The background color on this div will only show for the length of the content. If you'd like a dividing line instead, place a border on the right side of the #mainContent div if the #mainContent div will always contain more content than the #sidebar2 div.
Donec eu mi sed turpis feugiat feugiat. Integer turpis arcu, pellentesque eget, cursus et, fermentum ut, sapien.
