当页面的预定部分出现时，如何滚动到顶部按钮？

I've got a scroll to top button I implemented via code that I got from one of the numerous examples out there. It works great. I changed the timing function for its appearance so that instead of appearing after 100 pixels it appears after 1340. I did this because there's some boxes on the right and I don't want the button to appear until they've been scrolled by and empty space thus appears for the button to fade in. The timing looked perfect -- the button appeared after what I figured was 1340 pixels from the top of the page.I've got a scroll to top button I implemented v