An assembly includes an airbag inflatable to an inflated position. The airbag in the inflated position has a first portion, a second portion extending transversely from the first portion, and a corner therebetween. The airbag includes a stay fixed to and rigid relative to the airbag, the stay having a bendable portion at the corner and extending from the bendable portion along the first portion and the second portion when the airbag is in the inflated position.