消除相对定位div的水平滚动

I want to keep a div (id='fixedDiv') at the top of the window while the user scrolls up and down the webpage. The page has another much taller div (id='tallDiv'). I want the user to scroll the page up and down to see the content of tallDiv, and fixedDiv to always be displayed at the top of the window during the scrolling.I want to keep a div (id='fixedDiv') at the top