At level 18, Rogues get the Elusive class feature, which says:
No attack roll has advantage against you while you aren’t incapacitated.
However, if you have both advantage and disadvantage, they cancel out, and you are considered to have neither.
That being the case, if you have both advantage and disadvantage, and you attack a level 18 Rogue, does Elusive steal the advantage entirely and leave you with disadvantage? Or do they cancel out before Elusive is applied, causing you to roll a single d20?