"Back to top" buttons are a system-in-system effect (https://en.wikipedia.org/wiki/Inner-platform_effect https://en.wikipedia.org/wiki/Inner-platform_effect) an

"Back to top" buttons are a system-in-system effect (https://en.wikipedia.org/wiki/Inner-platform_effect https://en.wikipedia.org/wiki/Inner-platform_effect) and clearly an anti-pattern. Scrolling should be handled by the browser and if a user agent (for instance with a small screen) thinks it is neccessary to give help for scrolling, it should do so, customizable for the user. This is not the task of a website.