Pack the tmux windows, keeping the order.

repack-tmux

#!/bin/bash
# you can use it from tmux
# bind-key P run repack-tmux

ttt () {
        local bef=-1
        for i in `tmux list-windows | cut -d':' -f1`; do
                local b=$(($bef + 1))
                if [ $b -lt $i ]; then
                        tmux move-window -s $i -t $b
                        bef=$b
                else
                        bef=$i
                fi
        done
}

ttt