appkit:status-item

(appkit:status-item title &key menu (dock t))

Puts an NSStatusItem of variable width in the system menu bar, its title drawn by the button the status bar owns, and hangs menu — an appkit:menu — off it. :dock nil asks for the accessory activation policy: no Dock icon and no app switcher entry, the shape a menu bar program has, in which case appkit:quit is the way out. Keep the answer in a variable: outside the status bar it owns the item's only reference, and letting it be collected takes the item out of the menu bar. appkit:set-text and appkit:text accept it, so a timer can rewrite the title. Part of the appkit package, a Cocoa widget layer written in rontolisp over objc and loaded on first use: macOS (java -jar, the rontolisp binary, or a compiled .class / .jar; never a .wasm), with a display. See the macOS GUI guide.

CL-USER> (defvar *item*
    (appkit:status-item "λ" :dock nil
                        :menu (appkit:menu (list (list "Quit" #'appkit:quit "q")))))
#<objc NSStatusItem>
CL-USER> (appkit:set-text *item* "λ 42")
"λ 42"