At the outset of her career, Kacey Musgraves bent country tradition to her will, writing songs that evoked the sound of classic country but were infused with progressive ideas that reflected her millennial perspective. "Merry Go Round" and "Follow Your Arrow," the big singles from her 2013 major-label debut, Same Trailer Different Park, crystallized this gift and helped establish Musgraves as a major voice in country music: she was the rare country singer who could cross over to a rock audience without playing pop. By the end of the decade, she expanded her musical horizons so...