This takes up more space than the previous UI! This is damning for my experience that previously relied on using StarterGui:SetCoreGuiEnabled(Enum.CoreGuiType.All, false) to disable the top-right button entirely and allow me to place my own buttons up there. I really liked the ability to simply disable all CoreUI (except the Roblox icon) and am devasted to see this ability go. (as anything outside MRU appears to be required)
Created a topic about this: The new CoreGui is a downgrade