General developer forum

I haven't heard that, but that is not surprising, I really do not contribute code. Glad to hear it is going into core though, a great resource for some subjects. However, I would suggest that two level side navigation is  sufficient, any more and you are adding a level of complexity to the code, but also to the entire navigation of a book. You can, and I have done this, go overboard with many levels and by the time you get to the end point the start can actually be lost. You might want to try this on a web page yourself sometime, check the design parameters. Write your web page, then your second level then third and forth and so on, you will see how easy it is to get lost. Imagine if you are a new user, do not know the site, see what happens? You lose where you started from and that gets frustrating. I suspect that more than two levels is asking for trouble...