I think it was when she leaned forward and balanced on one leg that Barbara Hannigan jumped the shark. It wasn’t just a question of physical agility, although that was impressive enough. Hannigan performed her on-the-spot acrobatics while singing; the results were projected on to a big screen by three remote-controlled cameras, which zoomed in on her eyes, merged blurry images of her face and occasionally froze, meaningfully, on a particularly arresting posture.