Make sure you're logged out and the page you're looking at has a vertical scroll bar (i.e. you need to scroll down to see the bottom of the page). Click on the watchstar icon, you'll see a new login drawer at bottom of the screen. Scroll down and you'll see the drawer scroll up. Happens with other drawers (for example, wikigrok version C) too.
Also set closeOnScroll to false in Drawer.js. Otherwise the Drawer just disappears on scroll.