Xcode had [mostly] single window mode since 2.0 or so. It wasn't default, though...

dchest · on June 22, 2011

For downvoters: Xcode 3 > Preferences > General.

Layout: All-In-One

"This layout provides virtually all operations within a single window, such as editing files and project content, filtering in the detail view, viewing the build results and build log, and debugging".

http://i.imgur.com/uW3SI.png

bonch · on June 23, 2011

It wasn't a true single window mode; there were still secondary windows, and Interface Builder was not integrated.