PosY = 255
PosY + 1 = 256 | > 256 ? false
PosY + 2 = 257 | > 256 ? true

But if we base off +1, surely at +2 it will assert?

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I've removed a condition checking that both X + CrossX = X and Y + CrossY = Y, which I don't think will ever evaluate to true, since there are no zeroes in gCrossCoords. Could you verify this?