I appreciate this is off topic and not a solution but on iOS you can tap the hole on top of your screen to scroll to the top on any application.
Some applications have neither a beginning nor an end, so scrolling to the beginning makes no sense. At best, you could make "scroll to the beginning" get back to the start of your navigation, but then your scrollbar wouldn't let you go further.
(also if you have both horizontal and vertical scrolling, you're still left with no horizontal options)