Limited Fixed Area using jQuery

Let's say that this is article area. The dimension of this area is 400 X 2000 pixels.

This is comment area. Keep the box fixed to the window but as soon as the window is scrolled to this area, stop the box.

This is the box. It should has fixed position toward the window and content area. However, it should stop scroll down when this box reaches the bottom of content area. Rather than using position: fixed, this box is positioned using position: relative and absolute combo.