Can a player use Stupefy in reaction to Mindsplitter after seeing which card is chosen, and what is the timing of card draw and discard?

Ruling: When Mindsplitter is played, the opponent can react with Stupefy before Mindsplitter resolves. You cannot get priority midway through an effect being resolved, so the opponent cannot wait to see which card is chosen before reacting.

Sequence:

• Player 1 plays Mindsplitter and its trigger goes on the chain
• Player 2 can react with Stupefy at this point
• If Player 2 plays Stupefy, it resolves first and they draw a card
• Then Mindsplitter resolves and can choose from the 2 cards (original hand + newly drawn card) to discard
• If Player 2 passes priority instead, Mindsplitter fully resolves without priority returning to the opponent

Nuances:

• The opponent cannot wait to see which card Mindsplitter chooses before deciding to play Stupefy