current-buffer returns a stale value immediately after display-buffer
returns.

The following snippet (saved in x.el), when loaded in 25.1.1 using
[google-emacs -Q -l x.el], generates the message:
*scratch* *scratch* *Help*
demonstrating that (current-buffer) has a stale value after describe-key
returns (I expect post to match the [run-at-time 0] value, not the pre
value).

(progn
  (setq display-buffer-alist '(("." (lambda (buffer alist)
(switch-to-buffer buffer)))))
  (setq pre (current-buffer))
  (describe-key (kbd "SPC"))
  (setq post (current-buffer))
  (run-at-time 0 nil (lambda () (message "%s %s %s" pre post
(current-buffer)))))

This is an extremely reduced test case that reproduces the problem that is
causing https://github.com/mina86/auto-dim-other-buffers.el/issues/10
 (AFAICT).

Cheers,
-a