Millikin

• WWizards of the Coast2022-12-08
  Official Ruling
  If an effect allows you to look at the top card of your library any time or to play with the top card of your library revealed, and you activate Millikin's mana ability while casting a spell, you can't look at the new top card of your library until you are done casting the spell. The same is true if you activate Millikin's ability while activating another ability.

Rulings via Scryfall