Tabbedinterface: .tabs-style-6 nesting and resize fixed

Bugs were caused by trying to size and position tab panels that
were hidden or contained in a hidden parent element. Fixes are:

* Nested, sliding, tabbed interfaces now work correctly
* On resize, sliding tabbed interfaces are properly sized/positioned
Merge pull request #2666 from patheard/tab-nested

Tabbedinterface: .tabs-style-6 nesting and resize fixed