Off the top of my head, the easiest one to think of is a shopping cart. A few e-commerce platforms I know of (Squarespace, some Shopify plug-ins, etc) utilize a

Off the top of my head, the easiest one to think of is a shopping cart. A few e-commerce platforms I know of (Squarespace, some Shopify plug-ins, etc) utilize a fixed, floating widget for the cart.