Can an opponent target Teemo, Scout with a reaction spell/ability after Teemo is revealed from hidden but before the chain resolves?

Ruling: Yes, opponents can cast reaction spells targeting Teemo, Scout after he is revealed from hidden. When Teemo unhides as a reaction, it starts a chain, and opponents can respond with their own reactions (but not actions).

Sequence:

• Teemo, Scout is played from hidden as a reaction, starting a chain
• Opponent can respond with reaction spells/abilities (cards that say "Reaction")
• The chain resolves in reverse order
• If opponent's reaction resolves first (like Gust), it can affect Teemo before his trigger completes

Nuances:

• Actions (like Void Seeker) cannot be played during a chain - only reactions can respond to reactions
• The opponent must wait for their turn or a showdown to play action spells