Whether by chance or bold design, the Royal Opera’s two Christmas shows were written at precisely the same moment, between 1857 and 1859, and both mark a high point of refinement in their respective traditions. Both Wagner’s Tristan und Isolde and Verdi’s Ballo in maschera sometimes give the impression of being entranced by their abstract musical fantasy; the drama on stage is suspended, drawn out, barely engaged with as the characters and audience peer down into the writhing or transfixed world being created in the orchestra pit.