Bertrand Bonello’s film comes on like a narcotic, presenting the life of late designer Yves Saint Laurent in a series of oblique narrative vignettes. It’s an alternative, at least, to the similarly numbing proliferation of fashion-focused documentaries, including the one Pierre Thoretton made about Saint Laurent a few years ago, L’Amour Fou. Achieving redemption for his risible young Lecter in Hannibal Rising, Gaspard Ulliel here at last looks the part — he seems quite at home in boxy glasses and fey ennui, partying through the social upheaval of 1968, now and then sketching a dress while some Mozart plays to help him concentrate. “I’d like to stop, but I can’t,” this Saint Laurent says, a long way in, and something in the movie’s metabolism seems to echo that sentiment. Bonello’s detail-minded yet distant approach seems vaguely kindred with Mike Leigh’s in Mr. Turner, an equally lengthy and unconventional biopic, and who really knows why two-and-a-half hours of discursive, deliberate mystery can be easier to tolerate in one artist’s story than another’s? Maybe it has to do with the difference between an approximation of style and an approximation of life. But Saint Laurent is aesthetically rich, with exacting production design and a semi-jaundiced sepia hue that periodically evokes the brittleness of an aging newspaper. Maybe that’s the idea: that decadence, over time, has a way of turning into sentimental clutter.