tmux: Add prefix mode

• Enter prefix mode (yellow) with the prefix key (type the prefix key twice).
• Exit mode (green) with Escape.

~/.tmux.conf

run-shell {
  tmux bind-key #{prefix} set-option key-table prefix
}

bind-key Escape set-option key-table root

Note: bind-key does not work with variables, hence using run-shell instead of simply:

bind-key '#{prefix}' set-option key-table prefix

You might want to display the current mode with a different color in the status line.

~/.tmux.conf

set-option -g status-style 'fg=black,bg=#{?#{==:#{client_key_table},root},green,yellow}'

Discussion: reddit • tmux • Add prefix mode

See also Make tmux modal.