On 2022-06-26 04:10, Eli Zaretskii wrote:
>> Regardless, given that the temporary buffer is modified in my
>> original example, shouldn't `after-change-functions' run?
> 
> Is the temporary buffer indeed modified, and is the hook set in that
> buffer?

Yes on both accounts.  See updated test below.

I changed the test to switch to the temporary buffer in the selected window and it now reliably inserts "x" into the temporary buffer.  It still doesn't run the `after-change-functions' hook, however.  Updated test code:

;; -*- lexical-binding: t; -*-
(require 'ert)
(require 'ert-x)
(defvar-local acf 0)
(defun acf (&rest _) (setq-local acf (1+ acf)))
(ert-deftest test ()
   (ert-with-test-buffer ()
     (let ((b (current-buffer)))
       (save-window-excursion
         (with-current-buffer-window b
             `(display-buffer-below-selected
               (body-function
                . ,(lambda (window)
                     (select-window window t)
                     (should (eq (current-buffer) b))
                     (setq-local acf 0)
                     ;; Note that LOCAL is t:
                     (add-hook 'after-change-functions #'acf nil t)
                     (should (memq #'acf after-change-functions))
                     (execute-kbd-macro (kbd "x"))
                     (should (equal (buffer-string) "x"))
                     ;; The above checks pass, this check fails:
                     (should (equal acf 1)))))
             nil)))))