top: mobileNavHeight + 'px !important'
with this one…
top: mobileNavHeight + 'px'
And it will work again on iOS6 in your responsive web design.
Responsive web design navigation
There are many ways of achieving good navigation on smaller devices in your responsive web design projects. There seems to be no set standard yet and many websites use navigation features such as drop down menus, responsive navigation bars that expand, pull down to reveal, slide in from the side navigation and many more. A lot of it comes down to the complexity of your responsive web design project and how the website design will look on smaller devices. It’s a case of working out whats going to work best for your website design. We do have a kind of standard forming on mobile navigation in that lots of people are starting to use the convention of the 3 line icon to denote a navigation element that will reveal a full nav bar on responsive web design mobile sites.