When Rek'sai's attack trigger happens, is she on the battlefield?

Ruling: Yes, Rek'sai is on the battlefield when her attack trigger happens. Attack triggers occur at the beginning of a combat showdown, which happens after a move is finished into an opposing battlefield.

Sequence:

• Move is finished into an opposing battlefield
• Combat showdown begins
• Attack triggers happen at the beginning of combat showdown

Nuances:

• Cards revealed from the attack trigger can contribute to winning the battle