Can I react to Teemo Strategist's trigger after the cards have been revealed, or only before?

Ruling: You cannot react after the cards are revealed but before damage is dealt - that all happens during resolution. You can only react before the trigger resolves or after it completely resolves (if there are still items on the chain or when you get priority/focus).

Sequence:

• Teemo is revealed and goes on the chain, resolves immediately (enters the board)
• Teemo's trigger goes onto the chain, target is chosen at this point
• Reaction opportunities available here (before trigger resolves)
• Trigger resolves: reveals 5 cards and deals damage to the target - this all happens during resolution with no reaction window in between
• If the chain is empty after resolution, the attacker gets focus and can play an action or reaction first
• If there are still items on the chain, you can add reactions to the remaining chain

Nuances:

• If Teemo is removed before the trigger resolves, the top 5 cards are still revealed but no damage is dealt because the trigger can no longer locate "here"
• Permanents (units and gear) resolve immediately after going to the chain with no reaction opportunity
• The target is chosen when the trigger goes on the chain, not during resolution