https://factorio.com/blog/post/fff-375
Hello, Today we are going to talk a little bit about a big feature in the expansion, Quality!
Quality of items in the expansion!
My bet (or hope) is that items stack with each-other still, even if they're of differing quality. Maybe when you hover over a stack of items in a slot, it'll show how many of each quality you have, and you can select/split specifically?
Though, inventory size is supposed to be further increased by higher-quality modular armours, which may also help...