Oh, boy! How dumb! Valentin, I guess it was your comment about the
default behavior being to display the toolbar when the file in the
active workspace is an .ly file. Since I'm a beginner, the first time I
saved a file, I made the unwarranted assumption that the .ly extension
would be added when I saved it, so my file lacked a file extension!
Since I loaded it from the "recent files" menu, it just continued to not
declare itself a lilypond file! Problem solved. Sorry for the bother.