Dropdown login problem

Hi everyone,

Im having a problem with this dropdown login in my navbar (Only in iPhone). I tested in iPhone 6 and iPhone 6 plus and I have the same problem with the simulator (https://appetize.io/demo). The problem is the following: When I tap the input text to login the marker goes down and cant focus well, but when Im at the bottom of the page it works properly (Picture) Anyone can help me? Thanks.