Error (use-package): beacon/:config: Symbol’s value as variable is void: after-focus-change-function about beacon

[purcell]

The culprit here is 0cf8b60. You can either use after-focus-change-function and require Emacs >= 27.1, or use the previous obsolete focus-{in,out}-hook. Probably an (if (boundp 'after-focus-change-function) ... ...) form could allow you to support either if that's desired.

[purcell]

Yep, seeing this too in my broken CI builds for some Emacs versions. Specifically, beacon.el has

        (add-function :after after-focus-change-function
                      #'beacon--blink-on-focus)

but in Emacs < 27.1, there's no such variable as after-focus-change-function, so dereferencing it in Emacs 26.1 (for example) will naturally fail, despite beacon.el declaring Emacs 25.1 as its minimal compatible version.

[philipp-classen]

I also get it on Emacs 26.3 (but not on Emacs >= 27).