This bug has been fixed in release 3.4 and later.
See bug 655489 comment 19 and commit
http://git.gnome.org/browse/gtkmm-documentation/commit/?id=43eea25b64623fd00a763d0b8b0c97bbb3f3249d

*** This bug has been marked as a duplicate of bug 655489 ***

But the bug is not fixed in the website, when is a new version of the website going to be published?

https://developer.gnome.org/gtkmm-tutorial/3.2/sec-custom-widgets.html.en
will probably never be corrected, but you can see the figure at
https://developer.gnome.org/gtkmm-tutorial/3.4/sec-custom-widgets.html.en
https://developer.gnome.org/gtkmm-tutorial/3.8/sec-custom-widgets.html.en
https://developer.gnome.org/gtkmm-tutorial/stable/sec-custom-widgets.html.en
https://developer.gnome.org/gtkmm-tutorial/unstable/sec-custom-widgets.html.en

I recommend https://developer.gnome.org/gtkmm-tutorial/stable/ unless you have
a good reason to prefer one of the other web pages. Don't expect that a
specific version of gtkmm-tutorial (gtkmm-tutorial/m.n) exactly describes
gtkmm-m.n. There is often a delay in the update of gtkmm-tutorial.

Thanks for your detailed answer Kjell, I understand it very well.

But I guess this means we need to improve how this is organized then. I arrived to that page because I googled some terms which led me to it. How should we deprecate an old-version page in favour of a "stable" (version-less) page? I guess this may be a question that kind of crosses the line to be asked to sysadmins. But should we stablish redirects too maybe?

https://developer.gnome.org/gtkmm-tutorial/stable/ is the latest stable
version. Now it's probably identical to
https://developer.gnome.org/gtkmm-tutorial/3.8/.

I guess you just have to accept that Google may lead you to both up-to-date and
out-of-date web pages. Few out-of-date web pages are clearly marked as such,
with a link to a newer page. This is something that does not concern only
gtkmm-tutorial. It concerns all or most of gnome.org, I think, and many other
web sites.