No matter if modals are presented or the user performs any type of segue.
Is there a way to keep the button \"always on top\" (not the top of the screen) throughout
You can use the zPosition property of the view's layer (it's a CALayer object) to change the z-index of the view, also you need to add this button or your view as subview of your window not as subview of your any other viewcontrollers, its default value is 0. But you can change it like this:
yourButton.layer.zPosition = 1;
You need to import the QuartzCore framework to access the layer. Just add this line of code at the top of your implementation file.
#import "QuartzCore/QuartzCore.h"
Hope this helps.