After experimenting a bit I think the issue may be related to differences related to app.xcodeworkspace and app.xcodeproj files. If I open the latter the Main storyboard it visible. If I open the former the Main storyboard is not visible. Since my app uses the CocoaMQTT pod, I need the app.xcodeworkspace to work. I also note that Version 15.2 (15C500b) is a relatively recent release. Perhaps the 15.3 beta will fix it -- I'm looking into that possibility.

@myuncle Good idea, thanks. I tried removing and then adding the file. Alas, no change. 15.3 beta does not fix the problem.

Creating a new storyboard does not solve the problem. Another clue: I notice that when I go to View>Show Library the normal UI elements are not listed. The tabs in the library dialogue are "code snippets", "media", and "color"