focus-out-hook is obsolete variable as of 27.1

[choffee]

Hi,

Just to note that the focus in and out hooks seem to be depreciated in 27.1. I think that the after-focus-change-function was only introduced in 27.1 so might need an if on version number to support that.

Note that it still seems to work on 28 but just throws a warning.

New frame focus state inspection interface.
The hooks 'focus-in-hook' and 'focus-out-hook' are now obsolete.
Instead, attach to 'after-focus-change-function' using 'add-function'
and inspect the focus state of each frame using 'frame-focus-state'.
https://www.masteringemacs.org/article/whats-new-in-emacs-27-1

[slotThe]

Indeed, after-focus-change is supposed to be "more accurate" in terms of when it fires (apparently, focus-{in,out}-hook had some problems where it would trigger in situations where no obvious user-facing focus action had taken place). It is, however, much more manual to use; we'd have to check each individual frame for its focus state and then decide what to do based on that.

It's still not removed in Emacs 29 yet (last compiled approximately a week ago) and with Emacs generally having quite a long "grace period" for these kinds of things, I think the current solution will continue to "work" for quite a while. Still, if someone wanted to convert it to use the new interface I would certainly not object!