Keep the Advantage

Progress is saved in this browser. A move keeps the advantage when it stays within 1.00 of the best move and at or above +1.00; it slips when the position stays at or above +0.50; otherwise the advantage is gone. For unlisted moves, the last listed evaluation is an upper bound at the database search depth. If that bound still leaves an advantage, an engine check is needed to distinguish slipping from losing it.