The initial attack and defense rolls are both simultaneously counted as the most current roll, although they are still considered separate. Otherwise, the current roll is the last set of dice rolled by any one player.
Outside of combat, the current roll is the last set of dice rolled by any one player. As soon as any other die is rolled, it becomes the most current roll.
Note: There are also many times when there is no current roll. After combat, after effects, beginning of each turn and after action cards: are all times when there is no current roll.
Diabolical: If a player plays Diabolical during the initial attack/defense (but before bombs/shields) they would choose which current roll to target, either the defense or the offense dice, and those dice would be rerolled.