overlay-testing.el

(defun ap/overlay-isearch-test ()
  (interactive)
  (with-current-buffer (get-buffer-create "*overlay-isearch-test*")
    (pop-to-buffer (current-buffer))
    (erase-buffer)
    (dolist (ov (overlays-in (point-min) (point-max)))
      (delete-region (overlay-start ov) (overlay-end ov))
      (delete-overlay ov))
    (insert "foo\n")
    (ap/insert-overlaid "bar\n" :transcluded-content t)
    (insert "baz\n")))

(defun ap/insert-overlaid (string &rest properties)
  "Insert STRING and overlay it, then return the overlay.
If PROPERTIES, add them as properties to the overlay."
  (let* ((beg (point))
         (end (progn
                (insert string)
                (point)))
         (ov (make-overlay beg end)))
    (when properties
      (cl-loop for (key value) on properties by #'cddr
               do (overlay-put ov key value)))
    ov))