6H7WAQUD2RDKO5PM2GCHRPNUE76UIT37NQ52ZWVDW46VQGIKHLBQC -- return (line, pos) of the screen line starting at or just below a-- given a y offset
-- return (line, pos) of the screen line starting near a given y offset, and-- y_offset remaining after the calculation-- invariants:-- - 0 <= y_offset <= Line_height-- - let loc, y_offset = schema1_of_y(pane, y)-- y - y_offset == y_of_schema1(pane, loc)