Content navigation breaks after first level

Currently I have setup a small test application and ran into some trouble lately. After some tries I still don't know where my mistake is.

Here is my pen: http://codepen.io/ins0/pen/wtELa

The problem is that after clicking the side navigation the content normally shows up, navigation updates. When I click on a link inside the content - the url state changes (eg. from #/settings to #/settings/about, xhr gets fired and received but the content doesn't get replaced.

I tried listening to all state events but no error is thrown.

I found the mistake by myself. In order to get the navigation correct you need to set a a viewtarget in your router configuration for childen pages that point to your parent view. Like this:

state('app.settings.about', { url: "/about", views: { 'content@app' :{ templateUrl: "about.html" } } });

see'content@app'. this tells the framework to render the about view in the content field defined in route app. I updated the codepen to a working example.