As its title suggests, “Happiest Season,” the second film from Clea Duvall, opens on a picturesque Christmas scene. Abby (Kristen Stewart) and her girlfriend, Harper (Mackenzie Davis) are on a neighborhood Christmas decorations tour, and Harper is trying to cajole Abby into the holiday spirit. After a wayward excursion onto a rooftop, the couple’s night ends in giggles and kissing. Caught in the moment, Harper asks Abby to spend the holidays with her and her family.