I just realized a qualifier on using the Undo function in Xcode. Such a head slapping moment deserves a brief mention. For context, I usually switch among files by clicking on a file in the Navigator area. Sometimes, I would switch files and immediately want to undo some typing, but there seemed to be no effect. It turns out that the Undo function depends on whichever window/view has focus. Since I just clicked on a file, the Navigator has focus. Hence, clicking in the main editor will allow Undo to function as I had expected – mystery solved.