I was intrigued by kmux but it doesn't seem production, or development, ready yet.
The installation - as a plugin - instructions were unclear to me, "kubectl plugin list" was listing the plugins I already had installed by krew, but not the scripts I installed in /usr/local/bin.
So I invoked kmux directly which complained about permission errors.
No doubt because I was already running in a tmux session - this should be detected if it's a problem.
I ran it in a non-tmux session, and all it did was kill my existing tmux session - ouch !
I'll come back when I have time to debug ...