Checking my projects for building against iOS 11/iPhone X, and wanting to ship a new utility app (more on that later), I had to use Xcode again. And I screamed in rage, and cried, and this is why Mark drinks.
Filed a "suggestion" in Radar:
Open a project in Xcode. Double-click a file in the Project Navigator to open it in a window, resize & place it somewhere to work, then close. Repeat. Note that sometimes it'll keep the same size, but never the same position as previously, but after a few files it returns to a window sized like the main project window, randomly placed.
There's a concept called "spatial memory", which both the Finder and Xcode actively sabotage now, an old but still valid complaint: About the Finder…, by John Siracusa
Suggestion: Record the position & size of each file's window, and reuse that when opening a window. When switching to an alternate file, change the window size & position, do not destroy the current file's position. Maybe make this a preference setting, called "Project Builder Mode".