Convert pane into a window
panes
// what it does
Ctrl+b ! runs break-pane, pulling the active pane out of its window and turning it into a brand-new window of its own. The panes you leave behind reflow to fill the gap. Use it when a pane has grown important enough to deserve full-window space.
// shortcut
Ctrl + b!
// gotcha
The new window is created at the next free index, which may not sit next to where you started. If the pane is the only one in its window, tmux refuses with "can't break with only one pane" — there's nothing to break out.