terminal.
To fix this, you need to change Kitty's clipboard to be no-append.
# ~/.config/kitty/kitty.conf
clipboard_control write-clipboard write-primary no-append