You are referring to DietBar. I LOVE this tweak too except it's glitchy in some apps. For whatever reason, in some apps the button in the top left of the navigation bar will look weird and sometimes will be hard to use. Other times the view just under the navigation bar will look really goofy. It's too bad because it's a very slick UI tweak which frees up a few rows of pixels. Not sure why those navigation bars are so big to begin with in the first place! Haha. Luckily a few months ago Ryan Petrich, the developer, pushed an update to DietBar which allows you to enable or disable the tweak in every individual app. This is at least better than nothing so if an app looks buggy with the tweak you can just turn it off for that app. I just wish it uniformly worked across every navigation bar!