Xcode File Browsing doesn't open files!

Previously when I browsed source code files using Xcode's file browser (located on left side), clicking on a source file would open it in the right window.

I must've accidentally changed some setting that stops this behavior -- now single clicking on any file doesn't do anything at all. I'd really like it to go back to the old behavior of opening the file.

Any ideas? This behavior is driving me crazy. To open a source file I have to double-click and open in a separate window.

Quit and restart Xcode.