It's because the input field will always appear at window level above the level ...

		ayrkm on Aug 9, 2015 \| parent \| context \| favorite \| on: Implementing a Sticky Input Field in iOS It's because the input field will always appear at window level above the level of key window. So, if you need to show overlay in key window, the input field will appear above the overlay. Example for such overlay could be a loading indicator.