When in the resource packs menu, the background texture is shifted to the right by two pixels on the top and bottom of the screen. I've made a resource pack that highlights the problem, which is used in the screenshots below.

Definitely not the sort of groundbreaking discovery worthy of the coveted custom cape (though it IS an 8-year-old bug... dirt cape?), but I'm doing my part to make Minecraft a better place one (or I guess two) pixels at a time.