Down comforters, pillows, featherbeds; any article of bedding or clothing that is filled with down or feathers, requires a special fabric, a "Down Proof" Lining Fabric, sometimes referred to as "Ticking", that will help prevent the down and feathers from leaking.
Proof Lining Fabric that is filled with feathers or a blend of feathers and down needs to be heavier and stronger than a fabric that is filled with only down. These heavier fabrics will also tend to be stiffer, and not as soft as a fabric that can be filled with just down, and no feathers.
It is the tiny pieces of broken down clusters, known as "fiber", and feathers with their sharp pointy ends that will inexorably work their way through the fabric and into your space. So, "fiber proof" or "feather proof" might be a more appropriate description, except for the fact that there is no fabric that is 100% fiber or feather proof.
Fabrics that are the most "down proof" (feather & fiber proof) are usually heavier and stiffer (not soft or cuddly), which is not a desirable attribute for a luxury down comforter or pillow.