Move a tmux pane from one window to another

Break the pane out via break-pane. This is bound to <prefix> ! by default.

Execute the tmux command:

:join-pane -t <window>

where <window> is the window number you want to move it into. You can optionally specify -h or -v to explicitly tell tmux to attach the current pane as a horizontal-split or a vertical-split on the target window, respectively.