On an iPad, the zooming behaves oddly because the header bar zooms at a faster rate than the body, so you get into situations where the crucial right hand side of the navigation bar is off the page and impossible to access.
Worse still, on the iPad when it has never seen the site before, its default state is needlessly zoomed in somewhat, and the zooming causes the nav buttons to be cropped and inaccessible.
You can see a variant of this behaviour on the desktop if you zoom in. Because the navigation bar is fixed to the window, you get a case like this:
Note that that is fully scrolled to the right of the window, but the navigation controls are still further right and inaccessible.
As I say, on the iPad it is particularly bad as unless the user thinks to pinch zoom to reset the scale, they cannot sign in at all and will be thoroughly confused.
Thankfully, once you have pinch zoomed the screen, it behaves normally when zoomed to fit the iPad screen.
I tried to look at the CSS to figure out where things are going wrong, but I’m CSS positioning it beyond me at the moment.