NowPlaying, QuizToolbar and SongControls become their own scenes, the auto
repeat switch moves into the music player scene, and downloading songs moves
into a SongLoader node. The quiz only wires the parts together. Every node
keeps its place on screen.
Screenshots of every UI state are pixel-identical to before, the layout test
still matches the original baseline, and each new part has its own tests.
Sansan
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