Div扩展了页面高度，但没有滚动条

I have a div along the left side of my page for navigation links. Clicking on a header expands a subset of links. I have this div set to 100% of the page height so that the column takes up the entire left side of the page. The problem occurs when all of the sub categories are expanded. The content of the div runs off the bottom of the page, but doesn't add a scroll bar. I have a div along the left side of my page for