I will of course try to share what I found out, @bennyb – sorry I haven't gotten around to it before
My problem, when trying to make an extra in 2.3 was the fact that I was unable to load the
modExtraManagerController properly. Prior to 2.3 I would just make a "general" controller in the root folder of the extra (core/components/test/index.class.php). Then, in the controllers folder, you would add your "page specific" controller files (i.e home.class.php). The latter seemed to automatically include the former.
I could not make this work in 2.3, and in the end I ended up skipping the index.class.php altogether. Instead I made a default folder inside the controllers folder, and I then put the "page specific" controller files there (i.e home.class.php). In each of these files I extended the class like this:
class TestHomeManagerController extends modExtraManagerController and
class TestEditManagerController extends modExtraManagerController.
There may be a simple way of making a general class file that can be included in the specific class files, but I haven't gotten that far yet
Hope this was helpful! Good luck