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-- 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)