colorcolumn is really useful. I would like a similar result to what it yields but only for the two lines that are at the top and the bottom of the screen (not the document, but the screen). Is there a way to achieve that?
More generally, is it possible to highlight or colorize a set of positions with row-column coordinates
(row_i, col_i), i = 1,...,N (again, those coordinates correspond to positions in the visible screen at any time, not the document)?