An unexpected delay when making text invisible

[Marcin Borkowski <mbork@mbork.pl>]

Hi all,

I'm not sure if this shouldn't go to bug-gnu-emacs, but I'll ask here
first, because it may happen that it's me who's doing something wrong.

I've been experimenting with the `invisible' text property, and
I noticed that after running `add-to-invisibility-spec' the text I want
to make invisible can still be seen - at least until I e.g. change
buffers.  I thought that I needed to add `(redisplay)', but that didn't
help, either.  Here is a piece of code which exhibits the issue.

--8<---------------cut here---------------start------------->8---
(defun invisibility-test-setup ()
  "Set up the buffer for invisibility testing."
  (interactive)
  (switch-to-buffer "*Invisibility test*")
  (erase-buffer)
  (insert "This line is always visible.\nAnd this one not.")
  (local-set-key (kbd "t") #'invisibility-test-toggle)
  (goto-char (point-min))
  (end-of-line)
  (put-text-property (point) (point-max) 'invisible 'invisible-test-property))

(defvar invisibility-test-state t)

(defun invisibility-test-toggle ()
  "Toggle the state of the second line."
  (interactive)
  (if invisibility-test-state
      (remove-from-invisibility-spec 'invisible-test-property)
    (add-to-invisibility-spec 'invisible-test-property))
  (setq invisibility-test-state (not invisibility-test-state)))
--8<---------------cut here---------------end--------------->8---

I checked this in emacs -Q, too.  Say M-x invisibility-test-setup and
then:

- if you say M-x invisibility-test-toggle, everything is fine - but
then, the buffer is changed from the minibuffer to some other one.

- if you press `t', nothing happens until you e.g. switch buffers (just
  pressing M-x to change to the minibuffer is also enough).

Am I doing something wrong?  I expected that pressing `t' would switch
between visible and invisible instantly.  Why does it not?

TIA,

-- 
Marcin Borkowski
http://mbork.pl