rule so often that it might rightfully be termed the best- established rule in appellate procedure. See, e.g., Metro. Reg'I Info. Sys., Inc. v. Am. Home Realty Network, 722 F.3d 591, 602 n.13 (4th Cir. 2013); Kensington Volunteer Fire Dep't, Inc. v. Montgomery Cnty., 684 F.3d 462, 472 n.4 (4th Cir. 2012); Mayfield