The little black note for "on top of" says that the "the" is absolutely required here. How would you say "This is on top of the screen", then?
As in a physical object you put on top of the monitor? I would say „to stoi na ekranie”.
'At the top of' and 'on the top of' and 'on top of' should all be accepted as answers as they all mean the same thing in English?
I consulted it with a British native, and from what I learned, only 'at the top of' should work ('on the top of' worked so far). What the Polish sentence means, is that "this" is in the upper part of the screen, like the highest row of icons on your desktop.
"on", as far as I understand, would treat "screen" like it was the monitor, and something physically put on it.