App Studio displays no Navigation Bar or left pane
We have some custom portals and wish to use App Studio to see how they would like in a mobile or tablet form factor. However, when we try to use App Studio, it is being fully replaced by our custom portal and there is no navigation bar or left pane where any settings can be selected. I managed to reproduce the issue with an OOTB portal. In the first partial screenshot you can see App Studio displaying without nav bar etc.
... and in the second screenshot you can see the nav bar and the left pane:
The difference between the two images is: the first one is from an application called "Clean" that I created in Pega 8.3.3 and the second one is from an application called "Clean" that I created in Pega 8.1.1. Both user logons are set up identically and both applications are layered onto CustomerService.
I can see some changes where made to pxExpress portal in the later version of Pega but cannot figure out what is going wrong. Has anybody else experienced this before? Or have any suggestions? Not being able to use App Studio affects our developers' productivity.