If Teemo Strategist is hidden and revealed as a reaction during combat (after the 'When I Defend' timing window has passed), does his 'When I Defend' ability still trigger?

Ruling: Yes, Teemo's 'When I Defend' ability still triggers. Attack and defend triggers now activate when a unit gains the attacker or defender designation, so when Teemo is flipped and gains the defender designation, his trigger goes on the chain.

Sequence:

• Opponent moves Yasuo to battlefield to start showdown
• Yasuo targets another unit with his 'When I Attack' ability
• Teemo is revealed as a reaction to the attacking ability
• Teemo gains the defender designation
• Teemo's 'When I Defend' trigger goes on the chain

Nuances:

• This rule change ensures units joining combat by any means still get their triggers, but still only once
• If Teemo is removed in response to his trigger (e.g., with Gust), his ability won't find "here" and won't ping anyone, though the trigger still resolves
• The "reveal 5" part of hidden still happens even if Teemo is removed, which can be relevant for cards like Nocturne