This package defines a global minor mode `companion-mode' whose purpose is to pair a companion popup buffer with a real buffer such when the real is displayed in a window, the companion buffer is also displayed and when it is not, the companion buffer disappears too.
companion-mode is a global minor mode. Use M-x companion-mode RET to turn it on or off. To set the companion of a buffer use M-x companion-mode-set RET.