title: How to make div's width not change with the browser window's
body: <p>There's a header div and it's width should be the same with the screen(not the window).I tried to set it's width to be 100%,
  but When the browser window became narrower the div became narrower too, so when I scroll to the right, the header div isn't long enough to the right.<br>How to make the div width not change with the window?</p>