When moving Travelling Merchant to a contested battlefield, can you respond to the move trigger by playing Rengar (who can be played at reaction speed to a battlefield you're attacking) to that battlefield, allowing Rengar to benefit from the discard/draw trigger and be an attacker?

Ruling: No. You cannot play Rengar to the contested battlefield in response to the move trigger because you do not have attacker status yet. The move trigger being on the chain prevents combat from being initiated (the game is in a closed state, not neutral open). You only gain attacker status when combat actually begins, which happens after the move trigger resolves.

Sequence:

• Moving to a battlefield applies the contested status and triggers the move ability
• The move trigger goes on the chain (game is now in closed state)
• Combat is staged but cannot initiate until the chain is empty and the game returns to neutral open state
• While the move trigger is on the chain, you are not yet an attacker
• Once the move trigger resolves and combat begins, attacker/defender designations are given
• By that point, Rengar would already be discarded

Nuances:

• Contested is a status applied to the battlefield, while attacker is a designation given to a player only when combat begins
• You can play Rengar to your base or to a battlefield you control during the move trigger, just not to the contested battlefield
• Applying contested to a battlefield leads to becoming the attacker, but these happen at different times