If you set the cart for displaying a popup when add to cart, it only works in PC desktop mode.
On mobile devices or small window, when you add to cart the popup is showing out of view and the sticky header crashes.
Thanks so much for reporting the problem, it occurs when the shopping cart is in menu widget.
I’ve fixed it for you by putting this code to the Theme editor module. .cart_popup #main_menu_widgets, .use_mobile_header .cart_popup #main_menu_widgets{display:block;}