What happened to my messages window?

I am using Mail 9.2 with the classic layout. I pulled down the message list window all the way to see more of the list, and now I can't get back to the split window with the message list above the messages. There is a slider that pops up when I hover the mouse over the small dot at the bottom of the window, but when I slide it up nothing happens. All the other resizing window option seem to be working properly. I have confirmed that I am using the classic layout and I restarted the iMac but I can't restore the split window with messages below the message list. How do I get back to the two-section view?