I chose Render > Render Image. I didn't like the composition and thought adjusting one of the menu items in the Blender Render window would resolve the issue (I forget which menu item).
But this removed the macOS Blender application menus & toolbars:
I quit Blender to start again. I didn't want to lose my edits, so I saved the file.
Blender displays fine for any other file, but when I open the file I was working on to resume work, the original issue occurs: the menus and toolbars are missing.