Thanks to Bill3D for a useful hint. I have added the information to my Feedback Assistant report. I will try your suggestions presently and will report here what happens.

MobileTen's link is very useful, but I could also use some information on how to access arguments to a C++ function call from assembler within the function. I think there is a special syntax for dealing with inputs and outputs between C++ and embedded assembler, but I do not know how to use it for CLang. Any pointers, anyone?

And yes, the storyboard is not just scrolled off view. There is no "Minimap" option in the Editor drop-down menu. I suspect that Xcode simply cannot find the right editor when I open the nib from within the project, though it does when I start with no Xcode running and use the Finder "Open with" option to open the file with Xcode. New projects do not have the problem. And the nib did open correctly in older Xcodes.

Responding to Claude31: Sorry to take so long ... When I select the project and open the file inspector, I see a panel at the right with the name and location of the project file correctly shown, but nothing else is visible there. There is no project-format information shown. The issues navigator shows many issues related to the code itself when I build or analyze -- this is an old project that needs lots of refactoring -- but nothing that suggests a problem with the project file itself. There is no option for "source code" in the "Open as" menu, but when I investigate with (e.g.) EMacs, I find that "MainMenu.nib" is a directory containing three ".nib" items; namely "designable.nib", "keyedobjects.nib" and "keyedobjects-101300.nib". All three are files. "designable.nib" is HTML, and the other two appear as binary files with the text "NIBArchive" the first thing in the file. The HTML for "designable.nib" starts with: