As a Roblox developer, it is not possible to put elements at the very top of the screen.
It is within my belief that by using SetCore to disable the top bar, developers would be able to fully customize the UI. This is not the case, as the topbar frame does not have it’s Active property changed to whatever bool is passed to SetCore (assuming using TopbarEnabled is the first argument). With the active property enabled, buttons cannot be clicked if placed there. That’s a whole 36 pixels gone to waste, save for if elements are text-only and are in the center-top of the screen or are banners for notifications or messages.