Minor bug, Beethoven's 5th was off by a measure for me. (Goldberg variation was spot-on).
Also, right/left arrow keys aren't skipping measures (as the keyboard page says), but are skipping whole sets of measures.
But overall, as a musician, I actually found it rather distracting to watch the line try to follow the notes exactly, as the line constantly sped up and slowed down due to rubato, etc. -- indeed, in particularly fluid pieces, it can even be possible that the right and left hands don't match up precisely in actual performance.
It might actually be more clear, and far less distracting, if you followed the time signature for the beat, and simply highlighted all notes in the current beat within a light-pink rectangle overlay, and then that box would instantly disappear, and the next one appear, upon the next beat.
That way, we can focus with our ears on the actual movement in the music, without being distracted by visual movement, but still have a perfect visual indicator of where we are.
But anyways, great work and great idea! Kudos, well-done!