From Wikimedia.
While I have not seen anything about it, I believe the flat panel over the Anne-Marie's head (and the heads of many others at this time) may be created by a piece of fine diaphanous cloth stretched between two fine wires over her head that support the pouffs of hair at the sides of her head to create the hurluberlu coiffure. This would transfer the load of her locks and any framework within them to her skull and allow the hurluberlu to be worn indefinitely. The hair in front of the flat panel is, as is often done, formed into curly bangs.
Keywords: de Seve, Anne Marie Louise d'Orléans, Duchesse de Montpensier, Princess, French, Bourbon family, Fronde, hurluberlu coiffure, veil, off shoulder scoop neckline, chemise, modesty piece, cuffs, elbow length puffed sleeves, full skirt, necklace, earrings, clasps, jeweled arm bands